1
UNIVERSITY OF HERTFORDSHIRE
Faculty of Engineering and Information Science
BSC (HONS) MEDICAL ELECTRONICS
Project Report
THE SIMULATION OF
BODY TEMPERATURE
STEVEN BALL
APRIL 2001
2
Abstract
Being able to simulate a physiological system is becoming an increasingly important
area in medicine, as it provides distinct advantages in both training of medics and
research opportunities. This report will discuss a recent project completed, which
involved producing a piece of hardware that can as accurately as possible simulate the
physiology of human body temperature. The main aims were to simulate body
temperature both under normal conditions and under fever. As the findings of this
report will show, a working model was completed capable of producing a constant,
stable temperature and therefore meeting most of the specifications laid down.
Included within the report will be comprehensive details of the design of the
hardware, and also a discussion into the improvements that could be made to enhance
the hardware further.
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Contents
1. Introduction…….……………………………………….…………………………1
1.1 Background………………………………………………………………..1
1.2 Objectives and Specification………………………………………………2
1.3 Overview of Report………………………………………………………..3
2. Human Body Temperature……………………………………………………….5
2.1 Homeostasis……………………………………………………………….5
2.1.1 Core and Shell temperature……………………………………...5
2.1.2 Obtaining a Heat Balance………………………………….…….5
2.2 Negative Feedback…………………………………………………….…..7
2.3 Temperature Measurement and it’s Diagnostic Value…………….………7
2.4 Fever………………………………………………………………….……9
2.5 Treating Fever……………………………………………………………10
3. Design……………………………………………………………………………..12
3.1 Design Concepts…………………………………………………………12
3.2 Past efforts……………………………………………………………….13
3.3 Control Circuit Theory…………………………………………………..13
3.3.1 Op-amp Switching……………………………………………..14
3.3.2 Comparator…………………………………………………….14
3.3.3 Darlington Pair…………………………………………………15
3.4 Temperature Box Design………………………………………………...16
3.5 The complete Hardware Design………………………………………….18
3.5.1 Power Supply…………………………………………………..18
3.5.2 Thermistor…………………….………………………………..18
3.5.3 Potentiometer…………………………………………………..19
3.5.4 The op-amp…………………………………………………….19
3.5.5 Transistors……………………………………………………...19
3.5.6 Heat Cell………………………………………………………..20
3.5.7 Circuit in Operation…………………………………………….20
4. Testing, Experimentation and Results………………………………………….22
4.1 Reaction time Experiments………………………………………………22
4.1.1 Methodology of Experimentation……………………………...22
4.1.2 Variables………………………………………………….…....23
4.1.3 Response time Results………………………………………….23
4.1.4 Analysis of Results……………………………………………..26
4.2 Circuit Performance and Testing………………………………………...27
4.2.1 Potentiometer – Calibration……………………………….…...27
4.2.2 Circuit Stability………………………………………………...28
4.3 Summary of Results……………………………………………………...29
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5. Conclusion………………………………………...……………………………...30
5.1 Overview of Project……………………………………………………...30
5.2 Outcome………………………………………………………………….31
5.3 Summary of Achievements and failings…...…………………………….31
5.3.1 Objective one…………………………………………………..31
5.3.2 Objective two…………………………………………………..32
5.3.3 Objective three…………………………………………………32
5.4 Improvements and Recommendations…………………………………...33
References……………………………………………………………………………35
Bibliography…………………………………………………………………………36
Appendix A - Glossary………………………………………………………………i
Appendix B - Parts List……………………………………………………………..ii
Appendix C - Time Management Plan……………………………………………..iii
Appendix D - Survey of Available Clinical Thermometers………………………..iv
Appendix E - Extract from 741 Op-amp Data Sheet……………………………….v
Appendix F - Extract from BD437 Data Sheet ……………………………………vi
Appendix G - Timing circuit theory………………………………………………..vii
Introduction
5
1. Introduction
1.1. Background
Simulating a human physiological system is a growing area of science in which a great deal
of work has been, and is currently being, undertaken. To be able to simulate one of the bodies
many systems provides an excellent facility in areas such as research and teaching where
there is a great demand to have a realistic working system, which scientists and clinicians are
able to work on or observe without the need to involve real patients.
In the Centre for Advanced Healthcare Technology, at the University of Hertfordshire, the
technology already exists whereby an ECG, SaO2 and blood pressure reading can all be
obtained from a life like manikin.
This facility is regularly used by paramedics and nurses for training in areas
such as resuscitation and cardiac observation. Here students have the opportunity of
interacting with the manikin by carrying out various procedures, and then observing the
physiological changes they cause in real time. Although much has already been achieved in
biomedical simulation, there are still plenty of areas where further development can take
place. If the simulating and modelling of more physiological systems can be completed then
these can be incorporated within the existing unit making a more advanced and life like
patient simulator.
One area where only a limited amount of work has taken place is the simulation of body
temperature. Generally speaking recording body temperature isn’t regarded that
important in emergencies like cardiac arrest, but as this report will discuss the measurement
of body temperature can be a primary indication of an infection or other illness. So being able
to have an electronic unit which copies the behaviour of body temperature would prove
a useful additional tool for an intensive care simulator.
Introduction
6
The main aim of this project was to do exactly that, and design and construct a piece of
electronic hardware that can as accurately as possible simulate human core body temperature,
both under normal conditions and due to fever. This simulation unit could then be embedded
in the manikins mouth allowing the user of the patient simulator to insert a probe and record
a temperature reading.
1.2. Objectives and specification
The maintenance and change of human body temperature is in itself, very complex, and
depends upon many physiological and pathological factors within the body. It would
therefore be very difficult to be able to simulate every aspect of body temperature,
particularly as this project has a limited budget of £50 and a limited amount of time in which
it must be completed. Therefore to prevent the project from becoming too complicated,
a set of objectives and a specification were drawn up, showing what it is hoped the completed
piece of hardware will be able to do.
