Maker Pro
Maker Pro

Carbon monoxide monitor calibrator

MIKE1

Oct 5, 2010
6
Joined
Oct 5, 2010
Messages
6
Hello my name is Mike and I am new to electronics. I have been a chief electrician for an underground coal mines for 10 years so I have a good electrical base just not much in electronics. Anyway here's my problem, I'm wanting to make a trouble shooting device using led's for a carbon monoxide monitor we use, to help some of my electricians when working on them. They have a 28 VDC supply so I have a good source for my led's. They have a pin on them that when read to ground has 235 millivolts at 0% CO. This is what I need to turn the led on because we have a potentiometer that we have to adjust occasionally. I thought of just simply using a transistor to turn on the led but then I found out that most of them have to see 700 millivolts to turn on (remember i'm new). Any help someone could give would be greatly appreciated thanks
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
The main problem with building equipment of your own that you use to calibrate other equipment (especially that where failure can lead to death) is that you not only have to calibrate it, but you have to somehow determine its stability, since the calibration of any instrument is no better than what you are calibrating from.

I would want to be very sure that I was not breaking any of dozens of rules and regulations in this area before I did this. In the worst case you could be found liable if someone dies as a result of CO poisoning.

Having said that, the solution is to use a comparator and have a stable source of 235 mV with which to compare the signal to. The output simply drives a LED.

Your 28V neatly becomes +/- 14V and as long as the CO detector's output is referenced to your 0V rail (the middle one) or as a second preference the -14V rail (0V in your original 28V rail) then you shouldn't have too much difficulty.

The hardest parts are the voltage reference (and only for stability and adjustability) and possibly the splitting of your 28V rail.
 

shrtrnd

Jan 15, 2010
3,876
Joined
Jan 15, 2010
Messages
3,876
I'm guessing you want the LED to make it easy for your techs to adjust the zero.
Doesn't this monitor have a visual display? Use that.
I used to repair these instruments for a mine in Arizona, and also for water and gas utility companies. Don't screw around with them, somebody dies, they'll come looking for you.
I don't know your situation.
If you're not now, you need to buy the calibration gases (nitrogen for your zero), and the
concentration of CO your instruments are set to. Whoever made the instrument, will
have the cal gases.
If you don't have a preventative maintenance schedule for the monitors, make one, and stick to it.
Mike1, you CAN'T use an outside calibration source for your instrument, only the factory can do that. YOU, as the end user, need to rely on the accuracy of the monitoring instrument itself; using the cal gases to verify proper operation.
Think about it. Something goes wrong with whatever LED device you're using, (anything at all, leaking transistor, voltage problem, anything, ...), and your guys set your zero at the wrong value, your alarm trip point will be wrong on the monitor.
I can see your concept just fine, good idea normally, to ease operational maintenance.
Can you see my point? ANYTHING goes wrong in cal, and something goes wrong in the mine or a confined space, and you lose somebody; how are you going to feel?
Rely ONLY on the instrument and the cal gas trip points, NO outside checks or attachments.
PS: Consider buying an electronics tech, with all your electricians.
 

MIKE1

Oct 5, 2010
6
Joined
Oct 5, 2010
Messages
6
Thanks for the posts, and trust me I know first hand about the dangers of tampering with our instruments. We have all of the factory calibration procedures in place and I personally calibrate them according to the manufacture and MSHA regs. The setting I am trying to help my men with is after you have applied a zero air source the pin I mentioned should read .235 vdc to the system ground. This is normally not a problem since I usually do all work on them but occasionally they will get out of adjustment. The sensor gives an alarm outside telling that there is a sensor fault at the location. A man will go to it, it will read 00 just like it should, the will check with their personal gas spotter to verify there is no CO then proceed to troubleshoot. Some of the men have no problems but then I have some that still won't switch to a digital meter. All of the adjustments are done exactly as the manufacturer states and this is their recommendation to fix this sort of problem. The only thing I am trying to do is replace the meter with a small box with some led's to tell someone they met the correct voltage. Thanks for any help and I am sorry if I made the first post unclear as to my intentions.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
A comparator is kinda like an op-amp but designed for comparing voltages rather than amplifying signals.

(practically speaking, you can use an op-amp as a comparator, or a comparator as an op-amp, but they won't be optimal if used that way)
 

shrtrnd

Jan 15, 2010
3,876
Joined
Jan 15, 2010
Messages
3,876
I figured there was a good reason you're the boss.
I don't mean to sound like a pain, but it's the same problem with relying on the LED's to tell your people if the zero voltage is right.
How's about, being the boss, you tell your people they can keep their analog meters, but if the do that, they can't work on the gas detectors. Or keep one DVM specifically available for work on the gas monitors. .235VDC is a pretty tight spec, and you could be sure it's met with the digital meter.
Just my two cents. Good luck with whatever you do.
 
Top