University of Warith
Ministry of Higher
Al-Anbiyaa
Education and Scientific
Faculty of Engineering
Research
Engineering of Refrigeration and Air Conditioning
Technologies
Control Circuits
Lecture 3
Sensor
Ahmed Ehsan
Temperature Sensors
In air-conditioning applications, temperature is typically the primary controlled
variable. In comfort HVAC applications, temperature is used as the surrogate for
human comfort because it is typically the primary factor affecting comfort.
Temperature sensors can be categorized by the effect used to generate the
temperature-versus-signal response:
1. Bimetal
First temperature sensor used for automatic control purposes was the bimetallic
sensor (or bimetal for short), as shown in Figure 4-2. This consists of two metal
strips joined together continuously by welding or other means. The metals are
selected so that each has a very different coefficient of expansion
Figure 4-2 Bimetallic Temperature Sensor
(Different rates of expansion relative to a change in temperature). Because one strip
expands and contracts at a greater rate than the other, a change in temperature will
cause the bimetal strip to bend, as shown in the figure.
• For all model of control two way or modulate
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• Used in cooling and heating
• Used to control of flow interned to system
To provide a firm closure, a small magnet is mounted to provide snap-action on
opening and closing
Another form of the bimetallic sensor, one that is only two position, is shown in
Figure 4-3.
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2. Fluid Expansion
The bulb-and-capillary sensor (see Figure 4-5) utilizes a temperature-sensitive fluid
contained in a bulb with a capillary connection to a chamber with a flexible
diaphragm.
A change in temperature will cause a volume change in the fluid, which will cause
the diaphragm to deflect. With the proper linkages, this can be used for either twoposition or modulating control, in electric, electronic, or pneumatic systems. It is
sometimes called a remote bulb sensor and is usually provided with fittings suitable
for insertion into a duct, pipe, or tank.
Capillaries most made from materials doesn't effect by temperature of surrounding.
This is done by using two dissimilar metals for the capillary, one on the outside
(usually stainless steel) and another metal on the inside, with the sensing fluid in
between. The materials are selected so that the differential coefficient of expansion
of the two metals exactly equals the coefficient of expansion of the sensing fluid.
Thus, as the capillary expands and contracts due to changes in ambient temperatures,
it makes room for the fluid as it expands and contracts at the same rate. In this way,
the ambient temperature changes do not affect the fluid pressure signal.
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3. Electrical, Self-powered
Thermocouples are formed by a junction of two dissimilar metals that develop a
varying electromagnetic force (voltage) when exposed to different temperatures. For
example, an iron wire and a bronze wire can be joined at their ends to form a
junction. If the junction is heated to 55oC above ambient, about 3 milli-volts will be
generated at the hot junction (see Figure 4-6).
The most used thermocouple materials (and their industry standard letter
designations) are platinum-rhodium (Type S or R), chrome-aluminum (Type K),
copper-constantan (Type T), and iron-constantan (Type J). Accuracy for handheld
instruments ranges from +-0.3oC to +-3oC for a calibrated thermocouple.
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Advantage
• Ease of manufacture.
• Standard wide temperature range.
• It does not need an external power source to operate.
• It does not depend on a specific type of wire.
• The resistance of the used wires does not affect the quality of the
measurement.
Disadvantage
• Small values of electro-thermal current generated.
• Difficulty in correcting the value of the error in the measurement.
• Difficulty in welding wires consisting of these metals.
• The danger that dirt or rust takes place between the object to be measured and
the pair, so the measured value will be affected.
4. Electrical Resistance
Modern analog electronic and digital control systems generally rely on devices that
resistance changes with temperature. Listed roughly in the order of commonality and
popularity, these include thermistors; resistance temperature detectors (RTDs); and
integrated circuit temperature sensors.
RTD (Resistance Temperature Detector) is a sensor whose resistance changes as its
temperature changes. The resistance increases as the temperature of the sensor
increases. The resistance vs temperature relationship is well known and is repeatable
over time. An RTD is a passive device. It does not produce an output on its own.
External electronic devices are used to measure the resistance of the sensor by
passing a small electrical current through the sensor to generate a voltage. Typically,
1 mA or less measuring current, 5 mA maximum without the risk of self-heating.
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The materials used Cu, Pt, Ni etc. the chose of materials depend on accuracy
required.
The advantages :
•
•
•
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High accuracy
Low drift
Wide operating range
Suitability for precision applications.
Disadvantages
• Doesn't used above 660oC in industrial applications.
