Science Buddies: "Ask an Expert"

We have connected tightly various combinations of metal wire.
We are using .015 dia copper , iron ,chromel and constantan for the combinations.
We are testing all combinations at Room Temp , Heat one junction only , Cool one junction only and Heat/Cool a junction.
When using Analog and Digital multimeter connected to both ends of Material # 2 ( per diagram shown on project page ) , we get no reading at all for any of the combinations.
We are tightly wrapping with material # 2 wire around the probes of the meters and even tried using alligator clips
slipped over the end probes for better connection.
I've checked both meters and they are working just no reading when we measure for voltage between the junctions.
Do I need to place the probes somewhere other than the ends of material # 2 to get a reading for the
voltage between the two junctions?? ie clip the leads directly to material # 2 where the voltage is supposedly flowing?
Any advise would be greatly appreciated.
Thanks

islandelectrical - The voltages generated by twisted wire thermocouples is very low (as is the current). Obviously your combinations are of made up of different material wires in each thermocouple. You might have better luck with the lowest voltage setting using the digital multimeter. Many older analog meters have low impedences and of maybe only a few thousand ohms/volt sensitivity. That may appear as a 'short circuit' to your thermocouple where you are seeing microvolts, millivolts and microamps of signal. Use the lowest setting on your multimeter, in the millivolt ranges if possible. If there is a very low current setting on your digital meter, say 200 microamps, that may be able to detect the very low voltages. Remember also to keep a temperature differential between your twisted junction and the ends of the wire that you are connecting to. I don't think using alligator clips or just tight twisted connections is the problem. Good luck.

islandelectrical - Another issue, you mention connecting the meter to both ends of material #2. That won't work. I don't know if you mean you are connecting to the twisted portion of material #2or some place just behind the twisted junction. The meter should be connected to the ends of both dissimilar wires, #1 and #2, and the twisted junction is your sensor. I didn't have a link to your diagram, but that may be the problem.

Rick Marz

To further clarify previous answers:
There are at least 3 external circuit junctions involved in a thermocouple circuit. The "positive meter lead" to thermocouple material A, the thermocouple material A to thermocouple material B, and the thermocouple material B to "negative meter lead". Thermocouple material A and B must be different materials and the temperature of the A-B junction MUST typically be a couple of thousand degrees different from the "reference junctions" (meter lead to A and meter lead to B). This typically means the A-B junction is in a flame. If the A-B junction is not "gas tight", the A and B materials will oxidize and form a high resistance contact which will prevent electron flow between them. Switch the multi-meter to the Ohms scale and the see if the thermocouple circuit reads zero ohms (or precisely the same as touching the positive and negative meter lead if your meter has a zero ohms adjustment). I'm using "A" and "B" just to indicate different metals because I don't know exactly what you are calling #1 and #2.

Gas tight connections can be made by spot welding the different metals together or cleaning them and tightly twisting them together. If you use the twist method, pre-form the leads into the shape you want them before twisting their ends, clamp the wires in a vise and pull while twisting at least 4 full turns using a pair of side-cut pliers so the metals are stretched and deformed together. Once twisted together, you don't want to be bending the wires or putting any other forces on the wires that might affect the junction area. spot welded junctions can also be broken by rough handling so take care with them as well.