1
Temperature Measurements
2011
Temperature
Measurements
Lecture Notes
Systems & Biomedical Engineering Department Faculty of Engineering, Cairo
University
Prof. Bassel Tawfik
Biomedical Measurements
1/1/2011
Temperature Measurements
2
Lecture Outline
1.
2.
3.
4.
The Concept of Heat
Types of Temperature Sensors
Comparison of Different Types
Applications in Medicine
1. The Concept of Heat
Temperature
is
an
indication
of
the
thermodynamic state of an object or system. It is a
macroscopic description of the collective
microscopic kinetic energy in a given material. If
two bodies are at the same temperature, they are
said to be in thermodynamic equilibrium with each
other. This implies that if they were connected to
each other, there would be no net flow of heat
from one to the other.
Kinetic energy is a measure of
the activity of the atoms which
make up the molecules of any
material.
Therefore,
temperature is a measure of
the kinetic energy of the
material in question.
Interestingly, temperature is not a measure of the unit thermodynamic energy of a
body; unit masses of differing materials can require differing amounts of energy to be
added or removed to change their temperature by a given amount. Identical
temperature of two bodies merely implies there would be no transfer of heat between
the two, regardless of the actual energy stored as heat in each body.
Most temperature measuring devices use the energy of the material or system they are
monitoring to raise (or lower) the kinetic energy of the device. A normal household
thermometer is one example. The mercury in the bulb of the thermometer expands as
its kinetic energy is raised. By observing how far the liquid rises in the tube, you can tell
the temperature of the measured object.
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Temperature Measurements
2. Types of Temperature Sensors
2.1 RTD (Resistance temperature Detectors)
Introduction
RTD’s are electrical resistors whose resistance increases
with temperature. RTD’s are manufactured using
different metals as the sensing element. The most
commonly used material is Platinum. Platinum is
preferred to other materials because of (1) its high
temperature coefficient, (2) excellent stability, and (3)
repeatability (reproducibility). Other materials used to
make RTD’s are nickel, copper, and nickel-iron. These
materials are becoming less common now that the cost
of platinum RTD’s is coming down. RTD elements are
usually long, spring-like wires surrounded by an
insulator and enclosed in a sheath of metal. Figure
1 shows the internal construction of an RTD.
Characteristics
Since RTDs’ resistance increase as the
temperature increases, they are referred to
as a positive temperature coefficient
devices. RTD’S are manufactured with a
base resistance at some temperature point.
This temperature is most commonly 0°C
(32°F). The most common base resistance is
100, which means that if the RTD is at 0°C,
the resistance would be 100. There are
other base resistances at different
temperatures.
Figure 1
Figure 2
The relationship between the temperature
and the electrical resistance is usually non-linear and described by a higher order polynomial:
R(t) = Ro(1 + At + Bt2 +C(t – 100)3)
Where: Ro is the nominal (base) resistance, “t” is the temperature, and the coefficients A, B, C
depend on the conductor material and basically define the temperature-resistance relationship.
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Temperature Measurements
For temperatures above 0° C, the “C” coefficient equals zero. Therefore, for temperatures above
0° C, this equation reduces to a quadratic.
Another common term used with RTD’s is
temperature coefficient. This refers to the
rate of change of resistance with respect
to temperature. The most common
platinum RTD has a temperature
coefficient of .00385 //°C. This means
that a 100  platinum RTD will increase in
resistance .385  for every 1°C increase in
temperature.
The temperature coefficient, called alpha (), is
defined as the change in RTD resistance from 0
to 100°C, divided by the resistance at 0°C,
divided by 100°C:
 (//°C) = (R 100 - R0)/(R0 * 100° C)
Where
R100 is the resistance of the RTD at 100° C, and
R0 is the resistance of the RTD at 0° C.
The maximum temperature rating for
For example, a 100  platinum RTD with
RTD’s is based on 2 different factors. First
 = 0.003911 will measure 139.11  at 100° C.
is the element material. For instance,
Platinum RTD’s can be used as high as
650°C (1202°F). The other determining factor for temperature rating is probe construction.
