Yes it is the different contact material, no physical difference between TC types.
Something similar https://www.omega.com/en-us/search/?text=thermocouple+connector
Thanks - I had looked on their site but only found these PCB sockets:
https://assets.omega.com/pdf/connectors/thermocouple-and-rtd-connectors/PCC-OST-SMP.pdfThey have an optional bracket to hold a cold junction transistor sensor but they are only available to match specific thermocouple calibrations, eg, K, T or J types. Why? The whole point of a PCB socket is to connect the thermocouple leads to the copper circuit board forming the cold junction so why not do it in the connector?
However I did also find these connectors which can be used with all thermocouple types:
https://assets.omega.com/spec/PCC-SMD.pdfNot cheap at $30 for 20 individual contacts and they are only suitable for miniature plugs. They are made of Berylium copper, which makes sense, but they are Nickel plated! Nickel is terrible for thermal EMF (at least when connected to copper) compounding the problem of minimising errors due to small temperature differences between the two thermocouple leads to copper junctions. Ideally they would be gold plated without nickel but I guess that would be too expensive - I assume that it isn't considered important given the relatively low accuracy of the thermocouples themselves.
Surely there must be low cost universal PCB connectors/sockets which support standard and miniature plugs? Don't most digital thermometers better than a $3 TMC902 support most thermocouple types and hence need a universal socket?
If you look around that site, there is some design info that may help you. You want your chip as close as possible to where the connector solders to the board. Most chips have cold junction compensation to deal with the connector-solder-copper junctions, but if the chip is too far away, there may be several degrees difference between what the chip and connector, which messes up all that compensation. Using the wrong connectors will most likely do the same in most cases.
Agreed but it should be pretty simple to provide for clipping the temperature sensor directly to one of the contacts, or between them, in the socket, at no additional manufacturing cost (retail pricing is a totally different matter of course).