BASIC THERMOCOUPLE THEORY

Thermocouples are made by welding two pieces of dissimilar wire together and using the welded junction as a temperature sensor. A voltage is created that is proportional to the difference in temperature between the welded sensing end and the other end where the measurement is being made.

To make an accurate measurement, the temperature of the measurement end must be determined by independent means and this temperature must be added to the temperature determined by the thermocouple voltage.

This process is named cold junction compensation and is a necessary requirement for accurate temperature measurement with thermocouples.

THERMOCOUPLE TYPES

Certain materials for thermocouples have been identified which have characteristics that make them valuable for temperature measurement.

These thermocouple types have been defined by standards and their manufacturing is rigidly controlled to insure the voltages created by temperature measurement comply with tables of voltages published by the NIST (National Institute of Standards and Technology, used to be NBS (National Bureau of Standards)).

The NIST publishes Monograph 175, which is a complete set of data for the letter designated standard thermocouples. The document may be purchased from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325.

LETTER DESIGNATED THERMOCOUPLES

Thermocouples are generally referred to by their "calibration" (J, K, etc). "Special calibrations" are available which generally refers to a smaller initial measurement error than a "standard calibration".

THERMOCOUPLE PHYSICAL CHARACTERISTICS

Thermocouples are manufactured as plain wire, in many gages, with different types of insulation (fiberglass, teflon, etc). These wires can be used as is or installed in metal tubing of various materials.

Thermocouples are also manufactured with the thermocouple wire enclosed in a metal tube that is filled with an insulator (usually magnesium oxide). These types are available in diameters from 1/25 Inch to 3/8 inch. The tubing is available in many types of metals.

The thermocouple junction may be welded to the metal tube or to the point to be sense grounded junction, or it may be left isolated from the tube or measurement point.

Grounded junctions in metal tubes are less expensive to manufacture than isolated junctions or exposed isolated junctions.

SIGNAL CONDITIONING FOR THERMOCOUPLES

Thermocouples generate very small voltages and therefore require the signal conditioner to have a relatively expensive amplifier as the first stage.

Cold junction compensation requires several circuit components and adds to overall cost.

The input terminations must create an isothermal mass such that the two thermocouple wires and the cold junction sensor are at the same temperature (within the tolerance limits of accuracy). This restricts mechanical considerations for the packaging of the circuit.

No thermocouple has a linear output voltage versus temperature. If the signal conditioner includes a display or drives a temperature display, the signal conditioner must include linearization circuits if the temperature display is to be an accurate indicator of temperature.

Depending on the temperature range, display accuracy requirements, and thermocouple type, linearization may require a few simple straight line approximations to meet the requirement or a microprocessor that creates hundreds of straight line approximations to correct the response.

A thermocouple's linearity can be determined by evaluating its Seebeck coefficient. This coefficient is the sensitivity of the thermocouple expressed in microvolts/Deg C. A perfect thermocouple would have the same sensitivity at any temperature and a graph of its sensitivity would be a straight horizontal line.

Actual thermocouples have Seebeck coefficients that vary widely over their temperature range.

COST FACTORS OF A THERMOCOUPLE SIGNAL CONDITIONER

Before deciding to use a thermocouple sensor, evaluate an RTD as the sensor. They may offer cost savings.

Wilkerson Instrument Co.,Inc.
2915 Parkway Street
Lakeland, FL  33811
800-234-1343
www.wici.com