how does a thermo couple work?

Answer 1

In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy; system errors of less than 1 °C can be difficult to achieve.

In most applications, one of the junctions — the cold junction — is maintained at a known (reference) temperature, while the other end is attached to a probe. For example, in the image above, the cold junction will be at copper traces on the circuit board. Another temperature sensor will measure the temperature at this point, so that the temperature at the probe tip can be calculated.

Thermocouples can be connected in series with each other to form a thermopile, where all the hot junctions are exposed to the higher temperature and all the cold junctions to a lower temperature. Thus, the voltages of the individual thermocouple add up, which allows for a larger voltage.

Having available a known temperature cold junction, while useful for laboratory calibrations, is simply not convenient for most directly connected indicating and control instruments. They incorporate into their circuits an artificial cold junction using some other thermally sensitive device (such as a thermister or diode) to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation.

Usually the thermocouple is attached to the indicating device by a special wire known as the compensating or extension cable. The terms are specific. Extension cable uses wires of nominally the same conductors as used at the thermocouple itself. These cables are less costly than thermocouple wire, although not cheap, and are usually produced in a convenient form for carrying over long distances - typically as flexible insulated wiring or multicore cables. They are usually specified for accuracy over a more restricted temperature range than the thermocouple wires. They are recommended for best accuracy.

Type K (Chromel (Ni-Cr alloy) / Alumel (Ni-Al alloy))
Type E (Chromel / Constantan (Cu-Ni alloy))
Type J (Iron / Constantan)
Type N (Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy))
Type B (Platinum-Rhodium/Pt-Rh)
Type R (Platinum /Platinum with 13% Rhodium)
Type S (Platinum /Platinum with 10% Rhodium)
Type T (Copper / Constantan)

Answer 2

I think you mean a thermo-coupler. Normally this is an accessory on most heaters...usually a safety feature that turns the gas off when the pilot goes out. The heat from a lit pilot causes the metal in the thermo coupler to expand and thereby open a valve. If the pilot goes out or the thermo-coupler cools for some reason, the valve shuts and no more gas will come through the line.

Answer 3

Heat causes a valve to open and allow a small amount of combustable gas to flow, such as a pilot light on a water heater. If the heat is removed, such as the pilot light getting blown out, the valve will close thereby preventing further gas flow. This prevents a hazardous build up of unburned gas in the appliance.

Answer 4

www.britannica.com/eb/article-9059021/Peltier-effect

Dissimilar metals produce a weak electrical current when they have a hot and cold junction. The amount of current is proportional to the difference between the two junctions. The power is sufficient to operate devices like gas shut off valves etc.

So they are one type of flame safety device. The same effect will work in reverse you can force current through a thermocouple circuit and create a heat flow in the opposite direction.

Answer 5

Like this:

http://en.wikipedia.org/wiki/Thermocouple

Answer 6

Thomas K with Wikepedia says it all.