Brian123 and g.mccormick answered it in a simpler and more concise way than I could have.
When asked exactly how and why thermocouples work this is how I explain the theory....
Thermocouples work on a principle known as the Seebeck effect. A physicist named Seebeck first discovered when a loop was made from wires of dissimilar metals and one junction heated, an electric current would flow (technically he said it was a magnetic field). The key point to note is that only one junction was heated, therefore a temperature differential existed between the two junctions.
In my question I specified a J type thermocouple, but there is nothing special about that type other than it being a common type, so I will continue to use that type to explain. Using a piece of J type thermocouple wire (let's say 500cm long), strip both ends then twist the two wires together at each end essentially making 2 thermocouple junctions. If one end was kept at room temperature of 72F and the other end heated to 200F, a current flow would be induced in the loop, therefore a possible measureable voltage. Now if both ends were heated to 200F, the current flow would stop, therefore 0 volts would be generated. Now heat both ends to 300F and once again there will be no current flow nor voltage. No matter how hot you make the junctions, if both junctions are equal, there will be no current flow and no voltage.
This may bring to question, if the thermocouple voltage can be 0 volts at 200F as well as 0 volts at 300F, then how is it possible to measure temperature with a thermocouple? As several have mentioned, this is where a cold junction compensation element comes into play. If you were to look up the specifications for J type thermocouple, you will find it generates -8.095mV at -346F and 69.553mV at 2193F. The mV values found in standard thermocouple tables are typically based on one junction being at 32F(0C). If the "cold" end were to warm up while maintaining the same temperature at the measuring end, the voltage will decrease and if it were to cool down, the voltage would increase, the generated voltage is based on the temperature difference between the 2 junctions. The purpose of the CJC element is measure the temperature at the terminals connecting the thermocouple to the device and use that value to compensate for the temperature not being at 32F (0C), which is what the standard tables use for their voltages specified.
With that being said, if the device measures 0 volts on the input, it must assume the measuring junction of the thermocouple is at the same temperature as the end connected to the device, which would be the temperature measured by the CJC element. I did state in the question the device is setup for thermocouple which was intended to mean the device would have a CJC for accurate measurement.
If a piece of copper wire were used to jumper the input terminals, it would force the input to 0 volts, therefore a reading of the CJC temperature would be the expected display temperature. What if the terminals are not copper, wouldn't that create 2 thermocouple junctions that could generate a voltage? The answer is yes. However keeping in mind there must be a differential temperature to generate a voltage and the input terminals are typically so close together that the temperature difference is negligible.
What use does this tidbit of information really have? Consider a scenario in which you have a PLC with a TC card and it is giving suspected bad readings. A very quick & dirty (Q&D) test is to jumper the input with a piece of common wire and see if it is giving a temperature reading close to the temperature of the TC card. This can give an indication if the card is working properly.
An interesting piece of trivia.... The same effect that makes thermocouples work can also be used to generate electricity. Several space probes use what is essentially a bank of thermocouples and a radioactive material (for heat) to generate electrical power for the probe's electronics. The mars rover, Curiosity, uses thermocouples to generate 110 watts of electrical power.
When asked exactly how and why thermocouples work this is how I explain the theory....
Thermocouples work on a principle known as the Seebeck effect. A physicist named Seebeck first discovered when a loop was made from wires of dissimilar metals and one junction heated, an electric current would flow (technically he said it was a magnetic field). The key point to note is that only one junction was heated, therefore a temperature differential existed between the two junctions.
In my question I specified a J type thermocouple, but there is nothing special about that type other than it being a common type, so I will continue to use that type to explain. Using a piece of J type thermocouple wire (let's say 500cm long), strip both ends then twist the two wires together at each end essentially making 2 thermocouple junctions. If one end was kept at room temperature of 72F and the other end heated to 200F, a current flow would be induced in the loop, therefore a possible measureable voltage. Now if both ends were heated to 200F, the current flow would stop, therefore 0 volts would be generated. Now heat both ends to 300F and once again there will be no current flow nor voltage. No matter how hot you make the junctions, if both junctions are equal, there will be no current flow and no voltage.
This may bring to question, if the thermocouple voltage can be 0 volts at 200F as well as 0 volts at 300F, then how is it possible to measure temperature with a thermocouple? As several have mentioned, this is where a cold junction compensation element comes into play. If you were to look up the specifications for J type thermocouple, you will find it generates -8.095mV at -346F and 69.553mV at 2193F. The mV values found in standard thermocouple tables are typically based on one junction being at 32F(0C). If the "cold" end were to warm up while maintaining the same temperature at the measuring end, the voltage will decrease and if it were to cool down, the voltage would increase, the generated voltage is based on the temperature difference between the 2 junctions. The purpose of the CJC element is measure the temperature at the terminals connecting the thermocouple to the device and use that value to compensate for the temperature not being at 32F (0C), which is what the standard tables use for their voltages specified.
With that being said, if the device measures 0 volts on the input, it must assume the measuring junction of the thermocouple is at the same temperature as the end connected to the device, which would be the temperature measured by the CJC element. I did state in the question the device is setup for thermocouple which was intended to mean the device would have a CJC for accurate measurement.
If a piece of copper wire were used to jumper the input terminals, it would force the input to 0 volts, therefore a reading of the CJC temperature would be the expected display temperature. What if the terminals are not copper, wouldn't that create 2 thermocouple junctions that could generate a voltage? The answer is yes. However keeping in mind there must be a differential temperature to generate a voltage and the input terminals are typically so close together that the temperature difference is negligible.
What use does this tidbit of information really have? Consider a scenario in which you have a PLC with a TC card and it is giving suspected bad readings. A very quick & dirty (Q&D) test is to jumper the input with a piece of common wire and see if it is giving a temperature reading close to the temperature of the TC card. This can give an indication if the card is working properly.
An interesting piece of trivia.... The same effect that makes thermocouples work can also be used to generate electricity. Several space probes use what is essentially a bank of thermocouples and a radioactive material (for heat) to generate electrical power for the probe's electronics. The mars rover, Curiosity, uses thermocouples to generate 110 watts of electrical power.
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