Download Transducers(cont`d)

6.Temperature Transducer: Temperature transducers can be divided into four main
categories.
1. Resistance temperature detectors (RTD).
2. Thermocouples.
3. Thermistors.
4. Ultrasonic thermometer.
Temperature Transducer(cont’d):1. Resistance temperature detectors (RTD): Detectors of resistance temperatures commonly employ
platinum, nickel, or resistance wire elements, whose
resistance variation with temperature has a high intrinsic
accuracy.
 They are available in many configurations and sizes and as
shielded or open units for both immersion and surface
applications.
Temperature Transducer(cont’d):1. Resistance temperature detectors (RTD): The relationship between temperature and resistance of
conductors can be calculated from the equation.
R  R0 (1    T )
Where
R = The resistance of the conductor at temperature t (°C)
Ro = The resistance at the reference temperature, usually 20°C
 = The temperature coefficient of resistance
T = The difference between the operating and the reference
Temperature
Temperature Transducer(cont’d):1. Resistance temperature detectors (RTD): EXAMPLE:-
A platinum resistance thermometer has a resistance of 150 at
20oC. Calculate its resistance at 50`C ( 20 =0.00392).
Solution:R  R0 (1    T )
150  [1  0.00392 (50  20) o C ]
 167.64 
Temperature Transducer(cont’d):2. Thermocouples: One the most commonly used methods of measuring
temperature in science and industry depends on the
thermocouple effect.
 When a pair of wires made of different metals are joined
together at one end, temperature difference between this end
and the other end of the wires produces a voltage between
the wires Fig. (9).
Temperature Transducer(cont’d):2. Thermocouples: The magnitude of this voltage depends on the materials used
for the wires and the amount of temperature difference
between the joined ends and the other ends.
 The junction of the two wires of the thermocouple is called
the sensing junction. In normal use this junction is placed in
or on the material being tested, and the other ends of the
wire are connected to the voltage-measuring equipment.
Temperature Transducer (cont’d):2. Thermocouples: Since the temperature difference between this sensing
junction and the other ends is the critical factor, the other
ends are either kept at a constant reverence temperature or,
when the cost of the equipment is very low, simply
maintained at room temperature.
 When the other ends are kept at room temperature, the
temperature is monitored and the thermocouple output
voltage readings are corrected for any changes in room
temperature.
Temperature Transducer (cont’d):2. Thermocouples: Because the temperature at this end of the thermocouple
wires is a reference temperature, the junction here with the
equipment terminals or with other connecting wires is
known as the reference junction.
 It is also quite often referred to as the co/d junction. Because
the thermocouple is frequently used for measuring high
temperatures, the reference junction it such cases is indeed
the colder of the two junctions.
Temperature Transducer (cont’d):2. Thermocouples: Since any junction of dissimilar metals will produce some
thermocouple voltage, the wires and any metal terminals
between the sensing junction and the rest of the equipment
must be carefully controlled.
 Usually this means that the wires between the sensing
junction and the reference junction are of specific materials
provided by the thermocouple supplier, and the wires from
the reference junction to the measuring equipment are
copper.
Temperature Transducer (cont’d):2. Thermocouples: Thermocouples are made from a number of difference metals
or metal alloys covering a wide range of temperatures from
as low as -270°C(-418°F) to as high as 2700`C (about
5000°F).
Fig (9) Schematic representation of a thermocouple assembly.
Temperature Transducer (cont’d):2. Thermocouples: The magnitude of the thermal emf depends on the wire materials used
on the temperature difference between the junctions. Figure (11) shows
thermal emfs for some common thermocouple materials. The values
shown are based on a reference temperature of 32°F. The effective emf
of thermocouple is given as
2
2
1
2
E  c (T1  T2 )  k (T  T )
Where
 c and k =constants of the thermocouple materials
 T1 =the temperature of the "hot" junction
 T2 =the temperature of the "cold" or "reference" junction
Temperature Transducer (cont’d):2. Thermocouples:-
Fig (10) Thermocouples and thermocouple assemblies. (a) Uninsulated thermocouple. (b) Insulated
thermocouple. (c) Probe assembly. (d) Thermocouple well.
Temperature Transducer (cont’d):2. Thermocouples:-
Fig (11) Calibration curves for several thermocouple combinations.
Temperature Transducer (cont’d):2. Thermocouples: EXAMPLE :-
During experiments with a copper- constantan thermocouple
it was found that c = 3.75 x 10-2 mV/°C and k =4.50x10-5
mV/°C2. If T1 =100°C and the cold junction T2 is kept in
ice, compute the resultant electromotive force.
Solution:E  c (T1  T2 )  k (T 1  T 2)
2
2
mV
o
o
(
100
C