Finally, the tolerance or accuracy for RTD sensors is stated at one point only, which is usually 0°C
(32°F). ASTM publications recognize 2 grades of platinum RTD elements while DIN (Europe’s
version of ASTM, also called IEC/DIN) recognizes 2 classes of elements. They are as follows:
ASTM E-1137 grade B = ± .10% @ 0°C (32°F)
ASTM E-1137 grade A = ± .05% @ 0°C (32°F)
DIN 43760 class B = ± .12% @ 0°C (32°F)
DIN 43760 class A = ± .06% @ 0°C (32°F)
An RTD does not produce any voltage and so it requires a source of power for its operation. On
the other hand, RTD’s are generally more expensive to manufacture or purchase than
thermocouples because of the expensive nature of Platinum. Yet, Platinum is not without
defects. RTD elements become quite fragile at temperatures above 320°C (600°F). An RTD
sensor will not hold up well at these elevated temperatures if there is any vibration present.
Finally, it has been observed that the tolerance (accuracy) of an RTD generally decreases as
temperature increases.
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Temperature Measurements
Figure 3: Variable shapes of RTD’s.
RTD Circuitry
(1) Two-wire voltage-source (or current source) configuration
Since the RTD is a resistance which varies with
temperature, if we pass a current through it, the
voltage drop across it will reflect the amount of
resistance and hence the temperature. There is a
catch however! As you can see from figure 3 above,
the RTD sensor is “usually” placed at the end of a
Figure 4a: Simple RTD connection to a voltage
long wire to avoid placing the entire electronics near
source.
the source of heat, thereby avoiding harming these
sensitive components. Since the long wire has a
significant non-zero resistance, the following
situation arises when the RTD is connected to a
voltage source directly (as shown in figure 4).
Here, we are measuring the voltage across the RTD
at points R, R (Red and Red) at the tips of the 2 long
wires connected to the RTD. If the RTD resistance is
Figure 4b: 2-wire voltage source RTD in a
100  and each wire has a resistance of 5  (actual
Wheatstone bridge.
value is unknown), then the total resistance of the 2
wires and the RTD will be 110 . Ideally, it should be just 100 . Now there is an error of 10%
in measured resistance which translates to another error in the estimation of temperature. If
the relationship between temperature and resistance is linear, the error in temperature
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Temperature Measurements
estimation will also be 10% but this is strictly not true. This 2-wire voltage-source circuit
produces the largest error in temperature measurements. Notice that the same problem occurs
when using a Wheatstone bridge in which one arm of the Bridge is the RTD and the 2 lead wires.
(2) Three-wire voltage-source configuration
To reduce the error incurred by the 2-wire voltage source method, a 3-wire circuit is constructed
as shown in figure 5a. The idea is to add a third
wire with a resistance and material equivalent
to the first and second wires. By measuring the
resistance between the red and the white leads
and then subtracting the resistance between
the two reds, we end up with the RTD
Figure 5a: 3-wire RTD connection to a voltage
resistance only (100 ). The main assumption
source. R= Red, W=White.
here is that all three wires have exactly the
same values all the time.
The above circuit is fine except that it requires
mathematical subtraction in order to obtain
just the RTD true value. By using the
Wheatstone bridge shown in figure 5b, the
effects of the lead wires cancel each other
electrically. Using this method the two leads to
the sensor are on adjoining arms. There is a
Figure 5b: 3-wire RTD connection to a voltage
lead resistance in each arm of the bridge so
source using Wheatstone bridge.
that the 2 resistances are cancelled out (so long
as the two lead resistances are exactly the same). This configuration allows up to 600 meters of
cable. It remains to say, however, that the Wheatstone bridge shown in Figure 5 creates a nonlinear relationship between resistance change and bridge output voltage change.
(3) Three-wire current-source configuration
Another way to connect the 3-wire
configuration using a current source is shown in
figure 6a. The RTD element is shown to the left
with two terminals: Hi and Lo, while the signal
conditioning (SC) is the box to the right. The 2
lead wires are called W1 and W2, while the
compensating (third) wire is called W3. The
constant current Iexc flows from ehi to RTD Hi
through wire W1.
Figure 6a: Another configuration of the 3-wire RTD
circuit using a current source. The circuit details
are shown in figure 6b. The assumption is that the
resistances of the two lead wires are equal.
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Temperature Measurements
Figure 6b: Details of the circuit in figure 6a. The circuit to the right is a bit more simplified than that on the left but
they are essentially the same. The objective is to measure the voltage across the RTD. This is equivalent to
measuring Va – Vb (the right hand circuit). Va and Vb are accessible for measurement because they are at the
Signal Processing end not the RTD end.
(4) Four-wire Current-Source Configuration
The four-wire configuration is the most accurate
method. A constant current is passed through L1
and L4, while L2 (lead 2) and L3 (lead 3) measure
the voltage drop across the RTD. With a constant
current flowing in the outer circuit, very small
current flows into the inner circuit, thereby
rendering the voltage drop across L1 and L2
negligible. This configuration is illustrated in Figure
7. This configuration is slightly more expensive
than the 3-wire one because of the high cost of a
good current source.
