Resistance temperature detectors, popularly known as RTDs, are one of the conventional types of temperature sensors. Their working is based upon the “physical principle of the positive temperature coefficient of electrical resistance of metals”1 which means that as the temperature of a material increases, its electrical resistance will also increase in the direct proportion. In other words, RTDs have a property according to which their electrical resistance varies as a function of temperature. Resistance temperature detectors are basically wire wound and thin film devices. Materials used for construction of RTDs mainly include:
- Platinum: It is the most popular material amongst all and give very accurate results
However, the last two materials mentioned in the above list are rarely used. A typical RTD design is shown in the figure below:
Following are the key features associated with resistance temperature detectors:
- RTDs are basically active devices which need an electrical signal to generate a voltage drop across the sensor. This voltage drop is then determined with the help of a calibrated read-out device.
- Due to lead wires which are usually employed to connect the RTD to readout device, errors can take place in temperature measurement results. Particularly in remote temperature measurement locations where longer lead wires are used, chances of errors are more frequent.
- As per requirement, three wire and four-wire circuit designs are available for compensation of lead wire errors.
- Besides, the lead error can be limited by means of a temperature transmitter which is installed in proximity of the RTD element. These transmitters are competent enough to transform resistance measurement into analog or digital signals. These signals can then be communicated over long distances by wire or radio frequency (rf) to a data acquisition or control system.
- Unlike thermocouples, resistance temperature detectors operate in quite small temperature domain. Their temperature span ranges from about -200 °C to a maximum temperature of around 650 to 700 °C.
- Although, Copper and Nickel are the cheapest materials, they are considered unsuitable for construction of RTDs because of non-linearity problems (in case of Nickel) and wire oxidation problems (in case of Copper).
- The most suitable material for accurate temperature measurements is considered to be Platinum, since its temperature Coefficient of Resistance is nearly linear for its pure form.
Types of RTD
Different types of resistance temperature detectors i.e. RTDs available in the market are mentioned below:
- RTD Element: It is the most simple and commonly used form of RTD. It basically comprises of a core of ceramic or glass, around which a piece of wire is wrapped. Since RTD elements are very small and compact in size, they find their greatest use in areas where space is the major constraint. An exceptional type of RTD element called RTD surface element is constructed in such a thin size that it offers brilliant contact for temperature measurement in case of flat surfaces.
- RTD Probe: An RTD probe consists of an element which is protected against environmental effects by means of a special type of metallic tube, popularly known as sheath. The RTD probe also includes lead wires which gets terminated or connected at the measurement point. It is the most rugged and strong type of RTD. A typical RTD probe is shown in the figure below:
- Platinum RTD: These are the most widely used resistance temperature detectors. They meet the requirements of precision thermometry in the best possible way. They are also referred to as PRTs & PRT100s. These RTDs provide excellent linearity and stability. Besides, their repeatability and accuracy also happens to be best amongst all RTD’s. Their response is very fast i.e. a fraction of a second and almost linear over a broad span of temperatures.
- Thin Film RTD: These types of RTDs are made up of “a thin layer of a base metal embedded into a ceramic substrate and trimmed to produce the desired resistance value”.2 Typically, these RTD’s are constructed by applying a film of platinum on a substrate and then followed by encapsulation of both film and substrate. RTDs constructed via this method happen to be small in size and offer fast and accurate response. A thin film RTD is shown in the figure below:
Major benefits offered by resistance temperature detectors are mentioned below:
- RTDs prove to be most accurate temperature sensors. They provide very accurate measurements even over comparatively narrow temperature spans.
- They also give excellent stability and repeatability. They tend to provide stable output for longer periods of time.
- RTDs provide immunization against electrical noise. Therefore, they are considered ideal for temperature measurements in industrial environments. For example, around motors, generators and other high voltage equipments etc.
- RTDs are very simple to recalibrate.
As compared to thermocouples, the use of RTDs involve following limitations:
- Their overall temperature range is very small.
- Their application involves high initial cost.
- They are not rugged enough to be used in high vibration environments.