Hi All,

I had another look at Mums 'old but going to br replaced soonish., the
pilot won't stay alight' Ascot / New World (Main?) gas boiler again
yesterday and think I found the upper limit cutout / inhibitor (?) had
triggered? I pressed what looked like a rest button in the middle of
it but as didn't have any real tools / meter with with me (only the
trusty Leatherman as always) I couldn't measure anything so shorted
the extra stat out (for testing) and tried to light the pilot.

Hold in the contril knob, hit the piezo, nice little pilot flame, hold
knob in 30 seconds, release, pilot stays alight (hah huh, so it was
overtemp cutout ..?).

Turn the gas knob round to 'On', pilot goes out?

Wait 3 mins, relight pilot, turn knob to 'On' pilot stays on this
time, turn mains power to boiler, main burner lights up but then main
pilot and burner goes out? Try this a few times with similar
(intermittent) results. So, it could just be that the thermocouple was
'marginal' and as they are 'cheap' ....

So, I removed the thermocouple (amazing what you can do with a
Leatherman PST) and this morning picked up a replacement. It's not the
'correct' part (long since obsolete) but sufficiently close where it
matters to 'do'.

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?

I'll see what happens when I ahem, sorry, 'the Corgi bloke', put's
it back later... "You know anything about parachutes ...?" ;-)

All the best ..

T i m

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,

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

although the links seem to be broken.

I'll see what happens when I ahem, sorry, 'the Corgi bloke', put's
it back later... "You know anything about parachutes ...?" ;-)

Ah, yes, the old joke. 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

John Schmitt

--
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/

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

In an earlier contribution to this discussion,
T i m wrote:

Hi All,

I had another look at Mums 'old but going to br replaced soonish., the
pilot won't stay alight' Ascot / New World (Main?) gas boiler again
yesterday and think I found the upper limit cutout / inhibitor (?) had
triggered? I pressed what looked like a rest button in the middle of
it but as didn't have any real tools / meter with with me (only the
trusty Leatherman as always) I couldn't measure anything so shorted
the extra stat out (for testing) and tried to light the pilot.

Hold in the contril knob, hit the piezo, nice little pilot flame, hold
knob in 30 seconds, release, pilot stays alight (hah huh, so it was
overtemp cutout ..?).

Turn the gas knob round to 'On', pilot goes out?

Wait 3 mins, relight pilot, turn knob to 'On' pilot stays on this
time, turn mains power to boiler, main burner lights up but then main
pilot and burner goes out? Try this a few times with similar
(intermittent) results. So, it could just be that the thermocouple was
'marginal' and as they are 'cheap' ....

So, I removed the thermocouple (amazing what you can do with a
Leatherman PST) and this morning picked up a replacement. It's not the
'correct' part (long since obsolete) but sufficiently close where it
matters to 'do'.

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?

I'll see what happens when I ahem, sorry, 'the Corgi bloke', put's
it back later... "You know anything about parachutes ...?" ;-)

All the best ..

T i m

If the pilot stays on after releasing the button, it sounds as if the
thermocouple and flame failure device are working ok. If the pilot goes out
when the main burner comes on, it suggests that it's being blown out by the
sudden rush of air. In this case, the pilot probably isn't quite powerful
enough - either because its height needs adjusting (is there a separate
little regulator for this?) or because there's a bit of crud partially
blocking its jet.
--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.

T i m wrote:
Hi All,

snip
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?

I'll see what happens when I ahem, sorry, 'the Corgi bloke', put's
it back later... "You know anything about parachutes ...?" ;-)

The current isn't real.
The meter is applying a resistor across the leads so that at its nominal
200mv fullscale, it'll read full current, so 2mA range has a 100 ohm
resistor across it.

As the thermocouple has a resistance way under 100 ohms, it's just reading
out the voltage.

The voltage ranges can be handy for reading out very low currents (nA)
similarly.