There were three main objectives to the project, these were:
a) To produce a working heat cell fixed within a box that a temperature-measuring
probe could be placed into, to take a reading.
b) To design a circuit which will operate the heat cell, to demonstrate normal
core body temperature, and this temperature should be recordable using a
mercury thermometer within a practical, realistic time period.
c) To add an addional part to the circuit so that it can be switched from
demonstrating normal core body temperature to show the temperature change due
to fever in real time.
Within these three objectives a more precise working specification was derived. This
specification gives a clearer statement, including numerical values, of what exactly the
Introduction
7
simulation should be able to do.
a) When working in normal body temperature mode, the heat cell should remain at a
constant temperature of 37°C. (Designated as normal core body temperature
by General Medical Council.)
b) When working in fever mode the heat cell should increase from 37°C to 39°C, in a
time of two hours.
[It should be noted that fever is a general term used for a raised core body temperature, there
is no definitive value for fever or how fast it may occur, as this will vary from person to
person, and illness to illness. The above statement was formed to clarify what this
particular simulation will do.]
c) The temperature box in which the heat cell is contained, should be designed
to work in real-time, so, when a mercury thermometer is placed into it, an
accurate reading can be obtained within less than a minute.
While carrying out a project like this, which has a limited time period in which it must be
completed, it is important to have a time plan to follow so the deadlines can be clearly
identified and the project can be well managed. A time plan for this project can be found in
the form of a Gantt chart at Appendix C.
1.3 Overview of report
Before going into detail about the project itself and the design and development of the
hardware, it is necessary to have a good understanding of what exactly is meant by human
body temperature and what importance it has in medicine. Section two of this report will
discuss the physiology of temperature control and how negative feedback is used to maintain
a homeostatic state. Also this section will give an overview of fever and methods of reducing
Introduction
8
fever and returning the body temperature back to normal.
Section three of the report concentrates mainly on the design and construction of the
hardware. It shows the different stages of how the simulation was designed and built up
including all modification that were made with an analysis of why they needed to be made.
Section four is the results chapter. Here data is presented, summarising graphically the
measurements that have been taken during the project. These measurements are mostly
concerned with the speed of the heating and cooling of the temperature box.
The final section, section five is the conclusion, this includes an overall summary of the
project, its successes and failures and suggestions of future improvements and developments
that could be made, should similar projects ever be attempted.
Human Body Temperature
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2. Human body temperature
2.1. Homeostasis
2.1.1 Core and shell temperature
The body’s internal temperature or core temperature, needs to be kept fairly constant in order
for the body to work at an optimal rate. The various different organs and structures in the
body all function at their best if the internal environment they are based in, is kept at a
constant temperature of around 37°C. In contrast to the core temperature, the body also has
what is described as a ‘shell’ temperature. This shell temperature is basically the surface
temperature of the person and includes the skin and underlying fatty tissue. [1] The shell
temperature will tend to be of a lower temperature than the core, as the body’s surface is
exposed to the outside environment where air temperature and layers of clothing all have an
effect. Because of this temperature difference between the constant core temperature and the
outside environment, a temperature gradient will exist across the shell, and consequently,
following the laws of physics, a heat exchange will take place.
2.1.2. Obtaining a heat balance
For the body to maintain homeostasis (see Glossary), a heat balance is required ensuring
that the amount of heat produced is equal to the amount of heat lost. This heat balance is
determined by both physical and biological factors, and can be expressed as a simple
equation.
Heat balance for human body temperature. M = E +/- C +/- K +/- R [1]
The Metabolism (M) is the main way in which the body produces heat and is the total sum of
all the chemical reactions that occur within the body. Metabolism is usually expressed as the
Human Body Temperature
10
metabolic rate and this metabolic rate, the rate at which the body produces heat, will
vary with exercise, but will always be occurring, even when asleep, as there is always some
form of reaction/exchange occurring within the body. Key heat producing factors within the
body include thyroxine, a hormone that regulates the rate at which energy is released from
food to produce ATP (the bodies main energy), the liver which is always metabolically active
and adrenaline released by the body during stress situations. Adrenaline will , like
thyroxine, increase the rate in which ATP is formed. (see Glossary for ATP definition)
Evaporation (E), Convection (C), Conduction (K) and Radiation (R) are the four main ways
of losing heat.
Evaporation is when heat converts water into water vapour and in the case of a
human, this occurs when water passes through the skin to the surface as perspiration.
Convection occurs when there is a temperature difference between the skin and air; a heat
exchange then takes place to balance the two temperatures.
Conduction doesn’t play a big part in temperature loss, but occurs when the skin comes into
contact with a solid object, which is either hotter or colder than the skins temperature,
for example clothing.
Radiation is similar to convection in that if the body is warmer than the surrounding
environment, the body will radiate heat out of the body towards the cooler environment.
Heat is also lost from the body during the process of urination and defecation.
All of the above are processes, which are happening all the time in order to maintain
temperature homeostasis. However, there are occasions, such as under exercise and
exposure to cold conditions, when the body needs to take extra steps to keep the 37ûC.
These ‘extra steps’ are implemented and controlled by way of a negative feedback loop.
Human Body Temperature
11
2.2 Negative Feedback
Obtaining a constant core temperature of 37°C is achieved by the body in the way of a
negative feedback loop. This feedback loop consists of a series of sensors and
effectors, which are all controlled by, or report to an integrating centre. In the case of
temperature, the integrating centre is a part of the brain called the hypothalamus. The
brain receives information from the various sensors within the body about the temperature,
and if any deviation from the set point was occurring the brain would then
send messages to the effectors, so the effectors can act appropriately to return the temperature
back to it’s original set point, 37°C.
The temperature sensors in the body are in the form of cold receptors, which can be found
mainly in the skin and spinal cord. The effectors, which the hypothalamus will ‘activate’ if
there is a large deviation from the set point, involve various different types of responses from
the body. These include:
- dilation of the blood vessels, allowing a greater amount of heat transfer
to the skins surface (if above set point).
