I would like to make a very simple pyrometer for raku work. I know that most people do not regard this as necessary but I would like to include temperature as one of the many things I look at when playing around :-)
Can someone point me to a link/instructions on how to construction a simple pyrometer? I have a multimeter ("and I am not afraid to use it") and I think I need a k-type thermocouple. But what about wiring/calibrating/compensation etc?
I am based in the UK in case people can recommend suppliers of electrical bits. Thanks very much for any advice.
You will need to be measuring 40-50 mV, so your DMM (Digital Multi-Meter) will need a full-scale sensitivity of 100 mV. (It might be called 200 mV... goes to 199.9 mV max) Actually, 55 mV is about the top of a K thermocouple range, but DMMs don't have ranges like that. You don't typically do anything to calibrate a thermocouple, since they are supposed to be correct due to their physics. Some types get poisoned by metals deposited from the kiln atmosphere. I've usually heard that about platinum types (R and S), but you may want to put your K type in a ceramic sheath anyway, if you have one.
You will have to take your mV reading and look it up in the proper K-type thermocouple tables to find the corresponding temperature. Then you will have to add to that the temperature of the "cold junction", where your thermocouple leads connect to your DMM. This assumes that you have long leads from the kiln to the DMM, made of the same K-type wire and connected with the proper polarity so that it is just as if you plugged the thermocouple right into the DMM. If you don't have long enough K-type wires and have to switch to copper wires, then the cold junction is where the K-type connects to the copper. I use an inexpensive indoor-outdoor digital thermometer with the "outdoor" sensor touching (or very near) the cold junction.
It's perfectly OK to use heavy gage K-type wire for the thermocouple itself, and cheaper light gage K-type wire to connect to the meter.
You can download complete tables for any thermocouple type from the US NIST through
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Hope this helps!
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
I have now bought a k-type thermocouple (with ceramic sheath) and a digital thermometer. Cost so far USD50 - ebay is a wonderful thing.
Exactly how should I connect the k-type to the digital thermometer in terms of cable. It is heavy guage wire and the digital thermometer has a very light k-type bead thermocouple.
Can I cut off the bead and connect to my heavy duty one? If so what about the connection? How should it be made?
I'm not entirely clear on what you have. Are you SURE the digital thermometer is a K-type thermocouple? Typically they use semiconductors or thermistors unless specifically designed for high temperature ranges. If you are sure it is a K-type, then it would be OK to cut it off and connect directly to your thermocouple leads. If you get the wires reversed, you'll know it when you warm the thermocouple in your hands and the readout heads toward zero instead of up.
Note that you should not interconnect the thermocouple and the thermomenter with anything other than K-type wire pairs, or you will need to compensate for the extra cold junction temperature differences.
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
Forgot to mention about how to make the connection: Typical thermocouple connectors are screw-type terminals. You don't solder K-type wires (they probably wouldn't take solder anyway). In your case, if you are connecting K-type wires together more-or-less permanently (instead of plug and socket for quick replacement), then you can use any sort of screw-terminals or even wire nuts. You just need a tight junction. It doesn't matter if the connector is made of something else, as long as the wires are in contact. Ideally, you want both junctions (from the two K-type wire splices) to be at the same temperature, but obviously not electrically connected. But I wouldn't worry about any fancy thermal-conductive junction blocks here, as long as the two junctions are near each other so they both are seeing about the same room temperature.
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
Thanks for the info again Bob. Just what I wanted to know.
The unit definitely takes a k-type thermocouple (it is supplied with one) and has a range up to 1300 deg C. It looks just like a fluke 52, but is made in china and so it is cheap :-)
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