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EGT Thermocouple


SiWhite
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Anyone know about EGT gauges and associated Thermocouples (senders)?

I've bought a nice EGT Gauge from Fleabay (like THIS) and am wondering about which thermocouple to use.

Does anyone know if thermocouples are universal i.e will change resistance with temperature or if they need to be matched to a standard for different gauge?

FYI, the guaue doesn't have a 12v input, so appears to measure resistance across the poles.

Many Thanks!

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Thermocouples are completely different animals.

They don't work in terms of a variable resistance, in fact the disimilar metals used generate a (very small) electrical potential difference from a thermal potential difference.

There are many different types (different metals used), which give different outputs. The measuring circuits are tuned to the specifc thermocouple used, usually a bridge circuit is used to amplify the signal.

Type K is common, i think. other types include E J N B R S T (types shamelessly from wiki, my notes are filed somwhere)

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Good news Lewis! Wrong thread though?

Thanks Luke, hadn't thought about Wiki for engineering-type background stuff like this, nice one!

Seems I need to borrow WillWarne's truck and see if his thermocouple isa match for mine - unless I can do a bit of multimeter testing on my gauge and find out about the parameters for the different thermocouples (clearly in way over my head here - help!)

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I'm not sure how much you'd gain from measuring with a multimeter, there's (in theory) reference junctions and laws of intermediate temperatures to take into account.

As an example a K type has a PD of 6.137 mV at 150 degC.

I'd try and find manufacturers specs, data sheets etc, to establish which type your gauge is designed for.

plenty of reading here

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You could measure it with your multimeter, but you generate additional thermocouple junctions where the +ve and -ve probes touch the dissimilar metals. So long as you use probes made of the same material and keep the junctions at the same temperature, this would cancel.

However, the voltage from the thermocouple is very non-linear, so it might be the same (give or take 0.02mV) for two different types at room temperature, but that difference could be very different when you double the absolute temperature of the junction.

The question I think you need to be asking is, are all K-type thermocouples the same? Me, I don't know... :blink:

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Si, come round and have a look at my setup when I'm back. I can't totally remember how mines wired up but IIRC the probe plugs into a 'box of trick' which is connected to the gauge which you then wire up.

As Luke said, there are lots of differant types of probes however, once you get the correct type they're interchangable (Tony Cordell changed to a smaller diameter probe to give a quicker reaction time). However, you need to know what type it is to begin with.

FYI mine wasn't that cheap but reacts very fast and isn't too expensive when you think its hand built in your colour of choice and the probe alone's nearly half the cost. Have a look here

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Thanks for all the suggestions guys - some food for thought.

Red90 - is that the thermocouple you think I need? Great if it is - how did you work it out?

I design and operate industrial facilties including lots of test apparatus. This is very common and will work well in the application.

The important thing is to get 3mm. Any smaller and I would not trust it to last in the service. Larger ones will not have adeqaute response time. The sheath should be 316 Stainless or better to deal with the environment. The company listed above sell 310 and Inconel 600 sheathing, either of which or more than adequate. They sell direct online, http://www.tcdirect.co.uk/deptprod.asp?deptid=190 and the prices look fine to me.

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