- Shivering, which amounts to muscle twitching, will promote greater
heat production (if below set point).
- Sweating, increases the rate of evaporation (if above set point),
- Vasoconstriction reduces blood flow to the skins surface
(if below set point).
2.3. Temperature measurement and its diagnostic value
Taking a persons temperature is probably the most common and easiest physiological
measurement used in medicine, and can provide a good indication into whether the patient is
suffering from conditions such as hypothermia (a lowering of core temperature) or fever
Human Body Temperature
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(a raising of core temperature). A person suffering from an abnormal core temperature
such as a fever, can also provide initial evidence to a clinician that the person may also
be suffering from other conditions, which will require further investigation. These might
include infection and disease. Fever will be discussed further in section 2.4.
These days there are many different devices available to measure temperature, some of which
can be found in Appendix D. The traditional method, which is often still used today, is the
thermometer. The thermometer would normally be inserted into the mouth of the patient, but
other sites are sometimes used – such as under the armpit or in the rectum. A rectal reading is
regarded as the most accurate measurement of core temperature (not including implanting
probes within the body) but oral provides a reasonably accurate reading, and tends to be more
convenient to do. A thermometer placed in the patients mouth would normally take less than
a minute to give a reading.
Although 37ûC is always stated as the ‘normal’ temperature of a human being, over a 24 hour
period, the ‘normal’ temperature will actually fluctuate around this figure, and can deviate
by up to half a degree centigrade. The graph in fig. 1 shows this variation in normal
temperature through out the day and night. There is no exact explanation for this, but,
research has shown that the fluctuations are partly due to the work and sleep patterns of the
person. An abnormal temperature is normally regarded as a temperature above 38ûC and
below about 35ûC. The absolute extremes of a human’s internal temperature would be around
29ûC and 43ûC. Outside these limits the person would almost certainly die due to severe cell
damage.
.
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Temp
(°C)
37.5
37
36.5
20
24
4
Time (hours)
8
Fig. 1. Daily variations of core temperature.[1]
2.4 Fever
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Human Body Temperature
Pyresis or fever as it is more commonly known is regarded as an elevation of body
temperature above the normal core temperature of 37°C. The most common reason for this
elevation to occur is because the person is suffering from an infection or disease within their
body.
When a part of the body becomes infected, the infecting bacteria or virus sets off a
chain of biological events within the body, resulting in the hypothalamus raising the set point
of normal temperature to induce a fever. In order to maintain this new, higher set point
temperature the body has to take various action to increase heat production and reduce heat
loss - these include shivering, vasoconstriction and raising the metabolic rate (as described
in section 2.2). The metabolic rate will increase by about 7% per degree Celsius [2] and
it is this increase, which helps the bodies defence mechanism fight off the infection, as the
higher metabolic rate will speed up tissue repair, increase production of antibodies and the
extra heat produced can kill off certain types of bacteria.
Fever can last anywhere between a few hours to a few weeks, this is dependant on the initial
cause of the fever and if any antipyrese or fever lowering drugs are used. Fig. 2 shows a
graph indicating the major events of fever.
Human Body Temperature
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Other physiological factors notable during fever are a raise in heart rate and respiration rate
of the patient. It should also be noted that a person’s body temperature will rise by a few
degrees during exercise. This is a normal response because as the body exercises, it will
body temperature will return back to 37°C.
41
set point of fever
fever breaks
39
vasoconstriction
&
shivering
38
Body
Temp.
(ºC)
40
increase the metabolic rate and hence heat production. After the exercise is completed the
37
normal set point
fever begins
36
time
Fig. 2. Sketch graph showing the course of fever [2]
2.5 Treating fever
Although as previously described, fever is the body’s natural reaction to infection or disease,
it is not desirable for a person to have a high temperature for a long period of time. A person
with a high temperature tends to feel uncomfortable, they can get dehydrated and often suffer
other symptoms such as headache and nausea, and now with the advances in antibiotics to
fight infection, the body’s physiological response of raising the set point temperature isn’t so
essential. The body either with or without the aid of antibiotics can normally fight the
infection just as well without the rise in body temperature, and returning the patients core
15
temperature back to 37°C usually makes them feel physically better.
Human Body Temperature
(It should though, be noted that some research shows that antibiotics do work better at
fever temperatures[2].)
The main method of reducing a fever is with a family of drugs called antipyretics.
Other methods include sponging the patient with tepid water and the use of cold air fans,
however, recent studies have shown that the use of sponging and fans doesn’t have a
significant effect in the reducing of a high temperature [3]. Drugs are usually the preferred
option. Some common antipyretic drugs, which are used, include paracetomol, aspirin and
ibuprofen. Scientific experiments suggest one of the better, more responsive drugs is
ibuprofen. Tests carried out suggest that this drug starts to work in around 30 minutes with
peak effects at around 3 hours. The drug under test has also shown to have an antipyretic
effect for a duration of 6 to 8 hours before the effects wear off. [4]
Design
16
3. Design
3.1. Design concept
The general design concept behind the hardware is to use electronics to try and mimic the
bodies own methods of maintaining and controlling temperature. This control is achieved by
way of a feedback loop. The body itself uses a negative feedback loop as was described in
section 2.2, the design for this circuit will also use a feedback loop but using electronic
components to take on the roles of the effecter, sensor and control.
Fig. 3 uses a block diagram to illustrate this.
SENSOR
Thermistor
CONTROL
Op-amp
EFFECTOR
Heat cell
Fig. 3. Block diagram illustrating concept of circuit design.
The effecter is a power resistor, which will dissipate heat if enough voltage is
dropped across it. The sensor is a thermistor, and the control aspect is achieved using an opamp. (These components will be discussed in more depth in sections 3.3 and 3.4)
One of the key aims to this project was to make the hardware small enough to install into a
manikins’ mouth so that it can be used as part of the patient simulator. However, the whole
system is likely to occupy a bigger space than that desired, so to solve this problem the
control circuit will be kept separate from the ‘black box’ or temperature box. The temperature
box is where the thermometer is to be inserted to take a reading. Housed inside the
temperature box will be the thermistor, power resistor and a large amount of insulation to
Design
17
help stabilize the temperature. Both the control circuit and temperature box will be described
in more depth in sections 3.3 and 3.4 respectively.