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

On Tue, 06 Sep 2005 11:35:35 GMT, (s--p--o--n--i--x)
wrote:


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

True, but I had the meter clipped to the very end of the thermocouple,
the probe end of the thermocouple in the gas hob flame and was holding
the middle bit with my hand (and I'm not a woman so don't have
asbestos fingers!) ;-)

I did measure both a few times and there were some small voltage
variations depending how hot you got the probe. It seemed to balance
out as long as you left it in the flame for a reasonable time (say ~20
seconds).

All the best ..

T i m

On Tue, 6 Sep 2005 16:42:56 +0100, "Set Square"
wrote:



If the pilot stays on after releasing the button, it sounds as if the
thermocouple and flame failure device are working ok.

Well, I think it DID say on all the time as long as you held the knob
in long enough to start with and did nothing else? Once it *seemed* to
go out when I put the front back on (carefully), another when I closed
the control box lid and another when I turned the gas control knob to
'Full'? But when I tried any of the above again the pilot stayed on?

If the pilot goes out
when the main burner comes on, it suggests that it's being blown out by the
sudden rush of air. In this case, the pilot probably isn't quite powerful
enough - either because its height needs adjusting (is there a separate
little regulator for this?) or because there's a bit of crud partially
blocking its jet.

That's what I first thought but the flame was pretty strong?

Anyway, I fitted the new thermocouple, shorted the 'overheat stat'
connectors and lit the pilot .. so far so good ;-)

I turned the control knob to 'On' .. still pilot .. turned the power
supply to the boiler on then the boiler itself, up came the main
burner and the pilot stayed on ;-)

I tried this a few times and each time it was A ok ;-)

The only 'problem' now is the only replacement thermocouple that I was
supplied (that looked like it had the right ends AND the connector for
the 'inhibitor') places the inhibitor connectors inside the main
boiler (outside the heat exchanger g) area rather than inside the
control box? So, I'm going to have to crimp up a pair of longer cables
(silicon) to reconnect the inhibitor. I can't see that being a problem
as the connectors will be quite low in the casing (alongside the pilot
gas tube and the ignitor wire) and will be cooled by the input air
flow (balanced flue)?

Thanks to all who helped / advised / questioned on this one and an
extra point for those who said 'thermocouple' ;-)

All the best ..

T i m

p.s. Now Mum is talking about where to fit the new condensing boiler!

On 06 Sep 2005 17:49:40 GMT, Ian Stirling
wrote:



The current isn't real.

Doh!

The meter is applying a resistor across the leads so that at its nominal
200mv fullscale, it'll read full current, so 2mA range has a 100 ohm
resistor across it.

But if the thermocouple is generating a voltage then there will
presumably (unless it's output impedance is infinity) some current
generated?

As the thermocouple has a resistance way under 100 ohms, it's just reading
out the voltage.

So (confused here) the meter was supposed to be measuring the current
generated by the thermocouple (all be it into the input resistance of
the meter rather than a real load). The meter itself does this by
measuring the voltage applied across it's internal shunt resistor and
using various scale modifing restors to calibrate the mV (uV?) display
to read (display) Amps? Could you explain (and I'm an aging
electronics engineer remember) how / why that would be different from
reading any current in a cct (on a DMM) please?

The voltage ranges can be handy for reading out very low currents (nA)
similarly.

Hmm .. I think even I would throw *that* thermocouple away!

All the best ... ;-)

T i m

T i m wrote:
On 06 Sep 2005 17:49:40 GMT, Ian Stirling
wrote:



The current isn't real.

Doh!

The meter is applying a resistor across the leads so that at its nominal
200mv fullscale, it'll read full current, so 2mA range has a 100 ohm
resistor across it.

But if the thermocouple is generating a voltage then there will
presumably (unless it's output impedance is infinity) some current
generated?

Of course.

As the thermocouple has a resistance way under 100 ohms, it's just reading
out the voltage.

So (confused here) the meter was supposed to be measuring the current
generated by the thermocouple (all be it into the input resistance of
the meter rather than a real load). The meter itself does this by
measuring the voltage applied across it's internal shunt resistor and
using various scale modifing restors to calibrate the mV (uV?) display
to read (display) Amps? Could you explain (and I'm an aging
electronics engineer remember) how / why that would be different from
reading any current in a cct (on a DMM) please?

