Data Acquisition Web

By sensor type:

RTD’s are more stable than thermocouples. On the other hand, RTD’s temperature range is not as broad: RTD’s operate from about -250 to 850°C, whereas thermocouples range from about -270 to 2,300°C. Thermistors have a more restrictive span, being commonly used between -40 and 150°C, but offer high accuracy in that range.

Infrared sensors, though relatively expensive, are appropriate when the temperatures are extremely high. They are available for up to 3,000°C (5,400°F), far exceeding the range of thermocouples or other contact devices.

The infrared approach is also attractive when one does not wish to make contact with the surface whose temperature is to be measured. Thus, fragile or wet surfaces, such as painted surfaces coming out of a drying oven, can be monitored in this way. Substances that are chemically reactive or electrically noisy are ideal candidates for infrared measurement. The approach is likewise advantageous in measuring temperature of very large surfaces, such as walls, that would require a large array of thermocouples or RTD’s for measurement.

Bimetallic and liquid expansion devices both have mechanical outputs and are difficult to insert into electronic measuring and control systems. Bimetals are mainly used as on/off temperature limit switches an seldom for direct temperature reading. Liquid expansion devices are used for direct reading thermometers, but also are common in temperature limit switching. Hysteresis is related to limit switching temperature control.

Change-of-state temperature sensors consist of labels, pellets, crayons, lacquers or liquid crystals whose appearance changes when a certain temperature is reached. The change of state is usually irreversible.

By sensor characteristics:

Temperature resolution is the smallest temperature change that can be detected. The precision (or reproducibility or stability) is a measure of how closely the measured values are grouped. Accuracy is indicated by the difference between measured and true values of a parameter. The accuracy of a single measurement can be no better than the resolution, but it is degraded by lack of calibration and measurement errors.

The achievable resolution depends on 1) the sensor characteristics and 2) the measurement system resolution.

The accuracy of a temperature measurement can be evaluated using standard error analysis.

Sensibility is the change of sensor’s physical characteristic per ºC

Other considerations:

Sensor size, thermal mass, stability, response time, mechanical and enviromental resistance, interchangeability, measurement system simplicity, linearity, cost, noise and magnetic field effects, and resistance to ionizing radiation, among others.