One of the most common industrial thermometers is the thermocouple. It was discovered by Thomas Seebeck in 1822. He noted that a voltage difference appeared when a wire was heated at one end. Regardless of temperature, if both ends of the wire were at the same temperature, there was no voltage difference. If the circuit was made with wire of the same material, there was no current flow.
A thermocouple consists of two dissimilar metals, joined together at one end, that produce a small unique voltage at a given temperature. This voltage is measured and interpreted by a thermocouple thermometer.
The thermoelectric voltage resulting from the temperature difference from one end of the wire to the other is actually the sum of all the voltage differences along the wire from end to end.
Thermocouples can be made from a variety of metals and cover a temperature range of 200ºC to 2,600ºC.
Types of Thermocouples
Thermocouples are available in different combinations of metals or calibrations. The four most common calibrations are J, K, T, and E. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.
Some of the thermocouple types have been standardized with calibration tables, color codes, and assigned letter-designations. The ASTM E230, Standard Specification and TemperatureElectromotive Force (EMF) Tables for Standardized Thermocouples, provides all the specifications for most of the common industrial grades, including letter designation, color codes (USA only), suggested use limits, and the complete voltage versus temperature tables for cold junctions maintained at 32ºF and 0ºC.
There are four classes of thermocouples:
- The home body class (called base metal)
- The upper crust class (called rare metal or precious metal)
- The rarified class (refractory metals)
- The exotic class (standards and developmental devices).
The home bodies are the types E, J, K, N, and T. The upper crusts are types B, S, and R, all platinum to varying percentages. The exotic class includes several tungsten alloy thermocouples usually designated as type W.
The advantages of thermocouples are
they are capable of being used to directly measure temperatures up to 2600ºC; and
the thermocouple junction may be grounded and brought into direct contact with the material being measured.
The disadvantages of thermocouples are
temperature measurement with a thermocouple requires that two temperatures be measured, the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction). To avoid error, the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using a semiconductor, thermistor, or RTD;
thermocouples operations are relatively complex with potential sources of error. The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion, etc.;
the relationship between the process temperature and the thermocouple signal is not linear; and
the calibration of the thermocouple must be carried out while it is in use by comparing it to a nearby comparison thermocouple. If the thermocouple is removed and placed in a calibration bath, the output integrated over the length is not reproduced exactly.