Temperature measurement

RTD temperature probe

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.

How is a resistance thermometer constructed?

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.

What is a measuring insert?

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.

How can resistance thermometers be connected?

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.

Why can incorrect measured values occur with the two-wire technology?

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.

What is the benefit of the third line in three-wire technology?

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.

How does the four-wire technology work?

The thermometer is supplied with the measuring current via the supply lines. The voltage drop at the measuring resistor is tapped via the measuring leads. If the input resistance of the downstream electronics is many times higher than the lead resistance, this is negligible. The voltage drop determined in this way is then independent of the properties of the supply lines.

The connection instructions required for installation are shown in the following overview:

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