0
C)
o
C
mV
 4.50 x 10 5 o 2 (100 2  0 2 ) o C 2
C
 3.75 mV  0.45 mV  4.20 mV
 3.75 x 10  2
Temperature Transducer (cont’d):3. Thermistors.: The electrical resistance of most materials changes with the
temperature. By choosing materials that are very sensitive to
temperature, we can make devices that are useful in
temperature control circuits as well as in temperature
measurement.
 A thermistor is a semiconductor made by sintering mixtures
of metallic oxide, such as oxides of manganese, nickel,
cobalt, copper, and uranium. Thermistors have a negative
temperature coefficient. That is, their resistance decreases as
their temperature rises.
Temperature Transducer (cont’d):3. Thermistors.: Resistance at 25'C for typical commercial units ranges from
the 100 to over 1 M . A graph showing resistance versus
temperature for a family of Thermistors is given in Fig (12).
The resistance value marked at the bottom end of each curve
is the value at 25°C.
 In addition to the choice of resistance values, choices of power
rating, physical size and shape, resistance tolerance, and
thermal time constant are also available.
Temperature Transducer (cont’d):3. Thermistors.:-
Fig (12) Typical thermistor resistance-versus-temperature curves. (Courtesy Fenwal Electronics, Framingham, Mass.)
Temperature Transducer (cont’d):3. Thermistors.: Thermistors can be connected in series-parallel arrangements for
applications requiring greater power-handling capability.
 High-resistance units find application in measurements that
employ wires or cables with small quantities of lead. Thermistors
are chemically stable and can be used in nuclear environments.
 Their wide range of characteristics also permits them to be used in
limiting and regulation circuits, as time delays, for the integration
of power pulses, and as-memory units.
Temperature Transducer (cont’d):3. Thermistors.: Typical thermistor configurations are shown in Fig. (13) and
the electrical symbol of the device is depicted in the same
figure.
Fig (13) thermistor configuration and the electrical symbol for a thermistor. (Courtesy Yellow Springs Instrument
Company, Yellow Springs, Ohio.)
Temperature Transducer (cont’d):3. Thermistors.: A thermistor in one leg of a Whetstone bridge circuit will
provide pre temperature information. In most applications
accuracy is limited only by readout device.
 Thermistors are nonlinear over a temperature range,
although units today are available with a better than 0.2%
linearity over a temperature range of to 100°C. The typical
sensitivity of a thermistor is approximately 3 mV/°C at
200°C.
Temperature Transducer (cont’d):3. Thermistors.: EXAMPLE :-
The circuit of Fig. (14) is to be used for temperature
measurement. thermistor is a 4-k type identified in Fig. 1116. The meter is a 50-mA ammeter with a resistance of 3 , Rc
is set to 17 , and the supply voltage VT is 15 V. What will the
meter readings at 77°F and at 15oF be?
Fig (14) Basic thermistor circuit for measuring.
Temperature Transducer (cont’d):3. Thermistors.: Solution:-
The graph for the 4-k thermistor in Fig. ( ) shows that its
resistant at 77oF is 4 .Therefore, the current at 77°F is
VT
15V
I

 3.73 mA
RT 4000  17  3 
 At 150°F the graph shows the thermistor resistance to be
950. The meter reading at this temperature, therefore, should
be
VT
15V
I

 15.5 mA
RT 950  17  3 
Temperature Transducer (cont’d):4. Ultrasonic Temperature Transducers: Ultrasonics, which are sound vibrations above 20.000 Hz.
can be useful when we are concerned wish rapid temperature
fluctuations.
 Temperature extremes. limited access, nuclear, and other
severe environmental conditions and when we must measure
the temperature distribution inside solid bodies.
Temperature Transducer (cont’d):4. Ultrasonic Temperature Transducers: The need to measure simultaneously the distribution of
parameters other than temperature (e.g., flow) may also
justify an ultrasonic approach.
 Ultrasonics also offers possibilities of remote sensing and
sometimes can prevent penetration of the system
(nonintrusive) as shown in fig (15).
Fig (15) (a) Schematic and oscillogram illustrating ultrasonic temperature profiling. (y) Ultrasonic thermometer.
7. Piezoelectric Transducers:
 When a mechanical pressure is applied to a crystal of the
Rochelle salt quartz, or tourmaline type, a displacement of
the crystals causes a potential difference to occur.
 This property is used in piezoelectric transducers: in these
transducers a crystal is placed between a solid base and forcesumming member, as shown in Fig. (16).
Piezoelectric Transducers (cont’d): Externally applied forces exert pressure to the top of the
crystal. This produces an electromotive force across the
crystal proportional to the magnitude of the applied
pressure.
Fig (16) Elements of a piezoelectric transducer.
Piezoelectric Transducers (cont’d): For a piezoelectric element under pressure, part of the
energy will be converted to an electric potential that will
appear on opposite faces of the element, analogous to the
charge on the plates of a capacitor.
 The rest of the applied energy is converted to mechanical
energy, analogous to that of a compressed spring: When the
pressure is removed, the piezoelectric element will return to
its original shape and also lose its electric charge.
Piezoelectric Transducers (cont’d): From these relationships the following formulas have been
derived for the coupling coefficient k.
k
Mechanical energy converted to electrical energy
Applied mechanical energy
k
Electrical energy converted to mechanical energy
Input electrical energy
Piezoelectric Transducers (cont’d): An alternating voltage applied to a crystal causes it to vibrate at its
natural resonance frequency. Since the frequency is a very stable
quantity, piezoelectric crystals are used principally in high frequency accelerometers.
 The output voltage is typically on the order of 1 to 30 mV per
gram of acceleration. The device needs no external power source
and is therefore self-generating.
 The principal disadvantage of this transducer is that voltage will be
generated only as long as the pressure applied to the piezo electric
element is changing.
Piezoelectric Transducers (cont’d): EXAMPLE :-
A certain crystal has a coupling coefficient of 0.32. How
much electrical energy must be applied to produce an output
of 1 in of mechanical energy?
Solution:1 in. = in. x 1 ft x 1 Ib x 1.356 J
12in 16
= 7.06 x 10-3 J
1 ft  Ib