Figure 7: Four-wire configuration.
Theoretically, the current through RT is
constant. Consequently, no current flows
through L1, L2. Hence, no matter how high
the resistances of L1, L2 are, Vout is only a
function of RT. Notice that this is not a
bridge circuit.
Exercise (2-wire circuit)
If the lead resistance, RL, in each wire is 0.3, then, the total lead resistance of 0.6  causes an
error in the temperature measurement. For a platinum RTD with  = 0.00385, show that this
additional resistance causes an error in temperature of 1.6° C. [Hint: Assume a linear resistancetemperature relationship].
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Temperature Measurements
2.2 (NTC) Thermistors
Background
The word thermistor is derived from “THERMally sensitive resISTOR”. The NTC (Negative
Temperature Coefficient, i.e. exhibit a decrease in electrical resistance with increasing
temperature) thermistor can be more accurately classified as a ceramic semiconductor. The
most prevalent types of thermistors are glass bead, disc, and chip configurations.
On a historical note, Michael Faraday (1791-1867), the British chemist and physicist, best known
for his work in electromagnetic induction and electrochemistry, has a less familiar 1833 report
on the semiconducting behavior of Ag2S (silver sulfide), which can be considered the first
recorded NTC thermistor.
As the reliability of thermistors improved during the 1980s, the use of electronic thermometers
in the health care industry increased. The rising costs of sterilization and concerns about crossinfection among patients led to the demand for low-cost disposable temperature probes, for
which cheap thermistors were well suited. Throughout the 1980s and 1990s, the use of NTC
thermistors has continued to grow in the automotive, food processing, medical, HVAC, and
telecommunications markets.
Characteristics
Depending on the materials and
methods of fabrication, NTC
thermistors are generally used
in the temperature range of 50°C to 150°C, and up to 300°C
for some glass-encapsulated
units. The resistance value of a
thermistor
is
typically
referenced at 25°C (abbreviated
as R25). For most applications,
the R25 values are between 100
and 100 k. Other R25 values
as low as 10  and as high as 40
M can be produced, and
resistance
values
at
temperature points other than
25°C can be specified.
Figure 8: Over the range of -50°C to 150°C, NTC thermistors
offer a distinct advantage in sensitivity to temperature changes
compared to other temperature sensors. This graph illustrates
the R/T characteristics of some typical NTC thermistors and
platinum RTD.
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Temperature Measurements
Circuit Applications
The following are simple applications utilizing thermistors to indicate temperature. The first
circuit
Circuit # 1: Qualitative heat indicator
When the thermistor is cold (no heat applied to
it), its resistance is high, and no current passes
through it. Hence, no current reaches the base
of the Darlington pair.
When warm, the
resistance becomes small and the base is fed
with enough current to open the transistor pair
and current flows through the LED which is lit.
The preset resistor can be turned up or down to
increase or decrease resistance, in this way it can
make the circuit more or less sensitive to heat.
Notice that this circuit can be used as an
indicator light for the operation of the hair dryer
itself!
Circuit # 2: Measuring the thermistor’s
resistance
This circuit measures the resistance of a
thermistor or RTD and provides an
output voltage proportional to the
resistance. When built with common op
amps, the circuit functions best when the
thermistor has a resistance greater than
a few hundred ohms.
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Temperature Measurements
3. Comparison of Types of Temperature Sensors
Source: Temperature Product Group, 2670 Indian Ripple Road, Dayton, Ohio45440-3605 USA (www.meas-spec.com)
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Temperature Measurements
Source: Agilent Technologies, Application Note # 290, “Practical Temperature Measurements”.
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4. Applications in Medicine
4.1 Hemodialysis
Temperature measurement and control is important in hemodialysis because small
arterio-venous temperature gradients in the extracorporeal circulation can cause
considerable transfer of thermal energy between the patient and the environment. If
this transfer continues for some time, thermal energy accumulates. Accumulation of
thermal energy over a prolonged period of time increases body temperature and may
eventually lead to heat stress.
Almost all hemodialysis machines are equipped with a system to control for a desired dialysate
temperature. Until recently a dialysate temperature of 37°C was considered adequate for
everyday hemodialysis. Even if a dialysate temperature of 37°C is somewhat higher than the
average physiologic core temperature, the excess was accepted as a rough compensation for
unavoidable heat losses in the venous part of the extracorporeal circulation.