3.2 Past efforts
This project is not the first attempt at designing a body temperature simulator, attempts have
been made in the past and each one has used a slightly different method.
One method involved the use of a heat pump to stabilize the temperature inside the box and a
cooling fan to create environmental conditions, so as to simulate for example hypothermia.
Another attempt at the problem used digital electronics. The desired temperature required
was entered into a keypad and this then, by using a Peltier heat pump device, set the
temperature inside the “environment chamber”.
Since neither of these methods resulted in producing a working circuit (the reasons were
generally put down to problems with the complicated electronics involving in setting up the
heat pump) this project has used a different method in approaching the problem, staying away
from the Peltier heat pump device. It should also be noted that this design will be
considerably less expensive to build than past efforts.
3.3 Control circuit theory
The original idea for the control circuit was taken from the circuit design for the control of a
thermostatic chamber in a blood gas analysiser [5]. However, this circuit wasn’t entirely
appropriate for this project so a degree of modification needed to be carried out in order to
get the circuit operational. Extra components were also added to further enhance the circuit.
This section will look at some of the electronic theory behind the control circuit.
A complete circuit diagram and an analysis of how individual parts of the circuit works will
be explained in a later section (section 3.5).
Design
18
3.3.1 Op-amp switching
The control circuit is basically centred around an op-amp switching circuit.
An op-amp switching circuit involves the output of the op-amp switching almost
instantaneously, from one voltage level to another, and won’t give the normal output voltage
you get from an op-amp circuit (that is, the difference between the input voltages). The only
two voltage levels you will see from the output will be the saturation levels of both the
positive and negative power supplies [6].
To determine whether the output either saturates to positive or negative is done by comparing
the two inputs. When the voltage to the non-inverting input of the op-amp is higher than that
of the inverting input then the output will saturate to the positive power rail. It saturates to the
negative rail when the inverting input is higher than the non-inverting.
3.3.2 Comparator
One of the main purposes of this control circuit is to keep a constant temperature. By using
the op-amp as a comparator, and with the use of a thermistor, it is possible to achieve this.
Fig. 4 shows a basic diagram of how this can be done. It shows the use of a voltage divider to
determine a reference voltage at the inverting input. This reference voltage will remain fixed
at a certain value (this value will be determined during the calibration of the circuit to obtain
the set point temperature, see section 3.5.3). The voltage at the non-inverting input can then
be compared to the reference voltage. If the non-inverting input is the greater voltage, then
there will be a positive voltage at the output of the amp, and if it is a lower voltage than the
reference, then a negative saturation at the output is produced.
By inserting a thermistor at the non-inverting terminal, it provides a variable component,
which is a key part of the control loop.
19
Thermistor (high resistance, 100kΩ – cold)
(low resistance, 2kΩ – hot)
12kΩ
V1
If V1>V2 +ve output
If V1<V2 –ve output
+
-
output
.
V2a comparator.[6]
Fig.10kΩ
4. Using the op-amp as
10kΩ
7.5 V ref.
Design
Values given are as an example
to demonstrate the theory and are not used
on actual circuit.
3.3.3 Darlington pair
A pair of transistors, as shown in fig 5, where the emitter of the first is fed into the base of the
second, is known as a Darlington pair. This in effect creates a single resistor which has a very
high β value. This arrangement can be used for two different reasons. The first is to act as a
very high input resistance and the other is as a means of driving small load resistances.
In the circuit design for the temperature simulator, a Darlington pair will be used to drive a
small load resistance[6]. This load will be the power resistor (heat cell), which will be a
low value, high wattage resistor.
C
B
Q1
Q2
β total = β Q1 * β Q2
E
Fig. 5. Darlington pair configuration.
20
3.4 Temperature box design
Design
As previously stated, the temperature box is the part of the hardware in which a thermometer
can be inserted to take a temperature reading. It is also intended that the box should be
small enough to be fitted into the mouth of the patient simulator.
As well as meeting these two aims, the temperature box must also be designed to help
stabilize the internal temperature, so as to prevent any outside factors altering the
temperature inside the box.
The temperature box was designed from scratch, and was constructed from a thin plastic
casing. Plastic was chosen in preference to other materials as it was a fairly easy material to
work with and it isn’t a great conductor of either electricity or heat.
These factors provide both a level of insulation and safety from the heat, and avoids the risk
of short circuits. Another method of insulation was also included. This was a piece of foam
which was placed into the box. The heat cell and the thermistor were placed into the centre of
the box and imbedded into the foam. A path was cut down into the foam so an external
be seen in fig. 6).
thermometer could be placed alongside the heat cell (a diagram of the temperature box can
It was important that this amount of insulation existed as it provided protection against an
interference such as a blast of air or rapid change in room temperature.
Another key aspect to the design and success of the temperature box was the function of the
heat cell. For the heat cell, a power resistor was used. Depending on the type and value of
the power resistor, when an appropriate voltage was dropped across it, it dissipated heat.
Various types of power resistors were experimented with in order to find one which gave the
optimum response for its intended purpose.
21
Design
Temperature box hand drawn diagram.
Design
22
The ideal power resistor for this hardware design was one, which kept the internal
environment of the box at the desired temperature without constantly having to be
“switched on” (so the resistor should retain it’s heat well). Also the power resistor should
be a good conductor of heat, so, the response time of the thermometer should be of an
appropriate time, say 30 seconds, to obtain an accurate reading of the box (having started
from room temperature).
A large part of the project was the trying and testing of different power resistors, and section
4 shows the results of these experimentations with an analysis of which gave the best
response.
3.5 The complete hardware design
A schematic diagram of the completed, working circuit can be found at fig. 7 on page 21.
