1. Introduction to Temperature Sensors
ISAT 300
Instrumentation and
Measurement
02/17/1999

2. Temperature Measurement Devices
Electrical Effects
Thermocouples
Thermistors
Resistance Temperature Detectors (RTD)
Mechanical Effects
Liquid-in-glass thermometers
Radiation (non-contact)
Pyrometers

3. Introduction
Somehow, we need to translate the temperature of an object to a voltage that a data acquisition system can read.
A look at transducers that translate a change in temperature to a change in voltage or resistance.

4. Language Range of Operation Lowest to highest temperature
Active vs. Passive
Require external power source?
Intrusive vs. Non-intrusive
Will this affect the system?
Linearity
Is the conversion from voltage to temperature a line or a more complicated function?
Sensitivity

5. Temperature Transducers
Thermocouples
Voltage Device, passive, non-linear
-270 to 1372oC for type K
Thermistors
Resistive Device, active, non-linear
-100 to 300oC
RTDs (Resistance Temperature Detectors)
Resistive Device, active, linear
Large Range: to +850oC for Platinum
Low sensitivity: % per oC

6. Thermocouples Two disimilar metals are joined together
Change in temperature at junction generates voltage
Smaller size compared to thermistors
Fast response time (time constant as small as 1 ms)
Ease of fabrication, long term stability
Low sensitivity, small output voltage
Need reference temperature

7. Thermocouples Passive Device
Voltage is developed as the function of the the temperature of the junction
Seebeck Effect
A thermoelectric phenomenon
The sum of two voltage effects, namely Peltier effect and Thompson effect
Use in Computer-Based Instrumentation

8. Thermocouples - Background
Kirchhoff’s Current Law
Sum of all currents entering a node is 0.
Kirchhoff’s Voltage Law
Sum of all voltages in a loop = 0. + _
??V 2V
-3V 5V
5 mA
3 mA
?? mA

9. How Does Thermocouple Work?
Seebeck Effect
An electromotive force (EMF) exists in a loop made of two dissimilar metals when the two junctions are different temperatures. T1 T2 A B

10. How Does Thermocouple Work?
Peltier EMF
Voltage at the junction of two dissimilar metals
Denoted as pAB, pBA T1 T2 A B + _
pAB
pBA

11. How Does Thermocouple Work?
Thompson EMF
Voltage in a wire caused by a temperature difference between the ends.
Denoted as tA, tB T2 tA B + _ T1 A tB

12. The Ice Point - A Reference JunctionB T1 A T3
DVM
Copper + - T2
ICE

13. Using Thermocouples Removing the Ice Point
DVM shows the voltage due to the difference between T1 and T2
Use the same equation as before!! T2
Chromel
Alumel T1
DVM
Copper

14. Using Thermocouples Conversion Process in General
Measure the Temperature, T2
Measure the voltage using the DVM, VDVM
Convert the temperature of T2 to a voltage (table), VT2
Add voltage from DVM to the voltage for T2, VDVM + VT2
Convert the voltage sum to T1 (use a table) T2
Chromel
Alumel T1
Copper
DVM

15. Example: A type R thermocouple system with an ice reference has an output of 9.1 mV. What is the temperature of the sensing junction?
From OMEGA Type R Thermocouple Reference Table, ttp://
9.090 mV corresponds to 891oC and
9.103 mV corresponds to 892oC.
Linear interpolation gives a temperature 891.8oC for 9.1 mV.

16. Thermocouple Reference Table www.omega.com/techref/tctables/rc-2.html