RTD stands for the term "Resistant Temperature Detector" and refers to a temperature sensor that makes use of the interaction of ohmic resistance and temperature. Therefore, the sensor is also referred to as a resistance thermometer. Depending on the application, RTDs are available with different resistance elements.
Platinum chip temperature sensors are generally used in RTDs. From the user's point of view, platinum offers the great advantage of being very stable over the long term. A Pt100 sensor is usually used. The designation "Pt" stands for platinum and the number "100" for 100 Ω base resistance at 0 °C. The resistance of the Pt100 increases by about 0.38 Ω per Kelvin temperature increase. Pt1000 temperature sensors are also used in industrial applications. Here, the electrical characteristics are ten times greater (base resistance 1000 Ω and temperature coefficient about 3.8 Ω/ Kelvin).
A change in temperature has a direct effect on the electrical resistance of a metallic conductor and thus enables conclusions to be drawn about the temperature. The temperature coefficient or temperature coefficient of the platinum sensors (approx. 0.38 %/Kelvin) is based on the physical properties of platinum; the basic resistances result from specifications. The characteristic curve is fixed in the standard DIN EN 60751, so that the application of the RTDs is relatively simple. The RTD is connected to an evaluation unit and the field device determines the ohmic resistance. Usually, linearizations such as Pt100 and Pt1000 are available in the field devices, after which the device determines the sensor temperature from the ohmic resistance. For more information on the design and function of resistance thermometers, watch the video.
There is a multitude of different RTD temperature probes. The most common are RTD temperature probes with a terminal head or with a connection cable.
A RTD temperature probe with a terminal head has a modular design: It consists of the measuring insert, the thermowell, the terminal head and the connection socket inside it, and possibly flanges or compression fittings. Only that part of the RTD temperature probe is designated as the temperature sensor, which is mounted on which is directly affected by the measured variable.
For RTD temperature probes with connection cable, a measuring insert and the terminal head are not required. The temperature sensor is directly connected to the connection cable and inserted into the thermowell. For strain relief, the end of the thermowell is rolled or pressed in several times (protection class IP65). The space between the thermowell and the temperature sensor is usually filled with a thermally conductive material to improve thermal contact with the measured medium. The maximum measuring temperature is primarily determined by the temperature resistance of the sheath and insulation material of the connecting cable.
The connection head contains a connection socket for attaching the connection cable. The thermometer is fixed by a flange. Thermometers of this type allow measurement of up to 600 °C and are frequently used in furnace construction.
Screw-in thermometers allow the pressure-tight termination of the process. In the case of thermometers with connecting cable, the maximum temperature is limited by the cable. Maximum temperatures of about 400 °C can be measured.
Surface probes have the advantage that they do not require a process connection. They measure the temperature of a surface and thus allow conclusions to be drawn about the medium temperature in a pipe system or tank. However, precision measurements are not possible with them.
In order to allow easy mounting/dismounting of screw-in thermometers, it is often useful to obtain them with a connector plug. The connection systems shown below are frequently used.
Measuring inserts are ready-made units consisting of temperature sensor and connection base, whereby the temperature sensor is placed in an insert tube of 6 or 8mm diameter made of SnBz6 according to DIN 17 681 (up to 300°C) or nickel. It is inserted into the actual protection tube, which is often made of stainless steel.
The electrical resistance of RTD temperature probes changes depending on the temperature. To detect the output signal, the voltage drop caused by a constant measuring current is measured.
There are 3 connection types: two-wire, three-wire and four-wire.
With the two-wire technique, the evaluation electronics and temperature sensor are connected with a two-wire cable. For the three-wire technique, an additional cable is led to a contact of the RTD temperature probe. Two measuring circuits are thus formed, one of which is used as a reference. The four-wire technique offers the most optimal connection possibility for RTD temperature probes. The measurement result is not affected by the lead resistances or their temperature-dependent fluctuations.
With the three-wire connection, an additional wire connects the resistance sensor to the evaluation unit. The evaluation unit measures the voltage drop at the resistance sensor and the connecting leads (UM). With the help of the third conductor, the evaluation unit further determines the voltage drop at one conductor (UL½ ). The double amount of this voltage is subtracted from UM and thus the voltage drop at the resistance sensor is determined. If all wires have the same resistance, no error results from the line resistances and the resistance of the sensor is determined without error. The three-wire connection is sufficient for most applications.
The fourth wire is used to determine the exact voltage at the resistance sensor in the four-wire connection.
Like any other electrical conductor, the line between the temperature probe and the evaluation electronics has a resistance that is connected in series with the temperature sensor. This means that the two resistances add up, resulting in a systematically higher temperature reading. At greater distances, the line resistance can amount to several ohms and cause a considerable falsification of the measured value. In order to avoid the problems of two-wire technology described above and still be able to dispense with multi-wire cables, two-wire transmitters are used: The transmitter converts the sensor signal into a standardized, temperature-linear current signal of 4 ... 20mA. The transmitter is also supplied via the two connection cables, using a quiescent current of 4 mA. Due to the raised zero point, this is also referred to as "life zero". The two-wire transmitter also offers the advantage of significantly reducing the sensitivity to interference by amplifying the signal. There are two designs for the placement of the transmitter. Since the distance of the unamplified signal should be kept as short as possible to reduce the susceptibility of the signal to interference, it can be mounted directly in the thermometer in its connection head. However, this optimum solution is sometimes contradicted by design conditions or the fact that the transmitter may be difficult to reach in the event of a fault. In this case, a transmitter for rail mounting in the switch cabinet is used. However, the advantage of better access is bought by a longer distance that the unamplified signal has to cover.
With the three-wire circuit, the lead resistance can be compensated both in its magnitude and in its temperature dependence. However, the prerequisites for all three cores are identical properties and the same temperatures to which they are exposed. Since this is true with sufficient accuracy in most cases, three-wire technology is the most common today. A line compensation is not necessary.