• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Thermocouple Selector Switch

Status
Not open for further replies.

jrz126

Active Member
I have a thermocouple reader and I want to put a selector switch on it.

Do I need to use thermocouple wire for all the connections? Do the switch contacts need to be made of a certain metal for it to work?
 

dknguyen

Well-Known Member
Most Helpful Member
All interfaces between two different metals (bi-metallic interface) generate a voltage difference in the pieces of metal forming the junction sit across a temperature gradient. But...no temperature gradient, no voltage difference. Therefore, you can introduce any bimetal interfaces that you want as long as those pieces of metal do not sit across a temperature gradient, or a gradient small enough so that the voltage generated is negligible (either by having a zero or very low temperature gradient, or pieces of metal so small relative to the gradient that the temperature at both ends is approximately the same).

This means that:
The metal does not matter for the switch as long as the contacts are in a very small space so the temperature gradient is approximately the same everywhere around the contacts (which means the contacts have to be small relative to the size of the gradient which tends to mean they should not be sitting near heat sources where the gradient is highest).

If you are extending thermocouple readings across a temperature gradient you have to use thermocouple wire until the gradient has died down. Rephrased, this means that if you are using extension wires of a material different than the thermocouple, then you must ensure the temperature at both ends of the "segments of wire made of a different material" are at the same temperature, or else they will introduce their own voltage difference at the bimetallic junctions and distort the thermocouple junction reading.

So in all probability, the answer to your question is NO, the switch contact material does not matter since it's probably sitting far away from the heat source so the temperature gradient is very small, and the the contacts are probably very small reducing the temperature gradient found across the contact. You should use enough thermocouple wire to get away from the region of highest temperature gradient before you start introducing bi-metallic junctions.
 
Last edited:

Leftyretro

New Member
All interfaces between two different metals (bi-metallic interface) generate a voltage difference in the pieces of metal forming the junction sit across a temperature gradient. But...no temperature gradient, no voltage difference. Therefore, you can introduce any bimetal interfaces that you want as long as those pieces of metal do not sit across a temperature gradient, or a gradient small enough so that the voltage generated is negligible (either by having a zero or very low temperature gradient, or pieces of metal so small relative to the gradient that the temperature at both ends is approximately the same).

This means that:
The metal does not matter for the switch as long as the contacts are in a very small space so the temperature gradient is approximately the same everywhere around the contacts (which means the contacts have to be small relative to the size of the gradient which tends to mean they should not be sitting near heat sources where the gradient is highest).

If you are extending thermocouple readings across a temperature gradient you have to use thermocouple wire until the gradient has died down. Rephrased, this means that if you are using extension wires of a material different than the thermocouple, then you must ensure the temperature at both ends of the "segments of wire made of a different material" are at the same temperature, or else they will introduce their own voltage difference at the bimetallic junctions and distort the thermocouple junction reading.

So in all probability, the answer to your question is NO, the switch contact material does not matter since it's probably sitting far away from the heat source so the temperature gradient is very small, and the the contacts are probably very small reducing the temperature gradient found across the contact. You should use enough thermocouple wire to get away from the region of highest temperature gradient before you start introducing bi-metallic junctions.
Good precise answer. However it might be helpful to give an example of a problem installation.

There will always be a cold junction reference voltage generated at the measurement end of a TC because there is a TC metal to copper transition at the electrical component(s) used to measure the voltage. Now if up stream you have a switch that can select different TCs to the measurement wire AND it's using normal copper type switch contacts, then you have created more then one cold reference junction and unless ALL those reference junctions are at the same ambient temperature then there will be a measurement error introduced into the system. How big an error depends on the amount of temperature difference between the various cold junctions. And of course the percentage error reading depends on the absolute temperature being measured at the hot junction end of the TC. When measuring 1,000 degrees the slight error injected from multiple cold junctions will most likely not be noticed within the normal accuracy specifications of a TC. However if you are measuring something close to ambient then the error induced can be significant.

PS: There are switches and connection hardware available made from the same metals used in common TCs to deal this problem, however they are pretty expensive.
Lefty
 
Last edited:

SPDCHK

Member
Maybe to add to what Leftyretro replied, if the device you use to read the thermocouple sensors allows for "Cold junction" temperature compensation, place this cold junction sensor right next to the selector switch.

Most instrumentation devices use copper wire to run to the recorder/controller. The measurement is made with either type "J" "K" or whatever needed for the measured range required. The compensation leads from the sensor runs to a "cold junction box" where the TC wires are connected to the trunk cable copper wires. At this connection the "cold junction" temperature sensor is installed. (In our application it was a 47K NTC resistor).
 
Last edited:
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top