This section will give an explanation of how the circuit works during operation and some of
the components that make up the circuit. A parts list can be found at Appendix B, given more
3.5.1 Power supply
exact details of the components used along with a pricing guide.
The circuit operates from a 15-volt power supply, this value was chosen as it is a high enough
value so when dropped across the power resistor, it will cause heat to dissipate and also it is
an appropriate value to operate the op-amp by.
3.5.2 Thermistor.
The thermistor is a general purpose bead thermistor, specific in the use of measurement and
control of temperature. The value selected was 4k7Ω at 25°C. The thermistor is connected to
the non-inverting leg of the op-amp, this is accomplied by two other resistors, R5 and R6.
These are but in place to protect the thermistor from the higher value voltage and current
which may damage the component. A capacitor, C1 has also been connected in parallel to the
Design
23
thermistor, that is the colder the temperature, the higher the resistance.
3.5.3 Potentiometer
thermistor to act as a noise filter. The thermistor can be described as a negative coefficient
This is a 10K POT, and it’s purpose is to calibrate the circuit and alter the reference voltage
at the inverting leg of the op-amp. By altering the POT the temperature inside the box can be
changed (see section 3 for theory). Two other resistors have also been placed in series with
the POT.
Their purpose is to “cap” the temperature at 43°C. There is no requirement for the
temperature to be any higher than this value, therefore by placing a resistance to the value of
5820Ω in series with the POT, it prevents the POT from been ‘wound up’ to a higher
temperature, providing a safety feature to the circuit.
3.5.4 The op-amp
This is a standard 741, general purpose op-amp (see Appendix E for extract page from data
sheet). The op-amp is set up to have some feedback, provided by a 1 MΩ resistor, 100k Ω
and 10kΩ resistors organised in a voltage divider configuration. This produces only a small
amount of gain, as most of the feedback for the circuit is provided by the heat cell and the
thermistor.
3.5.5 Transistors
As previously described in section 3.3.3, the transistors are arranged in a Darlington pair
configuration, and are there for the purpose of driving a small load, the power resistor.
The first transistor, Q1, is a general-purpose transistor, the emitter leg of this transistor is
then fed into the base of the second, Q2 to produce a higher current gain, β value. Q2 is a
different transistor, a power transistor. This is able to cope with a higher current than the
general purpose one (see appendix F for out-take of data sheet for Q2). A resistor, R7 has
Design
24
been placed at the base of Q1, this is to act as a current limiter and C2 and R8 have been
placed across the collector legs to provide a noise filter.
3.5.6 Heat cell
The heat cell, H1, is a power resistor, which has a low resistor value. This type of component
has been chosen because, from the data sheets, it states that the resistor will dissipate a
temperature of 70°C when operating at maximum power. Therefore using the formula
P=V²*R, it is possible to work out, with the knowledge that there is a 15V power supply, an
appropriate value of resistor to produce the desired heat.
A variety of power resistors were tried out during the project to find one which gives an
optimum response for its intended application. The results of the tests can be found in
chapter four.
3.5.7 Circuit in operation
The heat cell and thermistor are placed in close proximity to each other, when the circuit is
switched on the op-amp will compare the voltage at each of the input legs of the op-amp.
If the voltage input at the inverting leg is less than that of the non-inverting leg then a
positive voltage will be seen at the op-amp output. This positive voltage will effectively
“switch on” the heat cell and consequently produce heat. As the heat is produced this will be
detected by the thermistor, and the resistance of the thermistor will decrease. Because of this
decrease in resistance, the voltage at the non-inverting leg of the op-amp will change. If the
voltage drops below that of the reference voltage at the inverting leg then the output of the
op-amp will go negative. This effectively “switches off” the heat cell. Because the heat stops
being produced, the resistance of the thermistor will increase, again altering the voltage at the
non-inverting leg. This action provides a constant feedback loop.
25
Design
Circuit diagram hand drawn.
26
4. Testing, Experimentation and Results.
Testing, Experimentation and Results
This section of the report presents details of any results obtained during the testing phase of
the project, with the purpose of the testing being to see if the circuitry achieves the aims
and objectives laid down.
This section will also give an account of further experimentation that took place to try and
improve the hardware in order to make it more accurate, and a more realistic simulation.
The first part of this chapter will concentrate on the reaction time of the thermometer. It
shows results recorded in relation to the reaction time of the thermometer after insertion into
performance of the hardware.
the temperature box, and the second part of this section analyses the stability and
4.1 Reaction time experiments
One of the main aims of this project is to produce a system that, as accurately as possible,
models the body temperature of a human. This means that when a thermometer is inserted
into the temperature box of the hardware, the time it takes the thermometer to reach the
appropriate temperature should be the same as it would be if inserted into the human mouth.
4.1.1 Methodology of experimentation
In order to do this first it must be established exactly how long it takes to record oral
temperature, and the quickest possible reaction time of the thermometer, so as to find the
time it is hoped to be achieved and the limitations of the thermometers performance.
The fastest response time of the thermometer was established by carrying out a small
experiment with a beaker of hot water. The thermometer, which started at room temperature
was inserted into the water, and the time it took to get to 37°C was recorded. This time was
actually so fast that an estimate of about 3 secs was all that could be obtained.
Testing, Experimentation and Results
27
The time taken to record oral temperature was obtained via product research. However,
as expected, this was found not to be an exact figure and actually depended on the type of
temperature-measuring probe used. Many probes have to be kept in the patient’s mouth for
up to 2 mins, but most of the modern temperature probes only have to be in the patient’s
mouth for a matter of 20 or 30 seconds.
In this project it was the intention to get the reaction time to that of about 30 seconds,
although as the water test showed it is theoretically possible to get the thermometer to react
faster than this.
4.1.2 Variables
To try to get this 30-second thermometer reaction time, two variables were experimented
with. These were the value of the power resistor used (the heat cell) and the effect of
insulation.
Six different values of power resistor were tried, these values were:
220 Ω - 1W, 180 Ω –1W, 100 Ω – 6W, 82 Ω – 2.5W, 82 Ω – 1W, 68 Ω – 1W.
