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

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

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.

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

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: -200 to +850oC for Platinum Low sensitivity: 0.39 % per oC

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

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

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

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

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

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

The Ice Point - A Reference Junction B T1 A T3 DVM Copper + - T2 ICE

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

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

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://www.omega.com/techref/tctables/rc-2.html 9.090 mV corresponds to 891oC and 9.103 mV corresponds to 892oC. Linear interpolation gives a temperature 891.8oC for 9.1 mV.

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