However, in the early eighties research has confirmed that a low dialysate temperature in the
range of 34 to 35.5°C improves intra-dialytic hemodynamic stability when compared to dialysate
temperature set at 37°C or higher. It has been demonstrated that lower dialysate temperature
improves cardiac contractility and increases venous tone.
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Temperature Measurements
4.2 Cryogenics
Cryogenics is the study of the
production
of
very
low
temperatures (below –150 °C, –238
°F or 123 K). Cryobiology is the
branch of biology involving the
study of the effects of low
temperatures on organisms. This
may be of interest in areas related
to the cryopreservation and
cryosurgery. When extremely fine
slices of tissue samples are required,
cryomicrotomes are used.
EQUIPMENT MAINTENANCE
Cryo Microtome – Courtsy Sakura,
Tissue-Tek® Cryo3®
Microtome/Cryostat
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4.3 Incubators, baby (http://www.freepatentsonline.com/6679830.html)
A. Incubators & Infant Warmers
Incubators and infant warmers
are enclosures used to maintain
the oxygen content, relative
humidity, and air temperature
surrounding an infant or baby at
appropriate levels.
Climate control within the
incubator is mainly performed by
controlling the amount of
circulating air flow (driven by
blower motor 22)
5
6
7
8
9
10
11
12
13
14
15
16
17
The Incubator
Head-end wall
Platform
Foot-end wall
Infant
Side wall
Canopy
18
19
20
21
22
23
24
25
26
27
28
29
30
Enclosure
Air circulation system
Heater
Blower motor
Blower
Control system
Sensors
IR Sensor
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Infant incubator control mechanisms operate in two
general modes: manual control and servo-control.
Selection of incubator control mode is often established
by unit procedural guidelines or the nurse's personal
experience or preference. The purpose of this
investigation was to determine the effectiveness of
servo-control and manual incubator operation in
maintaining adequate infant body temperature.
When using manual control, the nurse predetermines
the air temperature setting for the incubator and
incubator heat output is thermostatically controlled to
maintain this air temperature. Thermal neutral zone
charts are often used to estimate appropriate air
temperature, given the infant's weight and age, however adjustments are typically based on
infant's body temperature.
Manual operation may involve frequent temperature checks and resultant time-based cost of
care issues. With servo-control operation the heat output of an incubator is thermostatically
linked to a specified infant skin temperature (servo-control set point). Adequacy of probe
attachment, which is critical to servo-control operation, is often difficult in preterm infants who
have high transepidermal water loss. The servo-control probe entails cost of care issues as well
as consideration of skin integrity and additionally servo-control operation results in difficulty
interpreting evidence of thermal instability.
Data for this investigation were taken from a study of the maturation of infant temperature
rhythm. Subjects for the secondary analysis were selected on the basis of incubator operation.
Data collection occurred at 14 ± 2 days postnatal age. Servo-control was utilized in 16 infants
and manual control was utilized in 14 infants. Gestational age at birth, which ranged from 26 to
31 weeks, was comparable in the two groups. While infant oxygen consumption is the gold
standard for measuring the adequacy of the thermal environment, this measure is currently not
feasible in clinical practice. Consequently infant body temperature was used to determine the
effectiveness of thermal support provided by the incubator. While results address
appropriateness of infant body temperatures the actual metabolic cost of thermoregulation was
not assessed. Infant abdominal skin temperature and incubator air temperature were collected
at one minute intervals throughout a 24-hour day using a battery-operated physiological
monitor (Vitalog) and standard surface and ambient air temperature probes (YSI). Graphic
displays of infant and incubator temperatures reveal irregular air temperatures in the majority
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of the servo-controlled incubators, with a 6 °C range in air temperatures. Analysis, currently in
process, will determine the percentage of temperature measurement intervals that were above
and below recommended parameters for infant abdominal temperature (36-36.5 °C) for both
servo-control and manually operated incubators. Results will provide initial information
regarding the clinical debate surrounding use of servo-control versus manual control incubators.
4.4 Clinical Lab Equipment
A. Incubators1
Cell cultures require a strictly controlled
environment in which to grow. Specialist
incubators are used routinely to provide the
correct growth conditions, such as temperature,
degree of humidity and CO2 levels in a
controlled and stable manner. Generally they
can be set to run at temperatures in the range
28ºC (for insect cell lines) to 37ºC (for
mammalian cell lines) and set to provide CO2 at
the required level (e.g. 5-10%). Some incubators
also have the facility to control the O2 levels.
