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THE BASICS OF THERMOCOUPLES

by Bob Perrin

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

A mathematician, a physicist, and an engineer were at lunch. The bartender asked the three gentlemen, "what is this pi I hear so much about?"

The mathematician replied, "pi is the ratio of a circleýs circumference to its diameter."

The physicist answered, "pi is 3.14159265359."

The engineer looked up, flatly stated, "Oh, piýs about three," then promptly went back to doodling on the back of his napkin.

The point is not that engineers are sloppy, careless, or socially inept. The point is that we are eminently practical. We are solvers of problems in a non-ideal world. This means we must be able to apply concepts to real problems and know when certain effects are negligible in our application.

For example, when designing first- or second-order filters, 3 is often a close enough approximation for pi, given the tolerance and temperature dependence of affordable components.

But, before we can run off and make gross approximations, we must understand the physical principles involved in the system weýre designing. One topic that seems to suffer from gross approximations without a firm understanding of the issues involved is temperature measurement with thermocouples.

Thermocouples are simple temperature sensors consisting of two wires made from dissimilar alloys. These devices are simple in construction and easy to use. But, like any electronic component, they require a certain amount of explanation. The intent of this paper is to present and explain how to use thermocouples and how to design thermocouple interfaces.

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