Broken one hung low gas sniffer (pics added)

[jtap]

So I bought this cheap(er) Mastech gas sniffer a couple of years ago to help find a leak. It did work at the time. It has been sitting in the garage for a couple years. A use for it popped up and I go to test it and...it doesn't work anymore. With nothing to lose I crack it open. The behaviour is that it alarms when you turn it on. Before it would not alarm until you turned up the sensitivity dial.

I checked the dial pot. When it's off, the pot seems to work great (B503 label - from ~2 ohms up to ~.50k ohms). When the unit is turned on...the pot is open and turning the dial doesn't do anything. I start to get out of my depth at this point.

The pot has 3 legs. Two of them are connected to this voltage reference diode (D4 on the board) LM385 Z-2.5 (http://www.ti.com/lit/ds/symlink/lm385-2.5-n.pdf). Pins 1 and 2 are connected to the pot. The 3rd pin is open.

I don't fully understand what this voltage reference diode is, how to confirm if it's working or why the pot is open when the unit is turned on. My thought was that something was shorted to ground. I tried to check all the 3 legged transistors for shorts and I re-soldered all the legs as a shot in the dark.

It would be cool to fix this and learn something. I do feel it might be a bit beyond my knowledge though and I thought to reach out to this community and maybe someone can help me. I would appreciate it if so. I don't care much about the unit. It would just be nice to figure out what was broken and learn something in the process. Thanks for any guidance in what to do next.

[pbarton]

Welcome to the forum.
I don’t know the model of your Combustible GAS Detector, but the current one appears to be the Mastech MS6310.
You can download the manual from the ‘Support’ tab at…
http://www.mastech-group.com/products.php?cate=116&PNo=112#_
The manual states that if the alarm sounds continuously, then the sensor may not be stable.
Many Gas detectors utilize an ‘Electrochemical Fuel Cell’, I’m not sure that your does? As different detection technology exists. Perhaps you could confirm?
Typically Electrochemical Fuel Cells have a finite life, after two years it’s probably dead.
Here is an article on them…..
http://www.hazardexonthenet.net/article/128977/Understanding-gas-sensor-lifespan.aspx

[jtap]

Yeah, 6310. That's the same deal. Thanks for the reply.

Interesting. I guess the circuitry could be reporting that the sensor has become unstable. Thanks for pointing that out. Not a very good tool to buy if you aren't going to use it much during the first year or two then (like me). Whoops!

It kind of feels like they should make the sensor portion more replaceable...certainly without having to de-solder and solder in a new one.

Now this has me thinking about my home CO2 detector. I wonder how old that is and how often it needs to be checked/replaced.

[pbarton]

Slightly different GAS sensor technology, datasheet here…
https://www.pololu.com/file/0J309/MQ2.pdf
Read the last page, Para 2.3 ‘Long time storage’

[mikerj]

Yeah, 6310. That's the same deal. Thanks for the reply.

Interesting. I guess the circuitry could be reporting that the sensor has become unstable. Thanks for pointing that out. Not a very good tool to buy if you aren't going to use it much during the first year or two then (like me). Whoops!

It kind of feels like they should make the sensor portion more replaceable...certainly without having to de-solder and solder in a new one.

Now this has me thinking about my home CO2 detector. I wonder how old that is and how often it needs to be checked/replaced.

The datasheet implies that the drift will reverse when powered up, but will take a long time.

[fzabkar]

Now this has me thinking about my home CO2 detector. I wonder how old that is and how often it needs to be checked/replaced.

CO detectors are designed to last for 10 years, after which they are programmed to stop working. This finite lifetime is mandated by the standards.