The resistance of a metal changes with temperature with some metals, such as platinum, exhibiting a large change. This phenomenon can be utilised to accurately measure temperature. The most common form of achieving this is to wind a platinum wire round a former or a platinum 'wire' is deposited as a thin film on a ceramic substrate. These resistance devices have a Positive Temperature Coefficient (PTC) where an increase in temperature increases resistance. 'Standard' sensors are made to read a given resistance at 0°C, values of which are commonly 100 Ω, 500 Ω, 1000 Ω designated as Pt100, Pt500 and Pt1000 respectively. Stability and repeatability are very good. Platinum Resistance Temperature Detectors (PRTD) have a measuring range between -200°C and 1000°C.
Mechanical aspects
Mechanical aspects of RTD design and use have to be carefully considered to prevent damage or inaccuracies due to differential expansion of the materials used in the sensor construction, the platinum wire being very fine, and fragility. Thin film devices on a substrate with no protection other than a light epoxy covering where the lead out wires are bonded offer a very fast response time but are fragile whereas wire wound on a former offers more mechanical protection but tends to be larger. Physical size of both device and application will determine which is used.
Any temperature sensor fitted in a tube or other protective device will exhibit a time lag in responding to temperature changes. The sensor tip must be located as close as possible to the point being measured noting that the enclosure and mounting will tend to contribute to the device measurement inaccuracy - it is important to remember the temperature being measured is that of the sensor element.
Connections to RTDs are often welded due to the high temperatures being measured. Welding is usually performed using a Capacitor Discharge Welder - energy stored in a capacitor is discharged across the junction of the wires. Resistance and laser welding can also be used and solder for lower temperature devices.
Electrical aspects
PRTDs are not as susceptible to electrical pickupas thermocouples but it is good practice to twist wires and route away from possible interference. In order to generate an output and be able to measure temperature an electric current has to pass through the sensor. If this current is too high it will heat the sensor thus changing its temperature and producing an incorrect reading. In practice a current of 0.1 to 1mA is used depending on the resistance, the higher the sensor resistance the lower the current. PRTDs have a very linear response over a wide temperature range.
Lead resistance
2 wire. In its simplest form the sensor is connected to two lead out wires so conductor resistance may need to be allowed for when in use.
3 wire. By connecting an extra wire to one side of the sensor and using this wire for measurement the resistance of the two current carrying wires is compensated for resulting in a more accurate measurement.
4 wire. This configuration is used for obtaining the highest accuracy of measurement. Two wires carry the current and the other two are used for measuring the voltage drop across the sensor. Wire resistance for both current and voltage is almost eliminated.
The above is offered in good faith but Watch Hill Ltd cannot be held responsible for errors or values shown.
