Resistance Temperature Detectors (RTD) Working Principle Thermo-resistive Temperature Measuring Devices A change in temperature causes the electrical resistance of a material to change. The resistance change is measured to infer the temperature change. There are two types of thermo-resistive measuring devices:
- Resistance temperature detectors (RTD) and
- BALCO – A sensor constructed using a BALCO wire is an annealed resistance alloy with a nominal composition of 70 percent nickel and 30 percent iron. A BALCO 500-ohm resistance element provides a relatively linear resistance variation from –40 to 250° The sensor is a low-mass device and responds quickly to changes in temperature. When 1000 ohms is measured across the BALCO element, the temperature is approximately 70°F. As the temperature increases, the resistance changes 2.2 ohms per 1°F. This is called a Temperature Coefficient of Resistance Curve (TCR Curve). In a BALCO, as the resistance has direct relationship with temperature i.e. as temperature increases, the resistance increases proportionally. The usual range of temperature measurement with BALCO is -40° to 240°F.
- Platinum – RTD sensors using platinum material exhibit linear response and stable over time. In some applications a short length of wire is used to provide a nominal resistance of 100 ohms. However, with a low resistance value, element self-heating and sensor lead wire resistance can effect the temperature indication. With a small amount of resistance change of the element, additional amplification must be used to increase the signal level. Platinum film sensor on an insulating base provides high resistance to the tune of 1000 ohms at 74° With this high resistance, the sensor is relatively immune to self-heating and responds quickly to changes in temperature. RTD elements of this type are common.
- RTD’s are commonly used in sensing air and liquid temperatures in pipes and ducts, and as room temperature sensors. The resistance of RTD elements varies as a function of temperature. Some elements exhibit large resistance changes, linear changes, or both over wide temperature ranges.
- Varying voltage across the sensor element determines the resistance of the sensor. The power supplied for this purpose can cause the element to heat slightly and can create an inaccuracy in the temperature measurement. Reducing supply current or by using elements with higher nominal resistance can minimize the self-heating effect.
- Some RTD element resistances are as low as 100 ohms. In these cases, the resistance of the lead wires connecting the RTD to the controller may add significantly to the total resistance of the connected RTD, and can create an error in the measurement of the temperature. For instance, a sensor placed 25 feet from the controller has a copper control wire of 25 x 2 = 50 feet. If a control wire has a DC resistance of 6.39 ohms/ft, the 50 feet of wire shall have a total dc resistance of 0.319 ohms. If the sensor is a 100-ohm platinum sensor with a temperature coefficient of 0.69 ohms per degree F, the 50 feet of wire will introduce an error of 0.46 degrees F. If the sensor is a 3000-ohm platinum sensor with a temperature coefficient of 4.8 ohms per degree F, the 50 feet of wire will introduce an error of 0.066 degrees F.