Basics of industrial temperature measurement 4

Once again welcome to the new publication on this newsletter Instrumentation revisited.

Temperature measurement using thermocouples has been discussed briefly in our previous articles. Industry also uses other means to measure temperature, and the other most commonly used temperature measuring sensor is now the topic of this article.

Yes you have guessed it correctly. It is the RTD.

The RTD (resistance temperature detector) or resistance thermometers, as known alternately is a method of determining the temperature of any process fluid through change of resistance proportional to change in the temperature. It is more reliable for industrial use. It is a bit costlier than thermocouples, but its use is as common now as thermocouples, and widely used as a temperature sensing device where more accurate measurement is required. It has one major advantage over thermocouples that the platinum wire sensor is less prone to corrosion and wear.

Resistance temperature measurement is basically done on the principle of Wheatstone bridge. Since it is a passive device, hence a little amount of current is used in the circuit mostly less than a couple of mA, so the self heating of the element can be eliminated.

Basically it can be a simple two wire, but being not accurate in measuring the temperature is less common. In some places where the accuracy is not a big concern it is used, where the transmitter is in close proximity of the RTD sensor itself.

Most common is the three wire configuration. Here, the source EMF is applied using a third wire at the resistive sensor itself. This compensates for the resistance of the lead wires which are not compensated for if only two wire system is used.

Four wire configuration is also used in cases where more accurate temperature measurement is required, and is more costlier option as more wiring is required. However it is the most accurate type among all RTD measuring loops.

RTD weather 3 wire or 4 wire have advantage over thermocouples that these are more accurate, linear and as we have seen in thermocouple with magnetic material is free from the curie effect which can compromise the temperature reading in thermocouples around this specific temperature.

RTD is built on several standardized temperature vs resistance curves, the DIN standard most popularly used which specifies for Pt-100 a base resistance of 100 ohms at 0°C, and a temperature coefficient of .00385 Ohm/Ohm/°C, though the tolerance may vary slightly based on its sub classification. Some RTD use a base standard of 200 ohms at 0°C. The material of construction is Platinum but, Nickle and copper are also used in some cases.

Installation of the RTD sensors in field is mostly similar to that of a thermocouple where the sensing element is installed into a thermowell to isolate it from the actual process. However bead type RTD sensors are also used which are mostly used with rotary machines without thermowell. Motor winding temperature measurement use RTD embedded in the windings. or in gas turbines and compressors, bearing temperature sensors also using RTD without thermowell. No matter how or where these are installed, the basic measurement principle remains the same.

RTD sensors are manufactured as thin film resistors, coiled or wire wound resistors. The overall sensor material and size depend on the type of usage. For very low temperature carbon film resistors are also used instead of normal platinum RTD. Whatever the case the principle of resistance thermometry remain same. RTD with Pt-100 measures between -150 °C to 900 °C.

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