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TYPE OF TEMPERATURE TRANSDUCER
TYPE OF TEMPERATURE TRANSDUCER
Passive Transducer
Passive Transducer
Resistance changes with temperature
Need an electrical supply to give a voltage output
Example: RTD and Thermistor
Active Transducer
Active Transducer
Generates an electrical signal directly in response to the physical parameter
Example: Thermocouple
Temperature
Temperature
Absolute temperature scales
Absolute temperature scales
common absolute scales are Kelvin(K) and Rankine(°R)
The relationship are as following:
Relative temperature scales
Relative temperature scales
This scale differ from the absolute scales only in a shift of the zero axis
Celcius (°C) – related to the Kelvin; and Fahrenheit (°F) - related to the Rankine
The relationship are as following:
Temperature Relationship
Temperature Relationship
Relationship between Kelvin and Rankine:
Relationship between Celcius and Kelvin / Fahrenheit and Rankine:
Relationship between Fahrenheit and Celcius:
Passive Transducer
Passive Transducer
RTD
RTD
(Resistance Temperature Detector)
(Resistance Temperature Detector)
A temperature sensor that produces a change in resistance in a metal as a function of temperature
Basic principle of RTD
➔
RTD commonly use material are as following:
Most RTD use Wheatstone's Bridge or modify version of it, for example:
Thermistor
Thermistor
Thermistors are semiconductor materials that exhibit a large change in resistance with a change in their body temperature
Type of thermistor
Type of thermistor
Negative Temperature Coefficient (NTC)
Negative Temperature Coefficient (NTC)
exhibits decreasing electrical resistance with increases in environmental temperature.
Positive Temperature Coefficient (PTC)
Positive Temperature Coefficient (PTC)
exhibits increasing electrical resistance with increases in environmental temperature
Thermistor resistance-temperature graph
Thermistor resistance-temperature graph
With an NTC thermistor, when the temperature increases, resistance decreases.
For a PTC thermistor, when temperature increases, the resistance increases
More about thermistor
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Relationship between resistance and temperature
Relationship between resistance and temperature
- RTD and thermistor resistance at temperature T°C is given by :
R(T) = resistance of wire at temperature T°C
Rref = resistance at the reference temperature
α = temperature coefficient of resistance
△T = T - To = difference between operating and reference temperature
2. Dissipation Constant
2. Dissipation Constant
∆T= temperature rise because of self-heating
P = Power dissipated in the RTD from the circuit in W
Pᴅ = dissipation constant of the RTD in W/C
ACTIVE TRANSDUCER
ACTIVE TRANSDUCER
Thermocouple
Thermocouple
Thermocouples (TCs) are voltage-generating sensor in which an electromotive force (emf) produced is proportional to temperature.
How to use thermocouple:
How to use thermocouple:
a three-wire thermocouple system is used to measure a temperature.
the reference temperature must be known, and the reference junctions must be held at the same temperature.
The reference junctions can be moved to a remote location using extension wires of the same type as the thermocouple wires
TM is the measured temperature
TR is a reference junction, • metal A and B are thermocouple wires
metal C connects thermocouple to the measurement apparatus
Seebeck effect
Seebeck effect
Thermocouple is constructed by two different metals or wires which are connected on their both ends
The wires loop produces an electromotive force (emf) which is called the Seebeck effect.
The emf produced is proportional to the difference in temperature between the two junctions.
Seebeck emf produced
Seebeck emf produced
α = constant in V/K
T1, T2 = junction temperature in K
Type of thermocouple
Type of thermocouple
There are several standard types of thermocouples which are based on their wires material
Each type has their own range, linearity, and sensitivity
Graph shows each Voltage vs Teamperature
Thermocouple table
Thermocouple table
Thermocouple tables give the voltage that results for a particular type of thermocouple when the reference junctions are at 0°C.
Each type has their own table
Table below are the example of type J table
interpolation equation
interpolation equation
When the temperature or output voltage value is not given in the table, it is necessary to use interpolation equation
VM and TM is the measured voltage and temperature.
VH and VL is the higher and lower voltage in the table.
TH and TL is the corresponding higher and lower temperature.
Shifted reference junction
Shifted reference junction
If the reference junctions of thermocouple are not at 00C, then the table values must be shifted up or down.
If the reference junctions are at higher temperature than 00C, then all the table values will be shifted down
Example of shifted reference junction: ➔
More about Thermocouple
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EXAMPLES
EXAMPLES
1)What is the resistance of a platinum RTD at 60ºC if the resistance at 20ºC is 135Ω. Linear
approximation method is used.
Solution:
The resistance coefficient α of a platinum is 0.0039 as per table above.
R(T)=Rref(1+αΔT)
R(60 ºC) = 135 [ 1 + 0.0039(60ºC - 20ºC) ] Ω
= 156Ω
2)Find the output voltage for type J thermocouple at 1500ºC with a reference
temperature of 300ºC:
Solution:
VJ30 (1500C) = VJ0 (1500C) - VJ0 (300ºC)
= 8.00mA – 1.54 mA
= 6.46 mA.
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