Figure xx: Collection of lab incubators
Copper-coated incubators are also now
available. These are reported to reduce the risk of microbial contamination within the incubator
due to the microbial inhibitory activity of copper. The inclusion of water bath treatment fluid
(Prod. No. S5525) in the incubator water trays will also reduce the risk of bacterial and fungal
growth in the water trays. However, there is no substitute for regular cleaning. (Note “Sigma
Clean” Prod. No. S5525 is harmful by inhalation, contact with skin or if swallowed and is also a
severe irritant.)
1
The first incubators were used in ancient China and Egypt, where they consisted of fire-heated rooms in
which fertilized chicken eggs were placed to hatch, thereby freeing the hens to continue laying eggs.
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B. Ovens
C. Tissue Processors
Picture: Courtesy Shandon Corp.
Blue = Air, Pink=Reagent, Yellow=Wax
The Hypercenter XP is a computer-controlled,
fluid transfer tissue processor with automated
fixation, dehydration, cleaning and paraffin
infiltration of tissue specimens.
D. Refrigerators
E. X-ray Film Processors
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F. Biosafety Cabinets
A microbiological safety cabinet is probably the
most important piece of equipment since, when
operated correctly, it will provide a clean working
environment for the product, whilst protecting
the operator from aerosols. In these cabinets
operator and/or product protection is provided
through the use of HEPA (high efficiency
particulate air) filters. The level of containment
provided varies according to the class of cabinet
used. Cabinets may be ducted to atmosphere or
re-circulated through a second HEPA filter before
passing to atmosphere.
G. Steam Sterilizers (& Washers/Disinfectors)
A table-top steam sterilizer that can be
found in many hospital departments
such as dental operatory, labs, central
sterilization department (CSSD) and
dermatology clinics.
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4.5 General Device Design
A. Temperature Compensation
B. Protection against Overheating (Thermostats)
The most obvious overheating protection mechanism that can be appreciated by most students
is the fan that is placed on top of all computer processors.
cozy.caf.org/.../manfreds-thermostat-circuit.jpg
The following is taken from: http://web.mit.edu/3.082/www/team5/electronicdesign.html
The thermostat circuit is a circuit that cuts off power when the local temperature reaches a
certain set point. This circuit is needed for two reasons. One reason is to conserve as much
battery power as possible. The other reason is to ensure that the temperature of the heating
band stays at a comfortable temperature for the consumer.
The main component in the thermostat circuit is the thermostat integrated circuit (IC) from
Analog Devices. This device was chosen because the set-point temperature can be altered by
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adding resistors in place of Rset shown below. This device is also for a low voltage source, which
fits the battery being used. The thermostat IC sends a voltage to the transistor, which acts as a
switch and closes the circuit if the voltage sent is above a certain voltage (2V). The thermostat IC
also contains a built-in hysteresis of 4deg C so that the entire circuit does not turn on and off
rapidly. It is set so that the circuit turns on at 38deg C and then turns off when the temperature
reaches 42deg C. The capacitor at the top of the circuit is there to steady any oscillating
current/noise.
The following circuit is taken from: electronics4everyone.blogspot.com
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Appendix (A)
Response Time
Time Response Characteristics
The response of an ARi Resistance Temperature Detector is defined by 2 noticeable
characteristics when exposed to an instantaneous (step) change in environmental temperature.
These are:
A) Time Constant (): The time to reach 63.2% of the complete step change in temperature
B) Response Time: Time to reach within 0.5% of the final temperature in a step change. This
is approximately equal to 5 times the time constant
The response of a temperature sensor to a step change in ambient temperature tends to follow
a second order differential equation. However, this is approximate, since if the mass of the
sensor is small in relation to the mass of the fluid passing over it (such as in the case of a liquid),
the response may approach a first order differential equation. A typical response is shown
below.
Time constant has application for more common experiences in process control, i.e., ramp
change or sinusoidal changes in ambient temperature.
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The value of the time constant, , is related to the environmental conditions by the following
approximate relation (Ref NASA TN 2599):
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Appendix (B)
Summary of Application Note 3450: PT100 Transducer
(http://www.maxim-ic.com/appnotes.cfm/an_pk/3450)
Temperature is one of the most measured physical parameters. Thermocouple and resistance
temperature detector (RTD) sensors are adequate for most high-temperature measurements,
but one should choose a sensor that has characteristics best suited for the application. Table 1
offers general guidelines for choosing a sensor.
