How does a safety pilot work?

[despence]

I have a Best Fire 18" natural gas log set. It came with a safety pilot kit (pss101). My wife asked the other day "How does the valve know to turn off the gas if the flame goes out?" You gotta love a women who asks these kind of questions! Unfortunately, I didn't have a good answer. It was something like: you've got this pilot light that heats this thermocouple and when the thermocouple looses its heat sourse the flame goes out. That's sort of what's happening, I think, but a guy needs to be able to explain why it's happening! Can someone please fill me in on what is actually going on inside the safety pilot valve to make sure things stay safe!
Thanks in advance.
Don

 

[Shane]

The thermocouple is made of two dissimilar kinds of metal that expand & contract at different rates. This creates millivolt electricity which is used to power an electromagnet in the valve which allows the gas to flow. If the flame goes out the expansion/contraction stops, thus the production of millivolts stops and the magnet loses it's charge closing the pathway for the gas to flow.

 

[elkimmeg]

you did a good job describing no heat at the thremo coupler and the sesnsor signals the gas valve to shut off

 

[despence]

Thanks for getting back so quickly. That makes sense. Now I can proudly say "Hey Honey, I figured it out!" That's all right guys, after about an hour of her thinking I'm a genius, I'll fess up and let her know I asked the experts .

Thanks again,
Don

 

[hearthtools]

Just remember that the Thermocouple on your Gas log set is NOT the same as one on a HOT water heater or Older Wall heater.
They are much more sensitive than the ones use in the past.
They are what the industry calls a 30 second safety thermocouple.

 

[Shane]

Good to point that out Rod. You definately don't want a thermocouple that will be dropping out up to 2 mins later in a system like that!