Maximal allowed error for PT resistors class A/B/C and Thermocouple K class 1 and 2.
As the class A is defined only from -100 to +300°C and B from -200 to +600°C, the allowed error has steps at these values to the next lower class.
Thermocouples Type K have no defined error below -40°C according DIN / ISO standards. For Class 2 a typical specification from a manufacturer is given.
The most common way for temperature measurement in vacuum systems is the use of thermocouples or PT100 resistors. Here a short summary is given for the pros and contras for these two ways.
Thermocouples use the Seebeck effect: If you join two materials, a voltage is created. This voltage can be measured and, by looking it up in a table, the temperature is defined. (it's a little more complex in reality, you need a fixed known reference point etc, but this is all included in the actual displays and controllers). So the thermocouple creates the signal / voltage, which is measured.
A resistor works in a different way. Here the resistivity changes with temperature, so to measure it, you need an external power supply to do this. All electrical conductors change the resistivity with temperature. For precise measurements Platinum resistors are used. The reason is, that Platinum can be manufactured with very poor impurities and the material is inert to a lot of gasses and chemicals. (Other sensors are NTC / PTC resistors. The tolerances on these items are significantly bigger).
So the PT resistor has a very high stability against long term changes. The new 243-PT100-C1 resistor combines high stability, high accuracy and small size with an expanded usable temperature range up to 750°C.
As usual, each method had advantages and disadvantages. A thermocouple is very simple (just two wires) and so installation and mounting is easy. The connection to a display is sometimes complex. Each junction must be done with the right material. If extension grade cables are used, the connections must be at the same temperature. Otherwise an error of some Kelvin is possible.
For accurate measurements, a PT resistor has much higher stability and lower errors over a wide range. No hysteresis effects appear, no “Zero” is required. Allectra recommends to use 3- or 4-wire connection to compensate the cable resistivity. The limit for PT100 are high temperatures. Above 750°C type R and S are the most accurate alternatives.
Fixing a PT100 is more complex than fixing a thermocouple. For god thermal contact, Allectra offers Liquid Metal (317-TCL-1 / 317-TCP-20x20 / 317-TCP-38x38) as well as a clamp.
Allectra GmbH was founded in 2002 in Berlin and now manufactures and supplies a full range of HV (high vacuum) and UHV (ultra high vacuum) components including custom products all over the world. Allectra Limited was started in 2003 in Sussex to promote the products on a worldwide basis. The Allectra sales and service network is also supported by offices in France and Italy. Today, global customers include universities, major Synchrotron laboratories and High Technology companies.