Thermal Measurement Hardware
- Thermistors (NTC): best for measurement at moderate temperatures, from -90C to 130C
- Thermocouples: best for measurements at very high temperatures, from 500C to 2320C
- RTDs: best for very low (-200C to -90C) and moderately high (130C to 500C) temperatures
Resistance Temperature Detector
aka ‘RTD’, 'Resistance Thermometer', 'Platinum Resistance Thermometer (PRT)
"Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The RTD element is made from a pure material whose resistance at various temperatures has been documented."
"They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability."
"A thermocouple is a device consisting of two different conductors (usually metal alloys) that produce a voltage, proportional to a temperature difference, between either end of the two conductors."
"...thermocouples are self powered and require no external form of excitation."
"A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors."
A thermistor may have positive thermal coefficient (PTC), in which case resistance increases with temperature, or a negative thermal coefficient (NTC).
"Many NTC thermistors are made from a pressed disc or cast chip of a semiconductor such as a sintered metal oxide. They work because raising the temperature of a semiconductor increases the number of electrons able to move about and carry charge - it promotes them into the conduction band. The more charge carriers that are available, the more current a material can conduct."
"Most PTC thermistors are of the "switching" type, which means that their resistance rises suddenly at a certain critical temperature."
RTDs vs Thermocouples
"If process temperatures are between -200 to 500 °C (-328 to 932 °F), an industrial RTD is the preferred option. Thermocouples have a range of -180 to 2,320 °C (-292 to 4,208 °F), so for temperatures above 500 °C (932 °F) they are the only contact temperature measurement device."
"If a tolerance of 2 °C is acceptable and the highest level of repeatability is not required, a thermocouple will serve. RTDs are capable of higher accuracy and can maintain stability for many years, while thermocouples can drift within the first few hours of use."
"If the process requires a very fast response to temperature changes—fractions of a second as opposed to seconds (e.g. 2.5 to 10 s)—then a thermocouple is the best choice."
RTDs vs Thermistors
"Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C."