Temperature

January 1, 2005 By: John R. Gyorki, Iotech, Inc. Sensors

Temperature is the most-measured physical property, and thermocouples do most of the measuring. But you can't just plug them in and count on accurate readings. Here's what to look for-and look out for.

Thermocouples (TCs) are probably the most widely used and least understood of all temperature measurement devices. When connected in pairs, TCs are simple and efficient sensors that output an extremely small DC voltage proportional to the temperature difference between the two junctions in a closed thermoelectric circuit (see Figure 1).

 Figure 1. A basic thermocouple measurement system requires two sensors, one for the environment under measurement and the other, a reference junction, normally held to 0°C (32°F). Type T is one of the dozen or more common thermocouples used in general-purpose applications. Made of copper and constantan metals, it typically operates from –270°C to 400°C (–454°F to 752°F).

One junction is normally held at a constant reference temperature, while the other is immersed in the environment to be measured. The operating principle, known as the Seebeck effect, depends on the unique value of thermal electromotive force (EMF) measured between the open ends of the leads and the junction of two dissimilar metals held at a specific temperature. The amount of voltage at the open ends of the sensor and the temperature range the device can measure depend on the Seebeck coefficient, which in turn depends on the chemical composition of the materials constituting the thermocouple wire. The Seebeck voltage is calculated from:

(1)

where:
eAB = Seebeck voltage
ΔeAB=small change in Seebeck voltage
ΔT = small change in temperature at
thermocouple junction
α= Seebeck coefficient

Thermocouple junctions alone do not generate voltages. The voltage or potential difference that develops at the output (open) end is a function of both the temperature of the junction T1 and the temperature of the open end T19. T19 must be held at a constant temperature, e.g., 0°C, to ensure that the open-end voltage changes in proportion to the temperature change in T1. In principle, a TC can be made from any two dissimilar metals such as nickel and iron. In practice, however, only a few TC types have become standard because their temperature coefficients are highly repeatable, they are rugged, and they output relatively large voltages. The most common thermocouple types are J, K, T, and E, followed by N28, N14, S, R, and B (see Figure 2). In theory, the junction temperature can be inferred from the Seebeck voltage by consulting standard tables. In practice, however, this voltage cannot be used directly because the thermocouple wire connection to the copper terminal at the measurement device itself constitutes a thermocouple junction (unless the TC lead is also copper) and outputs another EMF that must be compensated.

 Figure 2. Common Thermocouple Types

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