On Tue, 06 Sep 2005 11:22:13 +0100, "John Schmitt"
wrote:

New device .. open cct voltage 30mV, short cct current (DMM on 2mA)
250uA.

Old device .. 23mV and 200uA ?

So, from that very crude test the old thermocouple isn't 'dead' but
maybe down on performance (especially when not in a 'strong' flame ie
only a 'pilot' not a low gas ring) enough to make a difference?

The difference in voltages puzzles me. There are various types of
thermocouple, each being composed of two metals (normally alloys) which
develop a potential difference, the Seebeck voltage, named after the
discoverer. This is characteristic for any pair of metals and I am
assuming that you have a very resistive meter.

Erm, it's a DMM and I thought even the cheapest of them was 10M ohm /
v ?

The amperage ought to be
proportional to the contact area. Linearity is not perfect, but in a
boiler, "hot" and "cold" are clearly sufficient.

Understood ..

I think it is type T
thermocouples which are used in boilers, but I will gladly be corrected on
this. For more information on thermocouples than you want,

http://www.thermometricscorp.com/the...thertypav.html

although the links seem to be broken.

Interesting thanks .. shame about the broken links .. ;-(

.. BTW a couple of years back I found that simply
giving the thermocouple a tighten broke through the oxide film and
restored the function. touches wood

And there's little to loose for a diyer .. ;-)

All the best ..

T i m