Hi,
One of the biggest problems in thermocouple literature and understanding thermocouples is the use of the word "junction". As soon as an electrical guy hears "junction" it brings up thoughts of two wires connected together and the ends that form a junction. That would be correct...correct had we been talking about an electrical circuit. However, when we talk about thermocouples we are not always talking about an electrical circuit usually we are referring to a thermal circuit or just a thermal conductor. This means when we talk about the HOT junction and the COLD junction we are not necessarily talking about an electrical junction.
So the first item on the list then is to figure out what exactly a JUNCTION really is.
There are two types of junctions used with the thermocouple. The first type is an ELECTRICAL JUNCTION, and the second type is a THERMAL JUNCTION. Let me list them here:
1. Electrical
2. Thermal
Now an electrical junction is usually the first thing we think of. That's where two wires are brought into close proximity to each other. In fact, they touch so that means an ideal junction has no voltage across it.
A thermal junction is the thermal counterpart of the electrical junction, but it is totally, totally different. A thermal junction is a place where two thermal conductors come into close proximity to each other. They dont have to touch though, just come very close to each other so that the temperature difference between them is zero.
So the electrical junction has no voltage drop, and the thermal junction has no temperature drop.
The big difference here is that the thermal junction can be formed from two metals yet never touch, while the electrical junction has to have conductors that actually touch. But because the wires are so close the electrical junction is also a thermal junction. So the electrical junction is both an electrical junction AND a thermal junction, yet the thermal junction is only a thermal junction.
Now we can talk about the HOT and COLD "junctions". Note the word "junctions" appears here and in all the literature.
The HOT junction is an electrical junction, so it is also a thermal junction.
The COLD junction is ONLY a thermal junction, so the wires dont touch yet they are at the same temperature.
The HOT junction has two wires that touch, the COLD junction has two wires that do not touch. So this means the HOT junction is an electrical connection, but the COLD junction is just a thermal connection with no ability to conduct current within it.
This brings us to the subject of using other metal conductors with thermocouples. We need to know the temperature of the cold junction in order to calculate what the temperature of the hot junction is. This is so because the two dissimilar metals produce a known voltage difference between the hot junction and the cold junction, and the difference between the ends at the cold junction allow us to measure this voltage difference.
However, because we MUST know the temperature of the COLD junction, we can not use ordinary wire between the COLD junction and the measuring device if that measuring device is also measuring the cold junction temperature at the ends of the ordinary wire. This is going to be the case with temperature meters that use a thermocouple wire with a connector at the end. That's because the meter depends on knowing the temperature of the actual wires at the connector. If we insert wire between the connector and the meter, we dont know that anymore. We need to know BOTH the electrical voltage difference AND the temperature at the cold junction, not just the electrical difference. Using ordinary wire, we would only know the electrical difference and loose the temperature measurement.
If we could somehow still measure the temperature of the ends of the dissimilar metal wires we could still do this, but usually that's not the case. That would mean we would need two more wires to send the cold junction temperature back to the meter.
There is a saving grace here however, and that is if the two extra wires can be kept at the same temperature at both ends. That would mean we could still know the true cold junction temperature. Usually this isnt that easy to do however because what is preventing that wire from getting one end heated more than the other.