• Doesn't used below -270oC in industrial applications.
• Need small voltage to work .
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Moisture Sensors
Humidity Sensor is one of the most important devices that has been widely in
consumer, industrial, biomedical, and environmental etc. applications for measuring
and monitoring Humidity.
Humidity is defined as the amount of water present in the surrounding air. This water
content in the air is a key factor in the wellness of mankind.
There are found different type of humidity sensor such as :
1. Capacitive Humidity Sensors
Humidity Sensors based on capacitive effect or simply Capacitive Humidity Sensors
are one of the basic types of Humidity Sensors available. They are often used in
applications low cost. In Capacitive Relative Humidity (RH) Sensors, the electrical
permittivity of the dielectric material changes with change in humidity.
A simple Capacitive RH Sensor can be made from an isolator (appropriate dielectric
material, whose dielectric constant varies when it is subjected to change in humidity)
capacitor as the moisture in the atmosphere changes its permittivity
The common method of constructing a capacitive RH sensor is to use a hygroscopic
polymer film as dielectric and depositing two layers of electrodes on the either side.
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Another way to use the capacitive RH sensors is to observe the changes in the
frequency of the oscillator constructed using a capacitor with RH sensitive test
subject as dielectric.
The test samples like medical tablets are placed between two plates (which form the
capacitor electrodes) to form a capacitor in the LC Oscillator circuit. The frequency
of the oscillator changes with humidity surrounding the test sample.
Let us see the construction of a thin thermostat polymer film based capacitive RH
Sensor. It is fabricated on a silicon substrate. On this substrate, two metal electrodes
made of either aluminum, platinum or chromium are deposited. The shape of these
electrodes is carved out such that, the electrodes form an interdigitated pattern.
On top of this layer, a dielectric layer is deposited. The following image shows a top
and cross section view of the capacitive humidity sensor.
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Advantages of Capacitive Humidity Sensors
• The output voltage is near linear.
• They provide stable results over long usage.
• Can detect wide range of RH.
Disadvantages of Capacitive Humidity Sensors
• The distance from the sensor and signalling circuit is very limited.
Applications of Capacitive Humidity Sensors
Capacitive Humidity Sensors are used in a wide range of applications including but
not limited to:
• HVAC Systems
• Printers and Fax Machines
• Weather Stations
• Automobiles
• Food Processing
• Refrigerators, Ovens and Dryers
2. Resistive Humidity Sensors (Electrical Conductivity Sensors)
Resistive Humidity Sensors are another important type of Humidity Sensors that
measure the resistance (impedance) or electrical conductivity. The principle behind
resistive humidity sensors is the fact that the conductivity in non – metallic
conductors is dependent on their water content.
The Resistive Humidity Sensor is usually made up of materials with relatively low
resistivity and this resistivity changes significantly with changes in humidity. The
relationship between resistance and humidity is inverse exponential. The low
resistivity material is deposited on top of two electrodes.
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The electrodes are placed in interdigitated pattern to increase the contact area. The
resistivity between the electrodes changes when the top layer absorbs water and this
change can be measured with the help of a simple electric circuit.
Some of the commonly used materials are salt, specially treated substrates, solid
polyelectrolytes and conductive polymers. The electrodes in the sensor are usually
made of noble metals like gold, silver or platinum.
Advantages of Resistive Humidity Sensors
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Low cost
•
Small Size
•
The distance between the sensor and signal circuit can be large (suitable for
remote operations).
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Highly interchangeable as there are no calibration standards.
Disadvantages of Resistive Humidity Sensors
•
Resistive Humidity Sensors are sensitive to chemical vapors and other
contaminants
•
The output readings may shift if used with water soluble products.
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3. Thermal Conductivity Humidity Sensors
Thermal Conductivity Humidity Sensors are also known as Absolute Humidity (AH)
Sensors as they measure the Absolute Humidity. Thermal Conductivity Humidity
Sensors measure the thermal conductivity of both dry air as well as air with water
vapor. The difference between the individual thermal conductivities can be related
to absolute humidity. Hence, two tiny thermistors with negative temperature
coefficient are used to for a bridge circuit.
In that, one thermistor is hermetically sealed in a chamber filled with dry Nitrogen
while the other is exposed to open environment through small venting holes. When
the circuit is powered on, the resistance of the two thermistors is calculated and the
difference between those two values is directly proportional to Absolute Humidity
(AH).
Advantages of Thermal Conductivity Humidity Sensors
• Suitable for high temperature environments and high corrosive situations.
