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IEC 60751-2008 tolerances RTD

IEC 60751-2008 explanation

RTD 2, 3 and 4 wire connection

Explanations

General about Pt100 sensors

Pt100 probes mainly are designed in two ways. Traditionally the detector is built into a protecton tube closed at the sensor ending. Via transition leads the joints to the extension cable are made in a transition junction. Here the maximum probe temperature is limited by the transition leads (normally up to approx. 250 °C) and the type of detector, see below. Connector as tube termination occurs but is not very frequently used as no special connectors for Pt100 sensors have been standardized.

An other way is to use metal sheathed cable (MIMS) to encapsulate the Pt100 detector. Such cable is available with e g nickel leads and normally the outer diameters 4.5 and 6 mm are used for Pt100 probes. Even 3 mm diameter probes can be built in certain cases. With the MIMS-cable the temperature limit is increased to about 600 °C. Above that isolation leakages can be disturbing to the measurement.

Pt100 detectors occur in two main groups, thinfilm detectors and wirewound detectors. Production of film detectors is highly automated. Due to the film techniques the standard IEC 60751 class A can only be fulfilled up to 350 °C. On the other hand at lower temperatures the film detector is a good alternative in most cases.

Wirewound detectors with "20% fixed platinum wire" have shown to be the most stable detector for industrial purposes. They contain the IEC class A over the entire range, -200 to 650 °C.

With cable

Independently of the probe being a protection tube or a metal sheathed cable (MIMS) the transition junction fixes the probe leads to the terminating cable. The junction is similar to the one for thermocouples, made in stainless steel, and cannot be parted. The cable most often has four leads, but in special cases three and two leads may occur. Four leads is preferably as it makes the measurement practically independent of cable defects. See The effect of 2, 3 or 4 wire connection using Pt100.

Process sensors

Process sensors is a common name for sensors intended for measurements in tubes, pipes and tanks. Often they have an outer housing with a terminal head in which a replaceable insertion probe is positioned. The signal is available from the terminal block of the probe or from a transmitter mounted in the place of the terminal block. Also there is a third way with flying leads to be able to make the terminal choice later. Different models of terminal heads are shown in pdf datasheets under this button.

High pressure sensors for flows of high velocities are available, too. They are using conical thermowells in different materials to suit frequent needs. Most process sensors for tubes and tanks are based on expired DIN standards, still customers want to use them.

Within the group Process sensors there are many models which are listed when you click the Process sensor button. More information about the models is available from the pdf data sheets which are also listed with the models.

Hygienic

Especially for food and pharmaceutical industry processes hygienic sensors are crucial. Under Hygienic Pentronic gathers sensors which are approved by the EHEDG requirements.

Surface

Pentronic keeps a number of sensor models in stock for temperature measurement on surfaces. There is a group of sensors where the detector has been built into a brass body with one or more sides of the body suited for different pipe diameters including flat surface. The brass bodies will normally be fastened by clamping bands.

Also sensor housings in two halves to be clamped around tubes are available especially for food and pharmaceutical industries where no penetration is allowed on inspected and approved processes. This model needs no extra insulation to make good measurements from the outside and it is easy to position along the tubing or moving to a separate tube.