General about Pt100/RTD
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 diameter 3, 4.5 and 6 mm is used for Pt100 probes. With the MIMS-cable the temperature limit is increased to about 600 °C.
With the MIMS-cable the temperature limit is increased to about 600 °C.
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.
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.
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.
Especially for food and pharmaceutical industry processes hygienic sensors are crucial. Under Hygienic Pentronic gathers sensors which are approved by the EHEDG and the American 3A requirements.
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.
Pentronic keeps stock of Pt100-sensors for measurement of air temperature. These sensors are protected by a plastic housing for indoor use and in a metal housing for outdoor use. There is also versions with connection head for installation with flange in e.g. ventilation systems. All air temperature sensors are fitted with a radiation protection at the sensor tip.
Tolerances as per IEC 60751
Industrial Pt100 sensors are devided into four tolerance classes under IEC 60751: AA, A, B and C. (Previously, there were only two classes A and B, the older expressions DIN A and DIN B are also sometimes used.) The new standard makes a difference between wire wound resistors and film resistors. Experience shows that film resistors can't handle as wide temperature ranges as wire wound resistors under the different tolerance classes.
Most manufacturers employ closer tolerances, but
these apply to just one temperature: 0 °C. This means,
for instance, that a tolerance of ± 0.03 °C applying to
“1/10 DIN” (Class B/10) is valid only at 0 °C. For other temperatures, the tolerances will follow the respective slope of curve for Class A or B, depending on which materials have been used. See diagramme to the right.
Thus, the Class-B value cannot be divided by ten: it is impossible to achieve this tolerance curve using the IEC platinum alloy.
As a standard, Pentronic uses wire-wound or thin-film elements according to class A. During the final inspection of finished temperature sensors, we sometimes check against class B, eg. if the temperature sensor is two-wire connected.
Tighter tolerances in other temperatures can only be achieved with regular calibration that determines the characteristics of the individual temperature sensor.
This is because the platinum quality, as prescribed in IEC 60751, is down-alloyed with bl. a palladium to match the traditional DIN standard. The alloy provides such deviations from the ideal curve that one must have a safety margin corresponding to the slope in curve A or in the worst case the slope in curve B.
Some Japanese and American standards provide for a cleaner platinum alloy that provides narrower tolerance over a larger temperature range. However, these sensors provide another output signal that does not fit European instruments. For competitive reasons, US and Japanese manufacturers typically include the IEC's Pt100 scale in their temperature indicators.
Temperature-resistance relationship as per IEC 60751
IEC 60751 describes resistance as a function of the temperature of a Pt100 which relationship was established in 1983.
Pt100 sensors can usually be used to measure temperatures up to approximately 250 °C. For higher temperatures, protective tubes, eg, with mineral-oxide insulation, are required.
There is little point in measuring temperatures in excess of about 600 C using a Pt100. This is because the resistance of the platinum wire is shunted unpredictably by the surrounding material.
Special detectors are available for measuring temperatures up to approximately 700 °C. In this temperature range, type-N and type-K thermocouples constitute a good alternative.