Sorry, I was not being clear.
Of course it's not different.
My point was that it's not a 'real' measurement, it's affected by the
test instrument to a great degree.
On 2mA, it'll read .25mA, on 200uA, 30uA, and on 20mA 20mA. (or so).


The voltage ranges can be handy for reading out very low currents (nA)
similarly.

Hmm .. I think even I would throw *that* thermocouple away!

True.
But I've found it really handy in the past to measure leakage currents,
when the uA range won't quite cut it.
A 3.99 meter with a 1M input impedance will resolve down to 100pA when
set on voltage.
(care does need to be taken that you're getting accurate measurements.)

On 07 Sep 2005 00:00:55 GMT, Ian Stirling
wrote:

Could you explain (and I'm an aging
electronics engineer remember) how / why that would be different from
reading any current in a cct (on a DMM) please?

Sorry, I was not being clear.
Of course it's not different.
My point was that it's not a 'real' measurement, it's affected by the
test instrument to a great degree.
On 2mA, it'll read .25mA, on 200uA, 30uA, and on 20mA 20mA. (or so).

Ah, gotcha ..

I've found it really handy in the past to measure leakage currents,
when the uA range won't quite cut it.
A 3.99 meter with a 1M input impedance will resolve down to 100pA when
set on voltage.
(care does need to be taken that you're getting accurate measurements.)

Good tip ;-)

Cheers Ian ...

T i m

On Tue, 06 Sep 2005 20:12:21 +0100, T i m wrote:

The current isn't real.

Doh!

Hmmmm. Yes and no. There is movement of electrons from one metal in a
differentially heated thermocouple to the other, so Yes there is a
current. I expect this varies with the imposed resistive load and tops out
somewhere, so meter readings may vary depending on their resistance.

Believe it or not my physics teacher had a gas-powered radio. It was
Russian-made (or at least the notation was in cyrillic) and had a bank of
thermocouples over a gas flame and a heat sink at the back to keep the
junctions at different temperatures. The radio used thermionic components,
naturally, and the case looked rather like the radio had been involved in
a shoot-out at Stalingrad. It was certainly a good way of impressing a
class of teenagers. The next week a gas-powered refrigerator was our
object lesson in thermodynamics.

http://www.kkn.net/archives/html/QRP.../msg01213.html

gives a useful detour possibility if you are in the midlands.

John Schmitt

--
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/

On Tue, 06 Sep 2005 09:49:37 +0000, T i m wrote:

Hi All,

I had another look at Mums 'old but going to br replaced soonish., the
pilot won't stay alight' Ascot / New World (Main?) gas boiler again
yesterday and think I found the upper limit cutout / inhibitor (?) had
triggered? I pressed what looked like a rest button in the middle of
it but as didn't have any real tools / meter with with me (only the
trusty Leatherman as always) I couldn't measure anything so shorted
the extra stat out (for testing) and tried to light the pilot.

Hold in the contril knob, hit the piezo, nice little pilot flame, hold
knob in 30 seconds, release, pilot stays alight (hah huh, so it was
overtemp cutout ..?).

Turn the gas knob round to 'On', pilot goes out?

Wait 3 mins, relight pilot, turn knob to 'On' pilot stays on this
time, turn mains power to boiler, main burner lights up but then main
pilot and burner goes out? Try this a few times with similar
(intermittent) results. So, it could just be that the thermocouple was
'marginal' and as they are 'cheap' ....

So, I removed the thermocouple (amazing what you can do with a
Leatherman PST) and this morning picked up a replacement. It's not the
'correct' part (long since obsolete) but sufficiently close where it
matters to 'do'.

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?

I'll see what happens when I ahem, sorry, 'the Corgi bloke', put's
it back later... "You know anything about parachutes ...?" ;-)

All the best ..

