industrial//instrumentation//temperature sensor

A temperature sensor is an instrument that turns the temperature of a body into an electrical quantity, and in a plant it is the most common measurement there is: reactor and oven loops, bearing and winding protection, cold chains. Two principles cover most industrial use. A **thermocouple** is a junction of two different metals that produces a small voltage depending on the temperature difference between that junction and the other end of the wires (a type K gives about 41 µV per degree). An **RTD** (resistance temperature detector) is a platinum resistor whose resistance rises with temperature; the Pt100 has 100 Ω at 0 °C and gains about 0.385 Ω per degree. A **thermistor** is a cheap semiconductor resistor with a large, strongly nonlinear response, common in electronics and battery packs.

A temperature sensor is an instrument that turns the temperature of a body into an electrical quantity, and in a plant it is the most common measurement there is: reactor and oven loops, bearing and winding protection, cold chains. Two principles cover most industrial use. A thermocouple is a junction of two different metals that produces a small voltage depending on the temperature difference between that junction and the other end of the wires (a type K gives about 41 µV per degree). An RTD (resistance temperature detector) is a platinum resistor whose resistance rises with temperature; the Pt100 has 100 Ω at 0 °C and gains about 0.385 Ω per degree. A thermistor is a cheap semiconductor resistor with a large, strongly nonlinear response, common in electronics and battery packs.

The choice is range against accuracy. A thermocouple is rugged, fast for its size and works far beyond 1000 °C, but its microvolts need careful amplification, and because it measures a difference, the temperature of the reference end must be known: cold-junction compensation measures the terminal block with a second sensor and adds it back. An RTD is more accurate and stable over a narrower range, at the price of passing a current through it, which warms it a little (self-heating) and biases the reading.

Both are slow. The sensor is a mass that has to warm up, usually inside a protective sleeve in the pipe, so it behaves as a first-order system with a time constant of seconds to tens of seconds, and it is sampled at 1 to 10 Hz. A temperature loop sees that lag as part of the process.

The microvolt signal is fragile over distance, which is why it is usually converted near the sensor by a head-mounted transmitter and sent over a 4-20 mA loop, with the cold-junction compensation done in that transmitter (measurement chain).

Its typical errors are the ones of the sensor error model: an offset from a wrong compensation or a tired thermocouple, a reading that lags a fast change, and a sensor in the wrong place, which measures faithfully the temperature of somewhere else.