RTDs the Golden Standard

Over the years laboratories and industrial processes have measured temperature using Resistance Temperature Detectors or RTDs.  A temperature sensor that contains a resistor that changes its resistance value as its temperature changes.  RTDs have developed a reputation for precision, test-retest reliability, and constancy.

Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core.  The element is usually delicate and is placed inside a sheathed probe for protection. The RTD element is made from a pure material whose resistance at various temperatures has been documented. 

The material has a foreseeable change in resistance as the temperature changes, with this foreseeable change that is used to determine temperature.  Sir Humphrey Davy discovered the resistivity of metals showed a noticeable temperature dependence. Then fifty years later, Sir William Siemens started using platinum as the element in a resistance thermometer.

The Construction of an RTD Probe is an assembly composed of an element, a sheath, a lead wire, and a termination.  Once the element is selected, the wiring and packaging requirements need to be determined.  There are several ways to wire the sensors. 

To measure temperature, the RTD element must be connected to wireless temperature monitoring or control equipment.  The temperature measurement is based on the element resistance any other resistance like lead wire resistance or connections added to the circuit will result in potential error. 

Except for the 2-wire configuration, all other wiring arrangements allow the monitoring or control equipment to factor out the unwanted lead wire resistance and other resistances that occur in the circuit.

Using a 3-wire construction which is the most common design, found in industrial process and monitoring applications. The lead wire resistance is factored out if all the lead wires have the same resistance.

When specifying the lead wire materials, attention should be taken to select the right lead wires for the temperature and environment the sensor will be exposed to in service. When selecting lead wires, the temperature is by far the primary consideration, however, physical properties such as abrasion resistance and water submersion characteristics can also be important.

RTD sensors are used more than thermocouple or thermistor sensors because each type temperature sensor has a set of conditions for which it is best suited. RTDs offer several advantages with a wide temperature range with approximately -200 to 850°C, Precision with better than thermocouples, good interchangeability and Long-term stability. With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes.

When made using metals such as platinum, they are highly stable and are not affected by corrosion or oxidation. Other materials such as nickel, copper, and nickel-iron alloy have also been used for RTDs.  These materials are not commonly used since they have lower temperature capabilities and are not as stable or repeatable as platinum. 

Laboratories, pharmacies, warehouses, hospitals and industrial processes have measured temperature using Resistance Temperature Detectors over the year and RTDs have developed a reputation for precision, test-retest reliability, and constancy.