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Thermocouple Design

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Thermocouple Design | 22 March, 2007

Anyone ever design an external thermocouple that connected back to an enclosed PCB? If so, how did you make the interconnect or solder joint to the board? Did you change metals to a solder friendly material?

The problem we have is an external t-couple feeding back to an enclosed housing. Temperature variation can only be +/- 1 degree C in a 700 degree C environment. The enclosed board must be environmentally closed and able to be submerged in water without leaking. So I would like to design using a male � female connector design. If I go this route I introduce several different types f metal which will cause temperature variant.

Just wondering if anyone else traveled this path before?

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Thermocouple Design | 22 March, 2007

Don't know a thermal couple that will meet your "+/- 1 degree C in a 700 degree C" requirement. Look here:

Anyhow, have you considered screwing the leads to a terminal block?

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Thermocouple Design | 25 March, 2007


And pay attention to CJC or cold junction compensation.

Normally the CJC isn't much but with the accuracy you demand you had better plan for it.

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Thermocouple Design | 25 March, 2007

Couple of things.

Type T thermocouples are solder friendly. However usually a header with screw terminals poses no problem as the temperature differential is nil.

Getting thermocouple leads wet causes problems because two dissimilar metals + conductive fluid == battery. So if you use an external connector it must be water tight.

There are a number of manufacturers that make connector pins out of the same material as thermocouples. However you may not need to go that route. Copper is a good conductor of heat so the temperature differential across the pins will tend to be nil.

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Thermocouple Design | 5 April, 2007

I have seen a method used where the pins on the connector are of the same materials as the thermocouple and the connector is connected via single strand wires of the same material to the PCB however the wires were spot welded to bare copper pads to reduce the effects of thermal noise as the noise becomes more noticable when the number of junctions is increased. Seems overly excessive. Standard terminal blocks are usually quite adequate for most applications.

You did not mention whether the connector needs be in the 700 deg.C environment or not. If not then you could use pins of the same material and also house a temperature sensor for cold junction compensation.

You should also avoid changing materials in high temperature environments especially 700C as the reaction between 2 disimilar metals will occur at accelerated rates possibly causing premature failure of the junction. I have found that this is a common cause of failure in thermocouples besides broken wiring.

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