Thermocouples often described incorrectly
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 10/1/2008 2:00:00 AM
| For reactions to this article, see Martin Rowe's blog posting, "Thermocouples raise reader temperatures." |
Perhaps the most common mistake told about measurement is the phrase “A voltage is developed across the junction of a thermocouple that’s proportional to temperature.” It’s repeated so often that even experienced engineers take it for granted. It’s also incorrect.
 Thermocouples generate a difference voltage proportional to a temperature difference. |
A thermocouple is made two of wires of different metals where the tips are connected (often welded) to produce a good electrical connection. This junction is the temperature measurement point. Figure a shows the incorrect assumption about thermocouples. There is no voltage developed across the junction, although there will be a voltage across the open ends of the wires (VT).
In Figure b, assume that the thermocouple junction is at temperature T1 and, for simplicity, assume that the open ends of the wires are both at T0 = 0°C. A wire whose ends are at different temperatures will produce a small voltage across those ends, called the Seebeck Effect. That voltage depends on the composition of the wire and the temperature difference across the wire’s ends.
The material of a wire affects the voltage across its ends, so wires of two different materials produce two different voltages (V1 and V2) for the same temperature difference. Thus, the voltage across the terminals is VT = V1 – V2. The voltage develops across the wires, not across the junction.
Because keeping the wire ends at 0°C is impractical, equipment manufacturers use a circuit to measure the temperature at the ends of the thermocouple wires and electronically compensate for that temperature. It’s called cold-junction compensation (CJC). Even with CJC, the thermocouple wire connections must be as close to each other’s temperature as possible to minimize measurement errors. Most connections to thermocouple measuring equipment, therefore, are mounted on a thermally conductive block.
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Jason,
You are incorrect. Rowe is right. The voltage is produced across the wire,
not the junction!!
Stanley Green - 2008-30-10 22:17:00 EDT -
Shame on you for giving incorrect information and saying that it corrects "incorrect information".
"The voltage develops across the wires, not across the junction."
This is incorrect. (or at least not strictly correct -- you make it sound as though the wires are producing this voltage rather than the junction) The voltage drop is produced by the junction. When you go to measure the wires with a circuit or DMM, you are creating additional junctions. If all the junctions are at the same temperature they cancel out and you are left with zero voltage. If they''''re at different voltages then you are left with a net voltage difference that is dependent on the temperatures at the three junctions. You have to do cold-junction compensation to correct for any changes in temperature of the two junctions at the reference terminals.
Jason S - 2008-23-10 10:06:00 EDT
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