Feature
Thermocouple RTD
Response time
Better
Maximum temperature Higher
Ruggedness
Better
Cost efficiency
Better
Accuracy
Better
Long-term stability
Better
Standardization
Better
Platinum's long-term stability, repeatability, fast response time, and wide temperature range
make it a useful choice in many applications. As a result, platinum RTDs are recognized as the
most reliable standard available for temperature measurement. The PT100 RTD is described by
the following generic equation, which makes obvious a nonlinear relationship between
temperature and resistance:
RT = Ro(1 + AT + BT² + C(T-100)T³)
Where:
A = 3.9083 e-3
B = -5.775 e-7
C = -4.183 e-12 below 0°C, and zero above 0°C
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Figure 1. This two-wire connection affects measurement accuracy by adding resistance in series with the
RTD.
Figure 2. An additional third wire to the RTD allows compensation for the wire resistance. The only
restriction
is
that
the
main
connecting
wires
have
the
same
characteristics.
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Figure 3. A four-wire approach enables Kelvin sensing, which eliminates the effect of voltage drops in the
two
connecting
wires.
You can connect a PT100 RTD to the measuring application using two wires, three wires, or four
wires (Figures 1, 2, and 3). Several analog and digital approaches are available for compensating
a PT100 RTD for nonlinearity. Digital linearization, for instance, can be implemented with a
lookup table or by implementing the previous generic equation.
A lookup table located in µP memory allows an application to convert (through interpolation) a
measured PT100 resistance to the corresponding linearized temperature. On the other hand,
the previous generic equation offers a possibility of calculating temperature values directly,
based on the actual measured RTD resistance.
A lookup table necessarily contains a limited number of resistance/temperature values, as
dictated by the required accuracy and the amount of memory available. To calculate a specific
temperature, you must first identify the two closest resistance values (those above and below
the measured RTD value), and then interpolate between them.
Consider a measured resistance of 109.73Ω, for example. If the lookup table has a resolution of
10°C, the two closest values might be 107.79Ω (20°C) and 111.67Ω (30°C). Interpolation using
these three values leads to:
This digital approach requires use of a microprocessor (µP), but the small circuit in Figure 4
performs accurate linearity compensation using the analog approach. It provides outputs of
0.97V at -100°C and 2.97V at 200°C. It may be necessary to add gain adjustment (span) and a
level shift (offset) to cover an output range of -100mV at -100°C to 200mV at 200°C, for
example.
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Figure 4. This analog circuit linearizes an RTD output
The suggested way to compensate for nonlinearity in the PT100 element is to apply a small
amount of positive feedback through R2. That feedback helps to linearize the transfer function
by providing a slightly higher output at high PT100 values. The transfer function can easily be
established by applying the superposition principle
Figure 5 shows the raw PT100 output vs. a linear approximation of the form y = ax + b, and Figure 6
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shows a linearized version of the circuit output vs. the linear approximation. Each figure shows the
calculated relationship between temperature and resistance compared to the output calculated from the
Figure 4 circuit. The graphs of Figures 7 and 8 illustrate the PT100 error before and after analog
compensation.
Figure
5.
Raw
output
of
a
normalized
PT100
vs.
a
linear
approximation
to
that
output.
Figure 6. Analog-compensated output vs. a linear approximation to that output, showing the error after
linearization.
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Figure 7. Normalized error, representing the deviation between raw PT100 output, and a linear
approximation
of
the
PT100
relation
between
temperature
and
resistance.
Figure 8. Normalized error, representing the deviation between the linearized output of Figure 4 and a
linear approximation of the PT100 relation between temperature and resistance. Normalizing the curves of
Figures 7 and 8 makes it easy to assess the performance of the Figure 4 circuit.
When calibrating an analog thermometer, it is always preferable to minimize the need for
adjustments and control measurements during production and calibration. Normally, it is best
to adjust only the offset and span at two PT100 values. That approach, however, requires a
linear relationship between PT100 resistance and temperature, which is not the case.
The exercise above shows that analog compensation can reduce PT100 errors by a factor of
approximately 80, assuming the transfer function exhibits a linear correlation between the
PT100 value and the measured temperature. Additionally, low power dissipation in the PT100
(0.2mW to 0.6mW) minimizes self-heating. Thus, PT100 signals linearized using the analog
approach allow an easy interface to ±200mV panel meters, for example, without additional
software overhead.
Figure 9. Digital approach: An ADC converts the RTD output to digital under control of a µP. Then, the µP
calculates
the
corresponding
temperature
using
a
lookup
table.