T i m

An open circuit voltage of 25mV would indicate a working t/couple.
The gas valve solenoid+t/couple might typically total 1 Ohm thus giving
about 25mA current.

If there is some doubt about the t/couple and it gives 25mV when heated,
I'd reckon on it being OK.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

On Wed, 07 Sep 2005 23:26:59 +0100, Ed Sirett
wrote:



An open circuit voltage of 25mV would indicate a working t/couple.

That's what I think I discovered when I compared old with new? I
thought I was going to see a vast difference (considering how old it
could have been) but apparently not?

The gas valve solenoid+t/couple might typically total 1 Ohm thus giving
about 25mA current.

If there is some doubt about the t/couple and it gives 25mV when heated,
I'd reckon on it being OK.

As I mentioned on the OP, the results were intermittent with the pilot
staying on for 10 mins (before we did summat that upset it) or not at
all?

In hindsight I don't like the condition of the spade connectors
between the thermocouple and the overtemp stat (something I can
resolve when making the new longer cables to fit the 'different
design' replacement thermocouple that I have tacked in (and and seems
to work?)).

I might even pop the origional one back in and see what happens (it's
all easy to get at).

All the best ..

T i m

In article ,
T i m wrote:
[snip]
Wait 3 mins, relight pilot, turn knob to 'On' pilot stays on this
time, turn mains power to boiler, main burner lights up but then
main pilot and burner goes out? Try this a few times with similar
(intermittent) results. So, it could just be that the
thermocouple was 'marginal' and as they are 'cheap' ....

Very similar symptoms to those we had with our factory
hot-air heater, trying to relight it after being off
all summer. Perched on the top of a 12ft ladder in
my case.

Press and hold button, light the pilot, wait, release
the button, looks ok. Turn on the main burner, looks
ok..... then the whole lot goes out. Repeat, and repeat,
and repeat, etc..... until it finally works.

After a few years of this I finally cottoned-on to what
was happening. The main feed pipe had been shut off at
the meter, for about 6 months, and air had crept into it.
The solution was to simply purge the pipe beforehand.

--
Tony Williams.

On Thu, 08 Sep 2005 09:35:29 +0100, Tony Williams
wrote:

In article ,
T i m wrote:
[snip]

After a few years of this I finally cottoned-on to what
was happening. The main feed pipe had been shut off at
the meter, for about 6 months, and air had crept into it.
The solution was to simply purge the pipe beforehand.

At least you found a solution ;-)

So, I popped into Mum's again this morning with the new spade
connectors and silicon wire to make some new leads to go between the
over-temp stat and the thermocouple connector. I made one lead and
first used it to short the thermocouple connectors just to make sure
all was still ok .. it was. I then went to 're-dress' the thermocouple
neatly when I felt it 'give' where the thin copper tube joins the
bottom of the thermocouple itself (doh .. don't you just hate it when
that happens ..sigh). I'm normally very carefull with such things
(nice easy bend radisus etc .. not sure what happened there ..) ;-(

Anyway, I ripped out the new thermocouple and re-fitted the old one
(still not convinced that the old one was 'that bad') shorted the
inhibitor connector with one of my new leads and all was still ok ;-)

Re-fitted it all neatly, ran the two new leads between inhibitor
connectors to 'over temp' / inhibitor device and everything was sill
aok. So, I think it must have been bad (hr) connectors?

We put back all the covers / panels etc and ran the bolier properly
till it had fully heated the cylinder and all was still fine..

So, fingers crossed I suppose on this one .. will it now last till
it's replaced with the new bolier soonish ... (I feel it will)?

All the best ..

T i m

John Schmitt wrote:
On Tue, 06 Sep 2005 20:12:21 +0100, T i m wrote:

The current isn't real.

Doh!

Hmmmm. Yes and no. There is movement of electrons from one metal in a
differentially heated thermocouple to the other, so Yes there is a
current. I expect this varies with the imposed resistive load and tops out
somewhere, so meter readings may vary depending on their resistance.

Of course the meter is measuring the current in the circuit accurately,
but the measurement is not a reflection of anything useful.