Part 1:
How thermocouples work.
TLDR: they measure the temperature difference between the hot end and the cold end. For this to be accurate attention must be paid to both ends.
Long Version:
Thermocouples exploit the way the potential difference generated at a junction between two metal wires changes with temperature. For type K thermocouples the two metals are two nickel alloys, one doped with chromium (chromel) and the other with aluminium (alumel).
All materials have a contact potential which is related to the work function, the energy required to remove a charged particle from the surface. If you join two dissimilar metals the difference in contact potentials becomes the junction potential. Since the work functions depend on temperature, the junction potential also changes with temperature and we can measure the temperature by measuring the junction potential and doing the appropriate calculation.
Alas it’s not quite so simple, the other ends of the two pieces of metal (typically wires) also have contact potentials so this needs to be included in our calculation.
The actual welded junction between the two wires is the “hot” junction, the other ends of the two wires are collectively the “cold” junction. The output is the potential difference between the junctions which is a function of the temperature difference between the two junctions. If you know the potential difference and one temperature you can calculate the other temperature.
This is an important point: to get accurate results we must know the cold junction temperature accurately.
The classical way to perform this measurement is to have the cold junction at a constant 0oC so the potential difference between the two wires represents the temperature difference between the hot junction and 0oC. This is both arithmetically convenient and corresponds to the most commonly used frigorific mixture, 50: 50 ice in water.

Note that in the pic the cold junction is where the thermocouple alloys are connected to the wires to the receiver (shown as copper here). This is an important point and is an inevitable consequence of how contact potential works. For brevity (Huh!) I’ve left out the explanation of why this is; if you want it just ask.
In practice nobody uses a constant temperature cold junction outside a lab environment, the usual way around it is to use a thermocouple receiver which measures the cold junction temperature and adds a bias voltage that compensates for the difference between this and 0oC. A typical example is the Maxim MAX31855 which simply uses the temperature of the chip itself,* so the datasheet
https://datasheets.maximintegrated.c...s/MAX31855.pdf
calls out maintaining the cold junction at the same temperature as the chip, preferably by ensuring they are thermally coupled.
A typical circuit board is constructed of a fibreglass material known as FR4 with 2 layers of 1 oz copper. PCB mount screw terminals and several other connectors such as 4mm banana plugs are typically made from steel. FR4 itself is almost as good a thermal insulator as it is an electrical insulator, it has a U value of 0.25 W m-1K-1. Copper is about 380 and ordinary steel is about 40, so steel is about 1/10th as good at conducting heat as copper and FR4 is about 1 / 1500th.
The upshot of this is that relying on conduction through these materials will not allow the chip to read the cold junction correctly: in a typical scenario the thermocouple amplifier will read high by a few degrees in the mildest of conditions and can be off by over 20 degrees in more severe conditions.
*I’m guessing it uses a simple temperature dependent bandgap reference in the chip architecture, this constrains the measurement to being that of the actual die temperature since the bandgap reference will be etched in the die.
TBC, I will explain how to design the cold junction connector so it works optimally.
How thermocouples work.
TLDR: they measure the temperature difference between the hot end and the cold end. For this to be accurate attention must be paid to both ends.
Long Version:
Thermocouples exploit the way the potential difference generated at a junction between two metal wires changes with temperature. For type K thermocouples the two metals are two nickel alloys, one doped with chromium (chromel) and the other with aluminium (alumel).
All materials have a contact potential which is related to the work function, the energy required to remove a charged particle from the surface. If you join two dissimilar metals the difference in contact potentials becomes the junction potential. Since the work functions depend on temperature, the junction potential also changes with temperature and we can measure the temperature by measuring the junction potential and doing the appropriate calculation.
Alas it’s not quite so simple, the other ends of the two pieces of metal (typically wires) also have contact potentials so this needs to be included in our calculation.
The actual welded junction between the two wires is the “hot” junction, the other ends of the two wires are collectively the “cold” junction. The output is the potential difference between the junctions which is a function of the temperature difference between the two junctions. If you know the potential difference and one temperature you can calculate the other temperature.
This is an important point: to get accurate results we must know the cold junction temperature accurately.
The classical way to perform this measurement is to have the cold junction at a constant 0oC so the potential difference between the two wires represents the temperature difference between the hot junction and 0oC. This is both arithmetically convenient and corresponds to the most commonly used frigorific mixture, 50: 50 ice in water.

Note that in the pic the cold junction is where the thermocouple alloys are connected to the wires to the receiver (shown as copper here). This is an important point and is an inevitable consequence of how contact potential works. For brevity (Huh!) I’ve left out the explanation of why this is; if you want it just ask.
In practice nobody uses a constant temperature cold junction outside a lab environment, the usual way around it is to use a thermocouple receiver which measures the cold junction temperature and adds a bias voltage that compensates for the difference between this and 0oC. A typical example is the Maxim MAX31855 which simply uses the temperature of the chip itself,* so the datasheet
https://datasheets.maximintegrated.c...s/MAX31855.pdf
calls out maintaining the cold junction at the same temperature as the chip, preferably by ensuring they are thermally coupled.
A typical circuit board is constructed of a fibreglass material known as FR4 with 2 layers of 1 oz copper. PCB mount screw terminals and several other connectors such as 4mm banana plugs are typically made from steel. FR4 itself is almost as good a thermal insulator as it is an electrical insulator, it has a U value of 0.25 W m-1K-1. Copper is about 380 and ordinary steel is about 40, so steel is about 1/10th as good at conducting heat as copper and FR4 is about 1 / 1500th.
The upshot of this is that relying on conduction through these materials will not allow the chip to read the cold junction correctly: in a typical scenario the thermocouple amplifier will read high by a few degrees in the mildest of conditions and can be off by over 20 degrees in more severe conditions.
*I’m guessing it uses a simple temperature dependent bandgap reference in the chip architecture, this constrains the measurement to being that of the actual die temperature since the bandgap reference will be etched in the die.
TBC, I will explain how to design the cold junction connector so it works optimally.



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