Which temperature sensor?

The principal types of sensors used for temperature monitoring and data acquisition are resistance temperature detectors (RTDs), thermocouples, integrated circuit temperature detectors (ICTDs), thermistors, and infrared sensors.

Important considerations when determining which type of temperature sensing component will work best for a given application are cost, how accurate the temperatures need to be, the environment in which the equipment will be deployed, and, especially, wiring.

ICTDs, for example, offer the simplest wiring scheme, as any twisted-pair cable will usually work effectively, and lengths can run thousands of feet with no loss of signal. RTDs, by comparison, are three- or four-wire. Also with RTDs, the gauge of the wire is much more important. Diameters must match, splices must be seamless, and there is greater susceptibility to noise.

Thermocouple-based applications, meanwhile, usually have the most stringent wiring requirements. Each thermocouple type utilises its own corresponding wire that has matching thermocouple material incorporated in its composition - this can be quite expensive, so short wiring runs are usually preferred.

RTDs

Resistance temperature detectors – RTDs – determine changes in the electrical resistance of materials in relation to temperature. RTDs deliver very precise readings (typically to 2–3 decimal places) and are manufactured in a variety of form factors. Though they are sometimes composed of nickel, copper, or other metals, historically, RTDs have been made of platinum - largely due to the fact that platinum’s resistance-temperature relationship is maintained in a very linear fashion across very broad temperature ranges. RTDs’ platinum composition also makes them somewhat expensive and unsuitable for applications involving temperatures above 660°C, as temperatures above this range compromise the inertness of the platinum and may cause it to become contaminated and deliver inaccurate readings.

Thermocouples

Thermocouples are bi-metal conductors that, when subjected to heat, generate voltages that correspond with given temperatures. They are the choice for applications involving intense heat. Like RTDs, thermocouples are most often used in industrial settings. They come in a variety of types (B, J, K, R, T, etc.) that offer varied sensitivities and temperature ranges. Thermocouple readings can be less accurate than those of RTDs.

ICTDs

Integrated circuit temperature detectors – ICTDs – are good, general, inexpensive all-purpose temperature sensors. These two-wire transistor-like devices work by applying a voltage (generally in the 5–30 V range), and the resulting current is linearly proportional to temperature.

Like RTDs, ICTDs are very noise immune, but they are more easily deployed than RTDs because they do not require resistance-measuring circuitry. ICTDs excel at simple, less industrial applications that require fairly accurate temperature sensing in the range of -50 to 100°C, such as HVAC, refrigeration, and room temperature monitoring.

Thermistors

Thermistors work by exhibiting resistance that varies with the temperature. In this and other ways, thermistors work very much like RTDs, but they differ in that thermistors use two-wire interconnections that are more sensitive to temperature. This makes thermistor readings somewhat less than precise. Additionally, the material is generally a ceramic or polymer, making them cheaper.

Thermistors are a good choice for high-volume temperature sensing applications in the range of -40 to 200°C where a certain amount of drift is acceptable.

Infrared sensors

Infrared sensors represent the newest instrumentation used for temperature sensing. Infrared radiation is used to remotely determine the temperature of objects through sensing the object’s electromagnetic radiation (sometimes called thermography or pyrometry).

Infrared sensing works well for detecting the temperature of fast-moving objects and also in highly volatile environments where the intense heat makes accessibility difficult if not impossible. They are used widely in many manufacturing processes, such as those for metals, glass, cement, ceramic semiconductors, plastics, paper, textiles, and coatings.

Products are available in the UK from System Devices in Cheshire.