Resistive temperature devices are also known as RTDs are used to produce a resistance that is a function of its current-voltage relationship. When the voltage is increased from zero, the device will decrease resistance until it reaches a specific point. At this point, the material undergoes a phase transition and becomes highly conductive for further increases in voltage.
This behavior makes the resistive temperature devices ideal for use as thermometers or thermal probes because they allow for easy measurements of absolute temperatures or small changes in temperature over time. To be able to use a resistive temperature device as an accurate thermometer, users must know what resistance vs. temperature curve it exhibits. It is also necessary for them to understand the nonlinearity and hysteresis error of their device.
The R – T Curve
An R – T curve shows a change in temperature affects the resistance across a material at the other end. If no current flows through a resistor with a positive temperature coefficient, its resistance will be equal to zero when it reaches its melting point. Below, this special value is called the null point; there is no decrease in resistance with any decreases in temperature below room temperature (25°C). In addition, if a material has a negative temperature coefficient, its resistance will be slightly higher than infinite when it approaches its freezing point.
The Two Standard R – T Curves
The two standard R – T curves are the straight line and the parabolic curve. There is no definite null point in some materials, or there is more than one null point on the curve. Equations that use temperature as an independent variable have been developed for each R – T curve type. These equations can then be used with any device to help determine how much resistance will change per degree Kelvin over a range of temperatures. The user should remember that these equations only provide a guide for calculations and not an exact answer without testing the specific device.
Amount Of Nonlinearity
A resistive temperature device’s nonlinearity is a deviation from a perfect calibration curve in which resistance increases linearly with the temperature being measured. The amount of nonlinearity is usually expressed as a percentage of full-scale error or as an absolute value for negative errors, such as when the actual value is lower than the minimum detectable limit.
Like accuracy, nonlinearity varies greatly across different models and manufacturers because it depends on many factors, including the heating element’s size, composition, and configuration. In general, nonlinearity has little effect on measurements that span only one or two decades below and above the null point. Still, it becomes significant when measuring conditions far away from this point.
Hysteresis Error
Hysteresis error refers to a difference between how much resistance changes when the device’s temperature is switched between two different setpoints. It can also indicate how much resistance will change with steady-state temperature changes at one level before and after another level has been reached. The hysteresis error of resistive temperature devices can be compensated by making minor adjustments to settings during calibration.
The resistive temperature devices (RTD) measure changes in electrical resistance as a function of the object’s temperature. Various materials are available depending on your application and environment, each with its unique advantages and disadvantages.