1 January 2002

Temperature measurement a matter of electrons

By Nicholas Sheble

At the most fundamental level, the concept of temperature is about motion.

An atom—made up of a nucleus orbited by a cloud of electrons, or a molecule, an aggregate of agreeable atoms of any substance—is continually in motion. We know, however, that there must be some attractive force that keeps them together, or nothing would ever exist that we could see or hold.

It turns out that this attractive force is electrical in nature. However, if the molecules come too close together, they repel each other due to the same-charge electrons whirring about their perimeters.

Thus, the molecules maintain an equilibrium distance from each other—not too far away and not too close.

In a solid material, the atoms or molecules hold together in a more or less fixed position. They are still in motion, though, as they vibrate around their fixed positions. In a liquid, the atoms move more rapidly such that they pass over and mingle with one another. In a gas, the attractive forces are so weak and the speeds so high that the molecules do not stay close together at all and will fill up any container in which they reside or just disperse completely.

Notice that the molecules' physical configuration changes as their phases move from solid to liquid to gas. These phase changes coincide with change in temperature from a lower degree to a higher one.

There are many kinds of thermometers, but their operation always depends on some property of matter that changes with temperature.

The earliest thermometers relied on the expansion of a material with an increase in temperature. These include liquid in glass, where the liquid moves up and down a tube or bimetallic strips that bend and contract with temperature change. Industry often requires even more precise thermometers that leverage electrical properties that vary with temperature.

These are thermocouples, resistance temperature detectors (RTDs), thermistors, and semiconductors. They employ the properties electrical resistance or voltage to measure temperature.

INDUSTRY CALLS ROLL

The resistance of metals increases with temperature. Intuitively, this makes sense. At higher temperatures, the atoms are moving around at a great rate and in a helter-skelter fashion. At lower temperatures, the atoms and molecules are systematically organized. Electron flow (electricity) slows as their disheveled states increase and their temperature rises.

That is, as temperature goes up, resistance goes up, and the flow of electricity goes down. This relationship is measurable, and over some temperature spans the change in resistance translates directly to temperature change.

RTDs contain sensing elements made of platinum, nickel copper, or nickel/iron alloy. The resistance change in RTDs is stable and repeatable over time. By applying a constant current or voltage to the sensor, one can determine the temperature by measuring the voltage or amperage that flows through the sensor.

Thermistors also correlate temperature to resistance. Mixtures of metal oxides comprise a thermistor, and the oxides provide sensitivity to temperature change that is more acute than that of an RTD. A thermistor, for instance, alters resistance (many ohms)/(degree of temperature change) compared with (tenths of ohm)/(degree of temperature change) for an RTD.

Semiconductors' resistance also varies in a repeatable way with temperature changes. They have a further advantage in that they allow for signal conditioning within the sensor package. If the user wants a digital output from the sensor, a semiconductor-sensing element provides it.

Thermocouples contain two electrical conductors made of different materials that join at one end and are in contact with the object of interest. The other ends of the conducting materials connect to a cold junction as a reference point. The different temperatures and the different conducting materials generate a voltage. The voltage varies with the temperature of the measured material and its difference in temperature from the reference (T).

Thermocouples come with myriad materials suitable for various usage conditions and temperature ranges. IT

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