I hear you - LOL had a similar conversation with the wife a few mins ago. However I think it may ve doable. This is a sticker type almost flat thermocouple used on car exhaust manifolds. We could stick them to the underside of the burners, usually there is a massive void under the burners once you take the top off. However at 43 a pop, its a bit steep - however this means that if we can find a cheap high temp thermocouple we could take it to the bottom of the burner with a high heat tape or other bonding stuff you get for cars. We only need to get up to 700 C or so.
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This may work - https://amzn.to/3hFWpVu - these have tiny heads and rated to 700C. And this adhesive - https://amzn.to/2X5J6nF
I may try it out next time I get some time.
I really like the door sensor idea because it detects gas leakage as well, but my range design doesn't allow for that.
I can already see the doghouse that I would inhabit if I spent $250 on thermocouples... 
Yeah, I like the contact sensors because if the gas is on but not lit it still gives us a warning. This mod is one of the few that she likes so I will take that money to the bank. 
I am just starting to play around with ST_Anything with nodemcu integrated into hubitat, and a whole new world of being able to sensor practically anything and feed it into hubitat is appealing. If people have found a solution to this problem and are happy with their solution, that is great. HubDuino and nodemcu have a pretty steep learning curve and it is not for everyone. (I am still trying to wrap my head around parts of it, but as an old dog I like the challenge, LOL)
I have a few HubDuino projects that are more pressing than a gas stove sensor. But once I am done, I will put a CO2 sensor completely hidden in the range hood and wire the sensor (completely hidden) to a nodemcu esp8266 board. To me this has several advantages as a solution:
- mains powered, no batteries
- Replace the stove, sensor still works
- And most importantly, respecting the fire rating of the gas appliance. Anything installed inside a gas stove has to be fire rated. If you look at one of the solutions in this thread, where contact sensors where used behind the control panel, everything behind this control panel is high temp rated, including the wiring. I guarantee a battery powered plastic contact sensor is not. Having said this, everyone has their own comfort level with code compliance and each to their own.
- Cost, hard to believe how cheap a esp8266 board actually is, along with the sensors. I am currently working on 5 window sensors, one water meter reader, and one smoke/co integration. Bought everything I need for this project, including the window sensors for $25 CDN. I imagine in the USA the cost would be around 15 dollars. When I get around to the CO2 sensor in the range hood, I will probably still have room on the esp8266 board so the cost will only be the sensor, probably find one for 5-10 bucks.
The two links I posted above for the glue and sensors are fire rated and 17 bucks for 5. I am on the same page as you on everything inside being fire-rated - my plan was to mount the controller outside. I like the idea of adding a c02 sensor.
Now to find the time. I recently bought parts to build a depth sensor for salt in my water softener, but havent had time to look at it.
Just FYI, here are what I learn when I am working on my project. I am hoping the help everyone here before you purchase your thermal component.
Thermocouple and Thermopile is mixed heavily in here and in marketplaces such as Amazon, Ebay, etc. To me they are different. The fundamental working of Thermocouple and Thermopile is exactly the same. Thermocouple generate mili/micro volt depending on the heat applied to it. Measuring the mili/micro volt with amplifier+adc can be used to calculate temperature applied to the probe. I am not aware any electronic design that can boost this small voltage today to be used to power an MCU. Thermocouple mostly use as temperature measurement device.
Thermopile takes many cells of thermocouple and put them in series. They are put in series such that it can generate much higher voltage. It seems today the availability of thermophile that generate 750 mili-Volt is abundant. I think probably because of the application in fireplace. Thermopile is not used to measure temperature in general. Keep in mind that 750mili-Volt is not enough to power any modern MCU as is. In addition, the current generated is very small. In another word, the power generated (P=IxV) is very small. Additional circuit and power saving strategy is needed to use the common thermopile. Having said that, thermopile is used as power generation. It is unlikely used as temperature measurement.
Unfortunately, many vendors advertise one while actually they are selling the other one. Please be careful and ask question to your vendor when you are buying either thermopile or thermocouple. It depend on what you want to do. If you want to detect your gas stove by measuring the temperature, you would get a thermocouple. You have to manage your power supply either by battery or DC power.
I personally use thermopile to power a device that I am working for so that I do not need to deal with battery or routing a DC supply.
There is also a "rod flame" sensor(not IR based). The rod becomes a diode when it is exposed to flame. It is act as open circuit when there is no fire. An AC voltage (high voltage, very small current) need to be applied to the rod. When flame is detected, because it become a diode (with proper electronic design called diode rectifier), it can convert that AC voltage to DC voltage. There should be a circuit that would detect this DC voltage to determine whether flame is detected.
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No one here is talking about powering an arduino/nodemcu board with a thermocouple. But rather just having the arduino/nodemcu board read the milli voltage (temperature) and have this sent to hubitat. An ad595 is a electronic device that is designed for this purpose.
How about MAX6675 instead of AD595? seems to be much cheaper.