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Temperature Measurements
An example of the digital approach (Figure 9) involves an RTD, a difference amplifier, a current
source, and an ADC controlled by a µP (not shown). A temperature measurement is
accomplished by driving a current of 1mA to 2mA through the sensor and measuring the
resulting voltage drop across it. Higher currents cause higher power dissipation in the sensor,
which in turn causes measurement errors due to self-heating. An internal 4.096V reference
simplifies
the
generation
of
excitation
current
for
the
sensor.
To prevent wire resistance from affecting measurement accuracy, four separate wires connect
the RTD to the differential amplifier. Because the sense wires connect to the amplifier's highimpedance inputs, they have very low current and virtually no voltage drop. The 4096mV
reference and 3.3kΩ feedback resistor sets the excitation current to approximately
4096mV/3.3kΩ = 1.24mA. Thus, driving the ADC and current source with the same reference
voltage produces a ratiometric measurement in which reference drift does not influence the
conversion
result.
By configuring the MAX197 for an input range of 0V to 5V and setting the differential amplifier
for a gain of 10, you can measure resistance values up to 400Ω, which represents about +800°C.
The µP can use a lookup table to linearize the sensor signal. To calibrate the system, replace the
RTD with two precision resistors (100Ω for 0°C, and 300Ω or higher for full span) and store the
conversion results.
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Temperature Measurements
Appendix (C)
Hemodialysis Machines: An Overview
Hemodialysis machine can be roughly divided
into custody alarm system and blood dialysis
fluid supply system in two parts.
The alarm system:
It includes blood monitoring, blood pump,
heparin pump, hybrid pulse pressure
monitoring and air monitoring;
Dialysis fluid supply system:
It includes temperature control systems, fluid
distribution system, in addition to gas system,
conductivity monitoring system, monitoring
and ultrafiltration Blood Leak monitoring
component.
Three hemodialysis machine parts function
1. The alarm system of monitoring blood
Blood alarm monitoring system for Gambro
AK 95 shown in Figure 2, for example,
function as part of the brief.
A. blood pump (Blood Pump)
Phoenix® Dialysis System - Courtesy: Gambro
To promote blood circulation to maintain
Corp.
hemodialysis treatment is carried out
smoothly. Generally speaking, often with blood pump some speed detection, used to monitor
the patient’s blood, and blood flow with the removal of toxins (shown in Figure 3), and the
blood pump cavity distance runner must set precise, and the need for regular adjustment of the
pump under the catch, generally set spacing of 3.2 to 3.3mm, not too loose, this will not be
allowed to flow detection; also not too strict, if too will cause pipeline rupture, the incident.
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B. heparin pump (heparin Pump)
Heparin pump equivalent clinical application of micro-injection pump, to keep the patients in
the blood injection of heparin. Since the patient’s blood in contact with air pump, it is prone to
clotting phenomenon, the use of heparin pump prevent coagulation.
C. hybrid pulse pressure monitoring
Arterial pressure monitoring to monitor dialysis machines thrombosis, solidification and
pressure changes. When inadequate blood flow, blood pressure will be reduced; When the
dialysis machines with coagulation and thrombosis, arterial pressure will be increased; Venous
pressure monitoring pipeline used to monitor blood pressure returned. When dialyzer clotting
or thrombosis, and the lack of blood flow to return shedding needles, intravenous pressure will
drop; If the return of profitable return needles or distorted plug to plug in, venous pressure will
be increased. The above situation occurs when the machine will automatically alarm.
D. air monitoring (Air Detector)
The road used to monitor blood flow and intravenous drip pot in the air bubble. Ultrasonic
detection by the general principle, in order to avoid air embolism in patients and set up. When
monitoring the air bubbles, the detection system will be driven move, blocking blood flow to the
road safe and prevent the occurrence of danger.
2. Part of dialysis fluid supply system
A. Temperature Control System
Including two heating and temperature measurement, in the normal dialysis, the treatment
would be consistent with the general standard of reverse osmosis water heated to 36 ~ 40 ° C,
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with the mixture of concentrated liquid temperature sensor by detecting temperature, which
makes dialysis fluid temperature control temperature and the temperature setting with general
dialysis fluid temperature control 37 ℃, according to the patients can be appropriately
regulated. With heat sterilization machine, in the heat sterilization, temperature can reach 100
℃.
B. Distribution System Solution
Preparation qualified dialysis fluid to carbonate dialysis example, the proportion of its hybrid: A
Solution: B liquid: water = 1:1.83:34. Many machines currently used ceramic pump ratio, by
adjusting the speed to achieve speed preparation of dialysis fluid precision.
