Introduction to Temperature Sensors

ISAT 300 Instrumentation and Measurement

02/17/1999

CoolBath

HotBath

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.

LanguageRange of Operation

Lowest to highest temperatureActive 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 TransducersThermocouples

Voltage Device, passive, non-linear -270 to 1372oC for type Ktype 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

ThermocouplesTwo disimilar metals are joined togetherChange in temperature at junction

generates voltageSmaller size compared to thermistorsFast response time (time constant as small

as 1 ms)Ease of fabrication, long term stabilityLow sensitivity, small output voltageNeed reference temperature

Thermocouples

Passive Device Voltage is developed as the function of the

the temperature of the junctionSeebeck Effect

A thermoelectric phenomenon The sum of two voltage effects, namely

Peltier effect and Thompson effectUse in Computer-Based Instrumentation

Thermocouples - Background

Kirchhoff’s Voltage Law Sum of all voltages

in a loop = 0.

+_??V

+

_

2V

+_

_+

-3V

5V

Kirchhoff’s Current Law Sum of all currents

entering a node is 0.

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

T1 T2

A

B

+

_+

_

AB BA

Peltier EMF Voltage at the junction of two dissimilar

metals Denoted as AB, BA

How Does Thermocouple Work?

Thompson EMF Voltage in a wire caused by a temperature

difference between the ends. Denoted as A, B

T2

A

B

+ _

+ _

T1

A

B

How Does Thermocouple Work?

The Ice Point - A Reference Junction

V V V V VT T T T DVM1 3 3 2 0

V V VT T DVM1 2 0

V V VDVM T T 1 2

B

T1

AT3

DVM

T3

Copper

A

+

+

+

T2ICE

Using Thermocouples

Removing the Ice Point DVM shows the voltage due to the

difference between T1 and T2 Use the same equation as before!!

T2Chromel

Alumel

T1 DVM

T2

Copper V V VDVM T T 1 2

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)

T2Chromel

Alumel

T1

T2

CopperDVM

Using Thermocouples

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.html9.090 mV corresponds to 891oC and 9.103 mV corresponds to 892oC.

Linear interpolation gives a temperature 891.8oC for 9.1 mV.

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