Foam was the only insulation material used, and the results show the effects both with and
without the foam. An aluminium foil covering inside the box was also tried, but due to
constant problems with short circuits, this had to be abandoned.
4.1.3 Response time results
The first graph shown in fig.8 shows the response times of the thermometers using four of the
different resistors as mentioned previously. Two of the resistors, the 82 Ω –1W and the 68 Ω
-1W had to be abandoned from testing due to over heating and smoke issuing from the
circuit. As well as the heating up times, the cooling down times were also recorded (see fig.
9) which will be of use if further work concerned with the simulation of fever is conducted.
28
38
36
34
32
30
28
26
24
22
220 ohm - 1W
180 ohm - 1W
100 ohm - 6W
82ohm - 2.5W
0
100
200
300
time (seconds)
temp (celsius)
Testing, Experimentation and Results
38
36
34
32
30
28
26
24
82 ohm -2.5W
temp (celsius)
Fig. 8. Graph showing response time of thermometer in reaching 37°C from room temp.
after insertion into foam insulated box using different valued resistors.
0
200
400
100 ohm - 6W
220 ohm - 1W
600
time (seconds)
Fig. 9. Graph showing cooling times after switch off of three different resistors.
The other variable experimented with was that of the foam insulation. Fig. 10 shows the
heating up of the 220Ω resistor with and without the foam padding, and fig. 11 shows the
Testing, Experimentation and Results
29
38
36
34
32
30
28
26
24
22
0
With insulating
foam
Without foam
200
400
time (seconds)
temp (celsius)
cooling down curves of the same resistor.
38
36
34
32
30
28
26
24
22
Without foam
insulation
With foam
insulation
temp (celsius)
Fig. 10. A comparison of the effects of the heating rate of a 220Ω resistor, in the
temperature box, both with and without foam insulation.
0
250
500
750
time (seconds)
Fig. 11. A comparison of the effects of the cooling rate of a 220Ω resistor, in the
temperature box, both with and without foam insulation.
Testing, Experimentation and Results
30
38
36
34
32
30
28
26
24
22
100 ohm (6W) with foam
100 ohm (6W) without foam
82 ohm (2.5W) with foam
82 ohm (2.5W) without foam
0
100
200
300
time (seconds)
temp (celsius)
resistors performed the best during the testing of the response timing.
Fig. 12 shows the results of the use of foam with both the 82Ω and 100Ω resistor. These two
4.1.4 Analysis of results
Fig. 12. The use of foam with both the 82Ω and 100Ω resistors.
From these results it has been established that the lower value resistors, with the higher value
power ratings gave the fastest response of the thermometer after insertion. The preferred
values from the selection tested were 82Ω – 2.5W and 100Ω – 6W. These gave a response
time of around 2 minutes, which wasn’t the value hoped to be achieve (30 seconds), but is a
reasonable, workable time for a patient simulator.
Because of the problem of overheating of the 68Ω – 1W, the results suggest that if further
lowing of the resistor value was to take place then the power rating would need to be
increased, and also a different type of transistor would need to be used to cope with the
high currents.
The use of foam as an insulator, from the results produced, seems to have a positive effect at
improving the response time of the thermometer. In the case of the 100Ω – 6W, by insulating
Testing, Experimentation and Results
31
the resistor with a foam covering it improved the response time by up to a minute. The foam
insulation does though decrease the speed in which the temperature box cools after being
switched off. The cooling of the box in the case of the 220Ω resistor took over 750 seconds to
cool back down to room temperature, and although this doesn’t effect the project as it stands,
if further developmental work was to take place in relation to the increase and decrease of
temperature due to fever, then this long cooling time could effect the fever timing circuit.
4.2 Circuit Performance and testing
Tests were performed on the circuit with the purpose of trying to identify the effectiveness
of certain components within the circuit, and the stability and operation of the circuit as a
4.2.1 Potentiometer (POT) - calibration
whole.
The reason for including the potentiometer within the circuit design was so the reference
voltage at the inverting leg of the op-amp could be altered. The reference voltage determines
the temperature within the temperature box, so by having a potentiometer attached it provides
a method of calibrating the temperature to 37°C. It also provides a way of increasing the
temperature manually. A multimeter was connected to monitor the voltage at the
potentiometer allowing a table to be produced, indicating what reference voltages relate to
which temperatures. This table can be seen in fig. 13.
The results from this table show that only a small voltage change, that is a small alteration of
the potentiometer is required to alter the temperature in the box. A 10kΩ POT was used and
the results may indicate that better accuracy may be achieved by using a different value of
POT.
Voltage at POT (node A – see fig.7 )
5.11
5.01
4.87
4.81
4.68
4.59
32
Temperature (°C)
35
36
36.5
37
38.5
39.5
Fig. 13. Calibration table.
Testing, Experimentation and Results
4.2.2 Circuit stability
Another part of the circuit design that was analysed, was the output of the op-amp. By
monitoring the voltage at the output pin, it was possible to observe the changes in voltage
levels during operation. This gave a good idea at how effectively the control loop was
working and the stability of the temperature inside the box. The greater the switching
between positive and negative voltages at the output pin, the greater the heat loss indicated,
resulting in the heat cell being switched on more frequently in order to maintain temperature.
It was observed that the higher value resistors, such as the 220Ω, which were physically
smaller in size, were less reliable at retaining heat, where as the 100Ω – 6W resistor, a larger
sized resistor and made of a ceramic material, gave good temperature stability during
operation.
During time measurement recording it was found that the 100Ω – 6W resistor, after initial
warming up, needed only to be “switched on” for 4 seconds, every 10 minutes to retain the
set point temperature. In comparison the 220Ω –1W resistor needed to be “switched on” for
about 12 seconds, every 4 minutes.
