Thermocouples

THERMOCOUPLES

What are thermocouples? • Thermocouples operate under the principle that a circuit made by connecting two dissimilar metals produces a measurable voltage (emf-electromotive force) when a temperature gradient is imposed between one end and the other.

• They are inexpensive, small, rugged and accurate when used with an understanding of their peculiarities.

A Demonstration of the Law of Intermediate Temperatures

emf

emf1-2+ emf2-3= emf1-3

C

emf23

emf13 emf12

Fe

T1

T2

T3

Let’s take a look at this circuit

Material EMF versus Temperature Chromel

With reference to the characteristics of pure Platinum

Iron Copper Platinum-Rhodium

emf

Alumel

Temperature

Constantan

Thermocouple Tables (EMF-Temperature) • Thermocouple tables correlate temperature to emf voltage. – Need to keep in mind that the thermocouple tables provide a voltage value with respect to a reference temperature. Usually the reference temperature is 0°C. If your reference junction is not at 0°C, a correction must be applied using the law of intermediate temperatures.

What thermocouple materials should be used? • Depends on requirements: – Temperature range – Required accuracy – Chemical resistance issues – Abrasion or vibration resistance – Installation requirements (size of wire) – Thermal conduction requirements

Thermocouple Material Vs EMF Types T, J, and K are most commonly used thermocouples (see Table 16.8 of the “Handbook”).

Law of Intermediate Metals 2) Insertion of an intermediate metal into a thermocouple circuit will not affect the emf voltage output so long as the two junctions are at the same temperature and the material is homogeneous. – Permits soldered and welded joints.

Law of Intermediate Temperatures T2

T3

T3

If a thermocouple circuit develops a net T1 emf1-2 for measuring junction temperatures T1 and T2, and a net emf2-3 for temperatures T2 and T3, then it will develop a net voltage of T2 emf1-3 = emf1-2 + emf2-3 when the junctions are at temperatures T1 and T3. T1

emf1-2+ emf2-3= emf1-3

If a thermocouple circuit of materials A and C generates a net emfA-C when exposed to temperatures T1 and T2, and a thermocouple of materials C and B generates a net emfC-B for the same two temperatures T1 and T2, then a thermocouple made from materials A and B will develop a net voltage of

emfA-B = emfA-C + emfC-B between temperatures T1 and T2.

• Sometimes useful in the calibration of different thermocouple wires.

Temperature Measurement Errors • • • • • •

Conduction Convection Radiation Response Time Noise Grounding issues and shorts, especially on metal surfaces