THE BASICS OF THERMOCOUPLES

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THE BASICS OF THERMOCOUPLES

Silicon Online by Bob Perrin

Start ı A Tail of Two Metals ı Breaking the Loop ı Measuring Temperature ı Practical Matters ı Circuits ı Closing Time ı Sources and PDF

PRACTICAL MATTERS

Thermocouples are given a letter designation that indicates the materials they are fabricated from. This letter designation is called the thermocouples "type." Table 1 shows the common thermocouples available and their usable temperature ranges.

Type	Materials		Usable temperature range in degrees Celsius
Type	Positive side	Negative side	Usable temperature range in degrees Celsius
B	Pt + 30% Rh	Pt + 6% Rh	0 to 1820
E	Ni + 10% Cr Chromel*	Cu + 43% Ni Constantan	-270 to 1000
J	Fe Iron	Cu + 43% Ni Constantan	-210 to 1200
K	Ni + 10% Cr Chromel*	Ni + 2% Al + 2% Mn + 1% Si Alumel*	-270 to 1372
N	Ni + 14% Cr + 1.5% Si Nicrosil	Ni + 4.5% Si + 0.1% Mg Nisil	-270 to 1300
R	Pt + 13% Rh	Pt	-50 to 1768
S	Pt + 10% Rh	Pt	-50 to 1768
T	Cu Copper	Cu + 43% Ni Constantan	-270 to 400
Chromel and Alumel are trademarks of Hoskins Manufacturing Company Table 1ıThere are a wide variety of industry-standard alloy combinations that form standard thermocouples. The most commonly used are J, K, T, and E.*

Each thermocouple type will produce a different open-circuit voltage (Seebeck voltage) for a given set of temperature conditions. None of these devices are linear over a full range of temperatures. There are standard tables available that tabulate Seebeck voltages as a function of temperature [1]. There are also standard polynomial models available for thermocouples.

Thermocouples produce a small Seebeck voltage. For example, a type K thermocouple produces about 40 ıV per degree Celsius when both junctions are near room temperature. The most sensitive of the thermocouples, type E, produces about 60 ıV per degree Celsius when both junctions are near room temperature.

In many applications, the range of temperatures being measured is sufficiently small that the Seebeck voltage is assumed to be linear over the range of interest. This eliminates the need for lookup tables or polynomial computation in the system. Often the loss of absolute accuracy is negligible, but this tradeoff is one the design engineer must weigh carefully.

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