• Very durable
• Higher resolution compared to other types
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Disadvantage of Thermal Conductivity Humidity Sensors
• Exposure to any gas with thermal properties different than Nitrogen might
affect reading measurement.
Applications of Thermal Conductivity Humidity Sensors
• Drying kilns
• Pharmaceutical plants
• Clothes dryers and drying machines
• Food dehydration
Important Considerations when Selecting a Humidity Sensor
The following are some of the factor that must be taken into consideration when
selecting any Sensor.
• Accuracy of the sensor.
• Calibration – requirements and methods
• Size of the sensor
• Cost of the sensor and cost of replacement
• Output repeatability
• Circuit complexity
• Resistance to contamination
• Reliability of the sensor
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Pressure Sensors
Pressure is almost always measured as a differential pressure, either the difference
between the pressures of two fluids or the difference in pressure between a fluid and
a reference pressure. When the reference pressure is atmospheric pressure, we refer
to the pressure of the fluid as gauge pressure.
1. Mechanical Pressure Gauges.
The Bourdon tube (see Figure 4-22) is the sensing element used in most pressure
indicating gauges. It is a closed, spiral tube, connected at one end to the pressure
being sensed, with atmospheric pressure as a reference. As the sensed pressure
increases, the tube tends to straighten, and, through a linkage and gear, drives an
indicating pointer. By adding a switch to the linkage, the device can become a sensor
with switching capability.
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2. Electrical Pressure Gauges
Digital pressure gauges make use of pressure sensors to convert pressure to an
electronic signal. Different types of pressure sensors exist, but the most used is a
piezoresistive pressure sensor. This sensor consists out of a diaphragm that is
equipped with piezoresistive elements. The medium pressure causes the diaphragm
to deflect, this deflection causes a change in cross-sectional area of the piezoresistive
elements that is directly coupled to the electrical resistance.
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Flow Sensors and Meters
The most common uses of flow sensors in air and hydronic systems are for energy
process control and energy monitoring (sensors with indication and/ or recording
device called meters).
Typical processes using flow control include:
• Measuring the variable flow in large, chilled water plants to facilitate making
decisions about flows and what equipment (typically chillers, pumps, and
cooling towers) should be running.
• Flow measurement to adjust the flow through variable volume boxes.
• Using flow to adjust variable speed fans to maintain the correct flow balance
between supply, return, and relief.
• Adjusting air flow through fume hoods to maintain the capture velocity under
changing conditions.
Differential Pressure Flow Meters
The differential pressure flow meter measures the volume flow in gases, liquids and
steam. They are particularly used in situations where high pressure, high temperature
or a large diameter play a role. They are mainly found in the chemical, oil, gas and
power industries.
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where V is the velocity, C is a constant that is a function of the physical design
of the meter, DP is the measured pressure drop, and r is the fluid density.
Displacement Flow Meters
Positive Displacement flow meters are the only flow measuring technology to
directly measure the volume of fluid that passes though the flow meter. It achieves
this by trapping pockets of fluid between rotating components housed within a high
precision chamber The process fluid must be clean.
A target meter (Figure 4-32), also called a drag-force meter, measures
flow rate by the amount of stress in the stem supporting a paddle or other
obstruction mounted in the flow stream. The higher the flow rate, the
greater the bending action, and the greater the stress. Stress is typically
measured using a strain-gauge located where the support stem is attached
to the meter body. One advantage of this device is that it has no moving
parts.
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Passive Flow Meters
One of this type is the ultrasonic flowmeter
uses the time difference principle of ultrasonic wave propagation in the medium to
measure the flow rate.It is mainly used to measure liquid, such as ultra-pure liquid,
chemical, raw sewage, reclaimed water, cooling water, river water, plant sewage,
etc.
Ultrasonic flow meters use sound waves at a frequency beyond the range of hearing
(typically 0.5, 1, or 4 MHz). Clamp-on ultrasonic flow meters allow users to
measure the volumetric flow rate of a fluid in a pipe without having to penetrate the
pipe which decreases installation and maintenance costs.
A typical transit-time ultrasonic liquid flow meter utilizes two ultrasonic transducers
that function as both ultrasonic transmitter and receiver. The ultrasonic flow meter
operates by alternately transmitting and receiving a burst of ultrasound between the
two transducers by measuring the transit time that it takes for sound to travel between
the two transducers in both directions. The difference in the transit time (∆ time)
measured is directly proportional to the velocity of the liquid in the pipe.
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