4.3 Summary of Results
In summary, out of all the testing and experimentations conducted, the 100Ω – 6W resistor
seemed to offer the best performance. It was anticipated that further testing of high power,
low value resistors was to take place but, unfortunately, the components were not
Testing, Experimentation and Results
33
immediately available, and due to lack of time further components were not tested.
Another line of testing that was hoped to be tried out was the evaluation of
different materials used in the construction of the resistors. The 220Ω resistor, was a carbon
film resistor, where as the 100Ω was silicone coated. This may have had a significant
effect on the response time, as certain materials may give a better method of heat transfer.
Conclusion
34
5. Conclusion
5.1. Overview of project
The main aim of this project was to develop a piece of hardware that can simulate the
core body temperature of a human being as accurately as possible both under normal
conditions and under fever. The hardware constructed consisted of a box, containing a
heat cell, in which a temperature-measuring probe could be inserted, and an electronic
circuit which drove and controlled the heat cell to the appropriate required
temperature. The aim of producing such a device was so it could be used in
conjunction with a patient simulator, in the University of Hertfordshire’s Centre for
Advanced Healthcare Technology. As the intention was to implant the device within
the patient manikin, and use it as part of nurse and paramedic training, it was
important that the device was stable enough to put up with any disturbances it might
encounter whilst in use. This meant a large proportion of the project concentrated on
the design and construction of the box in which the measuring probe is inserted. The
box needed to be well insulated and be capable of producing adequate heat dissipation
leading to workable temperature recording times, to make the simulation as close to
taking a persons body temperature as possible. Apart from the design of the box, the
other important part of the project was the development of an electronic circuit to
control and stabilize the temperature within the box. This involved
using components such as an op-amp, thermistor and power resistor (as the heat cell)
which were arranged so as to provide a control loop.
5.2. Outcome
As detailed in section 4, a working circuit was successfully produced which is capable
of
maintaining a steady, stable temperature of 37°C, which can be measured by inserting
Conclusion
35
a mercury thermometer into the centre of the 6cm by 7cm by 6cm heat box. On
completion of the final design (after alterations and improvements were made) the
heat cell (a 100 Ω, 6W resistor), within the box takes approximately 15 seconds to
reach the required 37°C. Although this sounds fast, actually taking the temperature
involves a longer period of time, as the response time of the thermometer when
inserted into the box alters the thermal equilibrium inside the box, thus the hardware
has to re-stabilize the temperature after having the cooler temperature probe inserted.
Another important factor which effected the response time of the thermometer was the
speed of heat transfer that took place between the thermometer and the heat cell. The
optimum response time of the thermometer achieved using this hardware was about 2
minutes. The hardware also enabled the temperature to be altered manually by using a
potentiometer.
5.3. Summary of achievements and failings
Although this project was reasonably successful, and produced a working model, not
all of the aims and objectives that were laid out at the beginning were met. This
mainly came down to lack of time and possibly setting unrealistic targets when
planning the project. Of the three main aims of the project, as stated in section 1.2, the
first was met and operated to the required specification and the second and third
objectives were both partly met, but did not achieve the initial specification.
5.3.1 Objective one
The first objective was to design and build a ‘temperature box’ which would house
the thermistor and heat cell. This needed to be small enough so it could, in theory, be
implanted into the manikins mouth. It also needed to have an appropriately sized hole
so a temperature measuring probe could be inserted, and the box needed to be well
insulated and stable so the that any disturbance wouldn’t effect the set point
Conclusion
36
temperature.
In large this objective was met. By filling the box with a foam material, and placing
the heat cell into the centre of the foam, it provide, enough insulation to prevent any
outside disturbance (like change in room temperature or blast of air) from altering the
internal temperature within the box. Also after experimentation with different power
resistors, it was found that the physically larger sized 100Ω, 6W resistor produced and
retained enough heat so as to maintain the thermal equilibrium within the box,
therefore when a cooler object, like the bulb of the thermometer, was inserted into the
temperature box, the temperature remained stable. As the box measured 6cm by 7cm
by 6cm it would be possible to install the device inside the manikin’s mouth as
desired.
5.3.2 Objective two
The second objective was to build a circuit, which can control and stabilize the
temperature in the box to that of normal temperature, 37°C, and this temperature
should be recordable using a mercury thermometer in less than a minute after
insertion. This objective was partly met. The circuit constructed did indeed produce a
temperature of 37°C in the box, and this was kept stable by means of a control loop.
However, after experimentation with different components the quickest recordable
temperature could only be achieved in a time of two minutes, longer than the desired
time. From the experimentation results though, there was a clear suggestion that the
desired time period could be achieved by using a different value of power resistor (see
5.4 for further details) but this could not be tested due to time restrictions and
component availability.
5.3.3 Objective three
The third objective wasn’t completely met to the desired specification.
Conclusion
37
37°C to 39°C in a time of two hours to represent a fever.
The aim was to automate the circuit so the temperature would rise at a linear rate from
Again due to lack of time and the foreseen problems of designing such a timing
circuit, this part of the project wasn’t attempted. (although the theory was looked into,
see Appendix G)
However, it was possible, with the aid of a potentiometer, to increase the temperature
manually, and although it doesn’t achieve the required aim with reference to the two
hour timing, fever can be simulated with this hardware.
5.4 Improvements and recommendations
As the project wasn’t completed to its intended state, there are plenty of ways forward
whereby this project can be extended and enhanced further, beyond its current
capability.
The first two major, recommended improvements to be made would be to improve the
Time in which the thermometer responds, and complete the design for the timing
circuit. The results obtained from the experimentation as detailed in section four,
suggest that a quicker thermometer response time can be achieved by using a lower
value, higher power rating, resistor. Suggested values from the results obtained would
be 68 to 82 Ω with a power rating of around six watts. This, however, would increase
the current through the transistor and therefore a different transistor may well be
needed in the circuit design to cope with the higher current. The material in which the
resistor is made may also need to be taken into account in relations to the amount of
heat transferred. In general though, if further testing with different components is
carried out it does look feasible to obtain the ideal response time for the simulation.
