How do thermocouples work?

[teetering-on-the-edge]

How can I test a thermocouple controlled 'gas valve' on a gas cooker .
I've just ( over a month ago now) installed a second hand range cooker, and just cant get the hob to stay lit.
i've tried a (two) new thermocouples , Universal ones from ebay £2 not the manafactures £30, but no joy.
Im doing the thermocouple up nice an tight at the valve end.
The hot end is sitting in the flame.
I know 'thermocouples' are not real (junction) thermocouples but can some clarify for me :-

1) Are they capillary tubes?
2) Do they push on a diaphram in the valve to keep it open?
3)When I hold the thermocouple in the flame should I see the other end move ?

If this is right , can I test the valve by pressing hard on the diaphram bit of the valve?
Thanks.
Dave.

 

[kevplumb]

http://www.thermocouplesensor.co.uk/

 

[teetering-on-the-edge]

Cheers kevplumb,
But these are the 'proper' junction thermocouples, not the copper tube thing Ive got.
Dave

 

[kevplumb]

ok if it knocks owt atween 30 and 13 mv out it should hold the valve in

 

[teetering-on-the-edge]

Kev
mv? Are you talking electrics?
We might be talking about different things here.
This is a 3mm copper tube with fat end that sits in the flame and the other end that goes into the gas valve is just flared out a little with a silver coloured middle and held in with a M8 nut.
Where do I measure?
Dave

 

[Bamber gaspipe]

Kev
mv? Are you talking electrics?
We might be talking about different things here.
This is a 3mm copper tube with fat end that sits in the flame and the other end that goes into the gas valve is just flared out a little with a silver coloured middle and held in with a M8 nut.
Where do I measure?
Dave

You could always go mad & do a gas course.If you don`t know how a safety device operates then maybe you should leave it to someone who does

 

[teetering-on-the-edge]

Im only asking how it works.
If it 13 to 30 millivolts between the inner and the outer just say so.
I will understand. If everbody has to do a course when they want to know how something works, whats the point of a forum?
I would love to do a gas course, plumping course, plastering course, cookery course but unfortunately I'm employed. Yep I would gladly leave it somebody else, but Im tired of being ripped off.
Dave

 

[Bamber gaspipe]

Your messing with a safety appliance & you are incompetent. Sore point around here Guy. Where is Water systems when you want the prat?

 

[teetering-on-the-edge]

Ok so you dont know.
So why bother replying?

 

[Bamber gaspipe]

Ok so you dont know.
So why bother replying?

I do. you don`t. I wasn`t replying, I was telling you how it is.

 

[kevplumb]

youve already had your answer felle

 

[sambotc]

A thermocouple when heated create's an electrical current which in turn powers a thermo electric valve by supplying enough current to keep the valve open, which is why when your flame is lit the valve stay's open.

I agree with the above comment, these kind of things shouldn't really be touched by DIY'ers but seeing as you haven't broken the gas suppy you are effectively not commiting any offence.

If your sure you have correctly fitted a replacement thermocouple then it would suggest your gas valve/thermo electric valve is knackered which would require you by law to get a corgi registered engineer in to replace as i will mean breaking the gas supply to replace.

Hope that helps, Sam (corgi installer for reference)

 

[kevplumb]

If your sure you have correctly fitted a replacement thermocouple then it would suggest your gas valve/thermo electric valve is knackered which would require you by law to get a corgi registered engineer in to replace as i will mean breaking the gas supply to replace.

*******S

 

[mehran]

Im only asking how it works.
If it 13 to 30 millivolts between the inner and the outer just say so.
I will understand. If everbody has to do a course when they want to know how something works, whats the point of a forum?
I would love to do a gas course, plumping course, plastering course, cookery course but unfortunately I'm employed. Yep I would gladly leave it somebody else, but Im tired of being ripped off.
Dave

when you are paying someone you are paying them to know stuff like this, all knowledge has a price.

a thermocouple is a safety device, in my view as it is involved in the safe burning of gas should not be messed around.

i don't begrudge you trying to save money but just a corgi engineer which is competent in cookers.

 

[doyle]

recomend you get a gas man to do the job, the thermocouple proberbly works, as you have already done it.
to test your thermo couple light the appliance and turn it off again a moment or two later and listen for a click, if you hear the click then that is the thermocouple working.
this can take a minute or so to activate, and closes the gas supply to the oven.

this is why you pay a corgi guy

is the thermocouple activating within a specific time scale permissable for cookers.
when you have tightened the thermocouple did you check for gas leaks if so how you re not allowed to use fairy as it is corrosive nor are you able to hold a flame to it.
the capilary tube contains mercury, which is hazardous and tube can be damaged

a corgi would know how to test the thermocouple and for any other problem and leaks.


the thermocouple is a safety device and is their for your safety call a corgi guy the law requires it.