How to Minimize Thermocouple to Transmitter Measurement Errors – 8500926

Preventing Temperature Measurement Error in Thermocouple to Transmitter Connections

The nature of thermocouple (TC) circuits is that they’re prone to produce error when mating them to a measurement device; unless you have some insight into how these TCs and their components work. This paper reviews thermocouple behavior and outlines common problems encountered when connecting them to measure temperature; thus helping you minimize thermocouple measurement errors and get the best possible performance from your TC temperature measurement system. This whitepaper is primarily for industrial users of TCs and TC transmitters; however, much of the information can be extended to any thermocouple instrument.

A thermocouple is a thermo-electric device used to measure temperature. Nearly two-thirds of US temperature measurement makes use of thermocouples. Most industrial applications use a TC to remotely sense temperature, then transmit its signal using TC transmitters to monitor and control a process. The TC transmitter amplifies, isolates, and converts the low-level signals to another signal suitable for monitoring and retransmission. Unfortunately, the interface between the TC and mating instruments is widely misunderstood, which often leads to measurement error. The focus of this paper is on the connectivity aspects of thermocouples and TC transmitters; to reduce error that can be extended to include connection to any TC instruments.

How Thermocouples are Formed

Thermocouples are formed using a pair of different metal wires joined at one end; referred to as the hot junction. At the opposite open end of this pair of wires, is the cold or reference junction. At the cold end, a low-level voltage, proportional to the difference in temperature between the ends, can be measured. Of available temperature sensors, the TC has the widest application range; giving accurate measurement of extreme temperatures in harsh environments. To extract the hot end temperature from measured TC voltage related to the difference; it’s necessary to measure the temperature at the open end, to determine the temperature at the the hot junction on the other end.

Because the TC voltage is not linear with temperature; its conversion from voltage to temperature normally requires a complex polynomial specific to the thermocouple type. (Or optionally determined using a standard lookup table of TC voltage versus temperature.) Originally, TC voltage was tabulated while holding its reference junction in an ice bath corresponding to 0°C; hence the term cold junction. Modern thermocouples still reference themselves to a cold junction of 0°C; with their standard table tabulating output voltage over temperature, with respect to 0.000mV at 0°C. The use of standard TC tables and a simple correction can reduce this polynomial conversion from voltage to temperature; to a combination of measured and tabulated voltage. For more on this, download the full How to Minimize Thermocouple to Transmitter Measurement Errors at the link below.

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For more comprehensive information about thermocouples, please refer to whitepaper “The Basics of Temperature Measurement Using Thermocouples.”