Thermocouples

A thermocouple is made by joining two dissimilar metals. Thomas Seebeck

discovered in 1821 that when such a junction is heated, it generates a tiny voltage.

The amount of voltage is dependent on which two metals are joined. Three

common thermocouple combinations are iron-constantan (Type J), copperconstantan

(Type T), and chromel-alumel (Type K).

The voltage produced by a thermocouple junction is very small, typically only

a few millivolts. A type K thermocouple changes only about 40 mV per _C change

in temperature; to measure temperature with 0.1 _C accuracy, the measurement

system must be able to measure a 4 mV change. Because any two dissimilar metals

will produce a thermocouple junction when joined, the connection point of the

thermocouple to themeasurement system will also act as a thermocouple. Figure 3.4

shows this effect, where a thermocouple is connected to a board using copper. The

wires leading to the amplifier could be either copper wires or the copper traces

on a PCB.

As shown in Figure 3.4, this effect can be minimized by placing the connections

on an isothermal block, which is a good conductor of heat. This minimizes the

temperature difference between the connection points and minimizes the error

introduced by the connection junctions. A common method of compensating for

the temperature of the connection block is to place a diode or other semiconductor

on the isothermal block and measure the (temperature-sensitive) drop across

the semiconductor junction. The amplifier used to increase the signal level from the thermocouple is

usually an instrumentation amp. The gain required to measure a thermocouple

is typically in the range of 100 to 300, and any noise picked up by the thermocouple

will be amplified by the same amount. An instrumentation amplifier

rejects the common mode noise in the thermocouple wiring.

Analog Devices makes a thermocouple signal conditioner, the AD594/595,

which is specifically intended for interfacing to a thermocouple. The AD594/

595 does not use an external semiconductor junction to compensate for connection

temperature; instead the part includes an internal junction that is expected

to be the same temperature as the connection. Consequently, the thermocouple

connection must be made on the PC board, close to the AD595/595 package.

The amplified thermocouple signal may need scaling, just like a thermistor, to

place it in a useable range for an ADC. Thermocouples are relatively linear over a

limited range of temperatures, but if the range of measurement is wide, the

software will need to compensate for nonlinearities. The formula for thermocouple

voltage is a polynomial, just like thermistor resistance formula.