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Another cooker grill problem


PineappleGuy

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Our Thetford Caprice Mark III grill is reluctant to stay on. I’ve checked that the thermocouple retaining nut is tight and that the prong is in the grill flame. I’ve crushed foil around the prong. I’ve even fitted a new thermocouple but whatever I do, it works correctly a couple of times before deteriorating steadily to the point where I have to hold in the knob for the entire time that the toast is cooking. Does anyone recognise the problem and know the solution?

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14 minutes ago, PineappleGuy said:

I’ve even fitted a new thermocouple but whatever I do, it works correctly a couple of times before deteriorating steadily to the point where I have to hold in the knob for the entire time that the toast is cooking.

 

You hold the knob in for a whole hour???!!!

 

But more seriously, there are only two components involved. The thermocouple and the electromagnetic valve in the gas tap. Given you've replaced the thermocouple, changing the electromagnetic valve would seem the next step. 

 

 

 

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5 minutes ago, PineappleGuy said:

Our Thetford Caprice Mark III grill is reluctant to stay on. I’ve checked that the thermocouple retaining nut is tight and that the prong is in the grill flame. I’ve crushed foil around the prong. I’ve even fitted a new thermocouple but whatever I do, it works correctly a couple of times before deteriorating steadily to the point where I have to hold in the knob for the entire time that the toast is cooking. Does anyone recognise the problem and know the solution?

We had similar problems with a Spinflo grill/oven combo, which is basically the same stuff and made by the same people. The problem turned out to be that the design relied of the cooker chassis to provide the return current path from the thermocouple to the gas valve. As the thing aged, the connections between the thermocouple and the chassis, the valve and the chassis, and the various bits of the chassis, increased in resistance. The voltage generated by the thermocouple is tiny, and it doesn't take much resistance to decrease the current below that needed to hold in the solenoid valve.

 

The fix was to run a wire from the mounting nut of the thermocouple to a mounting screw of the valve, which worked instantly. Be careful to use either uninsulated wire or wire with high temperature insulation or be very sure that the wire in securely held away from any components which will get hot. You don't want a PVC insulation fire.

 

After making this fix I happened to look at a new unit in the chandlers, and the same fix had been implemented in manufacture, so if your cooker is relatively recent, it may already have the earth return in place. In that case all you can do to be make sure the connections are clean and tight.

 

MP.

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1 minute ago, MoominPapa said:

The voltage generated by the thermocouple is tiny

 

Have you ever measured one? They typically bung out about 20V open circuit!

 

When short circuited by the short, fat electromag coil of approaching zero resistance I grant you, the voltage will fall to near zero. 

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Thank you for the instant replies. It's the fact that whatever I do seems to make it work properly but only temporarily. I'll look into the idea of running a wire from the mounting nut to the valve as I don't recall noticing one on ours (which is nearly 8 years old).

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16 minutes ago, MoominPapa said:

 

The fix was to run a wire from the mounting nut of the thermocouple to a mounting screw of the valve, which worked instantly. Be careful to use either uninsulated wire or wire with high temperature insulation or be very sure that the wire in securely held away from any components which will get hot. You don't want a PVC insulation fire.

 

After making this fix I happened to look at a new unit in the chandlers, and the same fix had been implemented in manufacture, so if your cooker is relatively recent, it may already have the earth return in place. In that case all you can do to be make sure the connections are clean and tight.

 

MP.

 

Couldn't you just have used some electronic contact cleaner? 

 

I recently had an issue with my gas fridge not staying on which turned out to be increased resistance between an electrical connection on a control knob. Apparently the voltage is so tiny that just a bit of oxidation of the connection can cause this to happen. 

 

In my case the fix was simply to hold in and turn the knob 20x to clean the connection, but the other suggestion from the gas fridge engineer I spoke to if that didn't work was to use some contact cleaner.

 

https://www.halfords.com/motoring/engine-oils-and-fluids/lubricating-and-penetrating-oil/halfords-electrical-contact-cleaner-500ml-621335.html

 

Edited by blackrose
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15 minutes ago, Tracy D'arth said:

Is the knob hitting the facia rather than going fully home on the magnet in the control valve? A ball of silver paper inside the knob to pack it out a bit will work if so.

Sounds interesting but can you clarify what you think might be happening? Does it explain why the behaviour tends to get worse each time I operate the grill?

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33 minutes ago, blackrose said:

 

Couldn't you just have used some electronic contact cleaner? 

 

 

Possibly, if I could have determined which contact was the problem. Given that the chassis is riveted aluminium there's dissimilar metal problems and corrosion to take into account too, adding a wire was the first thing I tried and it worked so I didn't explore other options.

 

MP.

 

45 minutes ago, MtB said:

 

Have you ever measured one? They typically bung out about 20V open circuit!

 

When short circuited by the short, fat electromag coil of approaching zero resistance I grant you, the voltage will fall to near zero. 

 

I've not. If they're 20v open circuit there must be quite a stack on junctions in there, individual thermocouple junctions are in the mV/Celsius range.

 

MP.

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6 minutes ago, MoominPapa said:

 

I've not. If they're 20v open circuit there must be quite a stack on junctions in there, individual thermocouple junctions are in the mV/Celsius range.

 

MP.

 

Interesting. That might explain their behaviour when failing. They don't fail in a binary manner, they work less and less well as they grow older. This I imagine, could be explained by junctions in the stack failing individually and shorting out, thereby delivering less current when heated to a given temperature. Is that feasible? 

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45 minutes ago, PineappleGuy said:

Sounds interesting but can you clarify what you think might be happening? Does it explain why the behaviour tends to get worse each time I operate the grill?

Pull the knob off and try it without, pushing the spindle fully in.  The metal facia can move out slightly when it gets warmed by the grill, stopping the spindle going in fully. 

Edited by Tracy D'arth
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2 minutes ago, MtB said:

 

Interesting. That might explain their behaviour when failing. They don't fail in a binary manner, they work less and less well as they grow older. This I imagine, could be explained by junctions in the stack failing individually and shorting out, thereby delivering less current when heated to a given temperature. Is that feasible? 

I don't know. I just tried to find a cross-section diagram of one to see if they do have a stack of junctions and failed miserably.

 

MP.

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30 minutes ago, MoominPapa said:

I don't know. I just tried to find a cross-section diagram of one to see if they do have a stack of junctions and failed miserably.

 

M

As far as I can remember its a single junction of nickle and some other metal, its been a long while since I did gas foundation!

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23 minutes ago, Tracy D'arth said:

As far as I can remember its a single junction of nickle and some other metal, its been a long while since I did gas foundation!

 

Silicon, surely?

 

I too thought they were one single junction but I dunno much about it.

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15 minutes ago, Tony Brooks said:

They are not semi-conductors, they are a pelter junction. Two metals as I understand.

 

Looks like we are both wrong. They are neither a Peltier cell nor a semiconductor junction, according to Wikipedia.

 

They work due to something called the Seebeck effect, this page says;

 

https://en.wikipedia.org/wiki/Thermocouple

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2 minutes ago, MtB said:

 

Looks like we are both wrong. They are neither a Peltier cell nor a semiconductor junction, according to Wikipedia.

 

They work due to something called the Seebeck effect, this page says;

 

https://en.wikipedia.org/wiki/Thermocouple

Correct, plain thermocouples but my list only includes temperature measuring thermocouples, not this type that generate a current.

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