DIY-Thermocouples

[john carroll]

Aircraft Spruce & Specialty (800.824.1930) sells a selection of EGT thermocouples for 25-30$.

I build serviceable thermocouples by welding the unterminated leads of thermocouple wire to the outside of the exhaust header.  I usually put them on the outside of a radius because of the unsubstantiated assumption that this will be the most responsive and hottest location.  The headers are usually very thin wall stainless.  This removes the junction from the corrosive effects of the exhaust stream.

Even if you prove that this installation does not read exactly same as a device directly in the hot gas stream, we mostly care about the changes and the changes will be the same, or at least proportional.

My welder consists of a large capacitor that is charged to a couple of hundred volts by a small high impedance transformer and a diode. A power resistor in the primary circuit protects the leads and the transformer.  One lead is attached to the pipe with a wire and an alligator clip.  The other lead is connected to a pair of needle nosed pliers with insulated handles.

The site is prepared by filing a small area to shiny metal.  Push the insulation on the end of the thermocouple wire back so that the pliers are able to grab it. Nip the wire so that you have a fresh cut surface.  Close the charge switch on the primary side of the transformer to charge the system, then spark the lead down where you want it. Weld the other lead down in a convenient nearby location.  Some have lasted for years.  Watch out for the other end of the wire.  If it loops back and stabs you somewhere during this operation it will really commemorate the day.

Should a lead eventually break off, cut the wire back and do it again.  I have used the leads that come on commercial thermocouples.  Just cut the leads back and use them.  It does take some practice, but it works very well.  I don't think that there is any magic voltage, just enough energy in the spark.  I added capacitors until it worked.  Capicitors are added in parallel.  BE VERY CERTAIN THAT YOU USE CAPACITORS THAT ARE RATED FOR THE VOLTAGE.  BE VERY CERTAIN THAT YOU OBSERVE THE PROPER CAPACITOR POLARITY.  Ignoring this can cause an explosion. 

The response time is that of the tubing.  I suppose that you could cut a hole in heavy wall tube and fill it with a thin stainless window for lead attachment.  

john carroll
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jac at wavecom.net
 03/13/97 17:29:04