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

On Tue, 06 Sep 2005 10:49:37 +0100, T i m wrote:

I just did some comparisons between old and new probe using my DMM and
the gas stove and noted the following with either (probe, not DMM) in
the gas flame (simulating pilot).

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

Also bear in mind that there are, in effect, juncions between the
meter probes and the thermocouple itself.

sponix