The other key improvement would be to include the timing circuit, giving an
automatic, timed increase from normal temperature to fever temperature. This wasn’t
Conclusion
38
and details of this research can be found in Appendix G.
attempted in this project, but the theory of how to design such a circuit was researched
Other potential improvements, which are outside the scope of this project but may be
of interest to other people attempting a similar task, would be to integrate the
temperature simulator to other physiological simulators such as blood pressure and
pulse. From the physiological literature, when a person develops a fever their heart
rate and respiration rate also increase. It may well be possible to try and integrate the
different systems to show how they relate and interact with each other.
Another area of research and development would also be to design a system, which
shows how antipyretic drugs work to reduce fever. An electronic unit could be
produced so that if a manikin had a simulated fever, then a medic could select a
suitable drug and dose, inject this into the manikin, and observe how quickly this
works to reduce the fever.
39
Man – Hot and Cold
By O. G. Edholm
Publisher: Edward Arnold, 1978
Chpt. 1. pp 1 - 12
[2] Human Anatomy and Physiology
By Carala, Noback & Harley
International edtion pp 755 – 759
Progress in Human Body Temperature Measurement:
IEEE Engineering in Medicine and Biology
Vol. 17 Issue 4, Jul – Aug 1998, pp 19 – 27
[4]
The effect of Fever in Humans:
The American Journal of Medicine
Vol. 106 Issue 5, May 1999
[5]
Handbook of Biomedical Instrumentation
By R. S. Khandpur
Publisher: McGraw – Hill, 6th ed. 1993
Chpt. 15. pp 350
[6]
The essence of Analog Electronics
By C. Lunn
Publisher: Prentice Hall, 1997
Chpt. 6. pp 131 – 135
[3]
[1]
References
40
Bibliography
Website: www.microclimate.com/medlinks.htm
Fever and Antipyresis : The role of the nervous system
By K. E. Cooper
Publisher: Cambridge University Press, 1995
ISBN 0 521 419247
Human Physiology
By S. I. Fox
Publisher: WCB, 5th ed. 1996
Project Report: A small controlled cell to simulate body temperature
By C. W. Foong
April 1998
i
Appendix A
Glossary
ATP – Adenosine Triphosphate, plays a vital role in cell energy.
Conduction – The transfer of heat from one medium to another.
Convection – The transfer of heat by circulation flow due to temperature difference
ECG - Electrocardiograph, a measure of the electrical activities of the heart.
Evaporation – The conversion of a liquid into vapour.
Homeostasis - The tendency of the internal environment of an organism to be
maintained constantly
Hypothalamus – Part of the brain that makes up the floor.
Metabolism – The sum-total of the chemical reactions occurring in the body.
Pyrogens – Bacterial polysaccharides which produce febrile reactions
SaO2 - Oxygen saturation
Radiation – The emission of energy from a source.
Appendix B
ii
Parts List
Price
£0.12
£4.10
£0.33
£0.33
£0.49
£0.01
£0.01
£0.03
£0.01
£0.01
£0.01
£0.04
Total component cost
All resistors taken from the E12 series.
£0.04
£0.01
£0.01
IC1 - 741 Op-amp
TH1 - Bead thermistor, 4k7 at 25°C
R3 - 10k Ω potentiometer
Q1 - General purpose transistor, npn, BC107
Q2 - Power transistor, npn, BD437
R6 - 1k Ω resistor
R4 - 3k3 Ω resistor
R5, R7, R8 - 4k7 Ω resistors
R11 - 10k Ω resistor
R10 - 100k Ω resistor
R9 - 1M Ω resistor
C1 - 0.01µF capacitor
C2 - 0.1µF capacitor
R1 – 5k6 Ω resistor
R2 – 220 Ω resistor
H1 – 220 Ω, 1W resistor
- 180 Ω, 1W resistor
- 100 Ω, 6W resistor
- 82 Ω, 2.5W resistor
£0.01
£0.01
£0.33
£0.29
£6.19
Jan
Dec
Nov
Feb
Mar
Apr
Planned target
Simulations
Ordering
components
Initial
design
Resource
analysis
Topic
research
Actual target
Time Management Plan
Modification
Testing
Construction
Oct
Appendix C
iii
iv
Appendix D
Survey of available clinical thermometers
Omron IT5 Ear (Tympanic)
Thermometer
Quick and simple to use. The Omron IT5 electronic thermometer gives accurate core
body temperature in just 3 seconds. More than 5,000 measurements per battery. Ideal
for babies, children and the elderly.
Digital Thermometers
Omron MC3B and MC63B - robust, safe and easy to read. This digital thermometer
even beeps when read. The MC63B has the additional advantage of being washable
for clinical use.
Appendix E
v
Data sheet for op-amp.
vi
Appendix F
Data sheet for power transistor
vii
Appendix G
This page looks at some of the possible ways of building a timer, which is capable of increasing the
temperature from 37°C to 39°C in a time of two hours.
Rf
A
_
+
B
C
D
Fig. 14 Summing amplifier
0V
output
voltage
Input
Voltages
From research conducted, one way of doing this would be based around a digital to analogue converter
- DAC.
There are a lot of pre-packaged DAC’s available to buy, however a simple DAC can be constructed
from a summing amplifier, see fig. 14
The formula associated with this circuit is:
Vout = -[ V1 * R/A + V2 * R/B + V3 * R/C + V4* R/D ]
As well as a DAC, other key elements of the design include a counter and comparator.
The counter could also be a pre-packaged chip. Counters are usually constructed from flip-flops, and
are driven from a clock, set at a particular frequency. The counter would be connected to inputs of the
DAC and would ramp up the voltage, at an appropriate speed, which is determined by the clock
frequency. A comparator would be used to provide a comparison with a reference voltage. This
reference voltage would correspond to the required voltage to maintain a temperature of 37°C.
One potential problem that would need to be overcome in this design is the frequency of the clock. This
needs to ramp up a fairly small voltage, in the region of 0.2 volts, in a time of two hours.
Appendix G
vii