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Thermocouples: safe & silent failures


Captain! The thermocouple is failing, the engines are down and we can’t start them until it’s fixed!

Sounds cool, right? This humble safety device isn’t that far-fetched. If you have a gas-powered hot water heater, you have one of your very own. It solves a dangerous problem with pilot lights. A pilot light is a small flame that is kept burning indefinitely inside your gas furnace or hot water heater. It gets a tiny trickle of gas from the main supply using barely enough volume to show up on your bill.

The engineers in the audience will ask, what happens when the pilot light goes out? Without a thermocouple, it will seep tiny bits of gas into the combustion chamber of the device. In a few hours, it can leak enough gas for a small explosion, probably just enough to shorten your eyebrows. If you were on vacation in Tahiti for a month and there is no circulation, enough gas could build up to some serious damage to homes and humans.

The engineering requirements are interesting:

  • no external electricity is available
  • has to be reliable
  • has to work in a multitude of physical environments, from indoor closets to filthy barns
  • must behave consistently across a large temperature range
  • cannot require regular maintenance
When you need something reliable, you need something simple. Enter the thermocouple.

The science of how a thermocouple works is described on Wikipedia. I wanted to send that to people, but it requires a lot of background in physics to even parse what the article says. While the physics in play are fascinating, at the end of the day it’s a simple device. The trick is in the first sentence on Wikipedia,

“In 1821, the GermanEstonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage.”

What does this have to do with hot water heaters? We need a way to detect when the pilot light has gone out. No electronics, or even electricity is available. Natural gas burns at 900-1,500° Celsius. You don’t even have to convert that to Fahrenheit to see that we have a considerable temperature delta at our disposal, so there’s an opportunity to use the Seebeck effect to create a safe and reliable device.

In layman terms, the thermocouple found in household hot water heaters is a probe that is placed inside a flame with two wires made of different metal. This creates a temperature gradient, since the metals disperse heat at different rates. This generates a few microwatts of current, which is boosted by a transformer connected to a solenoid. When current is present, the solenoid holds the gas valve open. When there is not enough current, the valve is pushed closed by a spring, so most failures will lead to the gas being turned off, including the thermocouple itself failing. The device itself is intended to act like a fuse or breaker, giving up before any other component resulting in a known-safe condition.