C. degassing system
In the water and condensed liquid in a certain air, the process of preparation of dialysis fluid due
to the presence of carbonate gas will also be the formation of these bubbles in dialysis fluid in
the air may cause blood clotting, reduce waste clearance rate, the impact on dialysis fluid flow
and pressure, thereby affecting the concentration of conductivity occurred, hence the need to
remove dialysis fluid in the air. When using vacuum degassing principle, in addition to general
set-600mmHg about pressure, but in high altitude areas due to lower negative pressure, such as
Lanzhou, Kunming and other places can be set-500mmHg.
D. conductivity monitoring system
General carbonate dialysis function of hemodialysis machine configurations are often two to
three conductivity monitoring module, the first of the concentration of A, A request to the
concentration of liquid absorption of B, then the conductivity detection is the actual dialysis
fluid conductivity. Conductivity monitoring module to monitor the conductivity value reached
CPU circuit, and set conductivity, compared to further concentrate control of preparation,
preparation to meet the requirements of the dialysis solution. Usually of dialysis fluid cation
concentration in the range of 13.0 to 15.0ms/cm, dialysis fluid concentration of 13.8 to 14.2 in
between.
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E. ultrafiltration (Ultrafiltration) Monitoring System
Use of transmembrane pressure (Trans-membranous Press, TMP) pressure control or capacity to
achieve control of ultrafiltration, remove blood in the water purposes. Transmembrane pressure
increased, the corresponding time in the ultrafiltration of the circumstances will determine the
increase (as shown in figure 4). Most hemodialysis patients kidney failure or complete loss of
body water can not be ruled out, ultrafiltration system in hemodialysis machine is very
important. The current market hemodialysis machine control system can be divided into
ultrafiltration flow sensor system and the balance of two cavity, the use of the Gambro is the
former, while others use most of it is the latter, advantages and disadvantages of each are
passed through the comparison of before and after dialysis difference to calculate the flow of
ultrafiltration, more accurate .
F. Blood Leak monitoring (Blood Leakage) system
Hemodialysis sometimes occur in the process of rupture of membrane dialysis phenomenon will
occur when Blood Leak, in order to detect the occurrence of Blood Leak generally hemodialysis
machine use optical detection of dialysis fluid in hemoglobin, the detection sensitivity of 0.25 ~
0.35ml heme / or a dialysis solution, in the dialysis process if precipitation or too dirty, high
incidence of false alarm, which requires the timely removal operators Blood Leak Detection
parts of stolen goods.
Four hemodialysis machine development
Hemodialysis machine and the maturing of the development, operation and design of human
nature are universal, as a treatment for type equipment, personalized treatment, safety
performance and modular design of each manufacturer is the most important issue to consider.
Actually, according to each patient’s electrolytes, can easily provide different dialysis fluid is
hemodialysis machine an important direction of development; In order to improve the safety of
hemodialysis, and some companies are linked to the concept of health; Engineering,
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hemodialysis machine design tends to modular design This maintenance to the machine’s
performance.
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Appendix (D)
2-wire circuit
Shown is a 2-wire RTD connected to a typical
Wheatstone bridge circuit. Es is the supply voltage;
Eo is the output voltage; R1, R2, and R3 are fixed
resistors; and RT is the RTD. In this uncompensated
circuit, lead resistance L1 and L2 add directly to RT.
4-wire circuit
4-wire RTD circuits not only cancel lead wires
but remove the effects of mismatched
resistances such as contact points. A common
version is the constant current circuit shown
here. Is drives a precise measuring current
through L1 and L4; L2 and L3 measure the
voltage drop across the RTD element. Eo must
have high impedance to prevent current flow in
the potential leads. 4-wire circuits may be
usable over a longer distance than 3-wire, but
you should consider using a transmitter in
electrically noisy environments.
3-wire circuit
In this circuit there are three leads coming from the
RTD instead of two. L1 and L2 carry the measuring
current while L3 acts only as a potential lead. No
current flows through it while the bridge is in
balance. Since L1 and L2 are in separate arms of
the bridge, resistance is canceled. This circuit
assumes high impedance at Eo and close matching
of resistance between wires L1 and L2. TEMPCO
matches RTD leads within 5%. As a rule of thumb, 3
wire circuits can handle wire runs up to 100 feet.
If necessary you can connect a 2-wire RTD to a
3-wire circuit or 4-wire circuit, as shown. As
long as the junctions are near the RTD, as in a
connection head, errors are negligible.
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Biomedical Measurements | Bassel Tawfik