Author Topic: Thermocouple sockets  (Read 502 times)

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Offline splin

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Thermocouple sockets
« on: May 17, 2020, 09:07:48 pm »
What are the differences between PCB mount sockets for the different thermocouple types? Is it about the contact materials? I can see this is important for an extension lead but in a thermometer type instrument I'd assume the contacts would be copper as the signals have to terminate on the copper PCB at some point, but obviously the cold junction measurement would need to be within or close to the socket.

Are there physical differences between say J, K and T types which only allow thermocouples of the corresponding type to be plugged in? I'm assuming not as there are plenty of thermometers which allow you to use almost any thermocouple type.

Where can you buy them? Searching for 'universal thermocouple connector' shows this for example:

http://www.tmswebshop.co.uk/buy/universal-thermocouple-connectors.html

Universal in this case seems to refer to accepting flat or round pins, standard or miniature but they still come in specific versions for each thermocouple type.
 

Offline bill_c

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Re: Thermocouple sockets
« Reply #1 on: May 18, 2020, 02:49:11 am »
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
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.
 

Offline splin

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Re: Thermocouple sockets
« Reply #2 on: May 19, 2020, 01:35:05 am »
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.pdf

They 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.pdf

Not 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?

Quote
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).

 

Offline bill_c

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Re: Thermocouple sockets
« Reply #3 on: May 19, 2020, 03:12:50 am »
If you look for type U (universal) copper/copper or any RTD connector that shares the same physical spec should work
Example: https://www.omega.com/en-us/sensors-and-sensing-equipment/temperature/thermocouple-and-rtd-connectors/smpw-smp-hmp-hmpw/p/SMPW-U-M
For "Sensing Type", select "RTD, Thermistor, or other non-thermocouple". For "Thermocouple Calibration Type" select "U"

How about this https://www.tcdirect.co.uk/Default.aspx?level=2&department_id=280/95
Type Cu should do what you want as long as the plug/socket interface temperature is close to the IC temperature.

You may have to do a bit of searching for better prices, Omega sells alot of rebranded items, no clue who actually makes what.
 

Offline mbless

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Re: Thermocouple sockets
« Reply #4 on: May 20, 2020, 02:24:05 pm »
They 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?

You could, but you have to understand your system. I've used the Omega PCB mount connectors where they are stuck through the case. That means the connector's metal is exposed to the external environment where you may not know the temperature. If the connector and TC are different metals, then your CJ is affected by external and internal environments (depending on heat transfer rates and length of contacts), and now you aren't accurately compensating if the CJ temperature sensor is inside the case. Now if the connector and TC are the same metal, then it's not a problem since the CJ is at the PCB where the connector is soldered.

If you can measure and guarantee the connector's temperature, then the connector metal doesn't matter.
 


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