Here at Becker Wholesale Mine Supply, we support fixed gas monitors in underground mines, and one complaint comes up more than any other. “The CO channel keeps alarming, but the handheld reads zero.”
We take that call seriously every time.
A monitor that alarms for no reason gets ignored, and a crew that has started to shrug at alarms is a crew in danger. A monitor that reads low is even worse. So here’s how we work through gas monitor troubleshooting, in the order we’d actually do it.
Key Takeaways
- Pull the alarm history before anything else.
- An electrochemical CO sensor can read hydrogen as if it were carbon monoxide, and charging batteries give off hydrogen.
- An expired test cylinder may no longer hold the concentration printed on its label.
- Every sensor has a shelf life, and no amount of cleaning brings back a cell that’s spent.
First, Leave The Settings Alone
We know the temptation. Raise the alarm point, make the noise stop, get back to work.
Please don’t.
Pull the alarm history before anything else. Our SmartSense Fixed Monitor stores calibration records, alarm history and logged data, so you can see when each alarm started, how high the reading went and how long it lasted. Look for a pattern in the times, because patterns point to causes.
For example, if every alarm starts within twenty minutes of a scoop battery going on charge nearby, you’ve just learned a lot.
Check For Cross-Sensitivity
Short version: a sensor can be honest and still wrong.
Sensors are built for one target gas, but plenty of them respond to others. This trips people up constantly.
An electrochemical CO sensor can read hydrogen as if it were carbon monoxide, and charging batteries give off hydrogen. Diesel exhaust carries nitrogen oxides that can push some sensors up or down. Catalytic methane sensors can respond to hydrogen and propane.
So the monitor may be telling the truth about its own cell. It just isn’t seeing CO.
To confirm, bring a second instrument with a different sensor type, one that won’t react to hydrogen. Park it beside the fixed unit during the next charge. If the fixed channel climbs and the second instrument stays flat, you’ve found your culprit.
But keep checking anyway, because one cause can hide another.
Look at Calibration Next
We see this one a lot in Greensburg calls. Somebody swears the unit was calibrated. It was. Last spring.
Open the calibration record and ask two questions. When was the last calibration, and what gas was used?
An expired test cylinder may no longer hold the concentration printed on its label. That shifts every reading that comes after it.
Calibration has two moves. First, clean air or nitrogen sets the zero point. Then a certified test gas at a known concentration goes on, and the monitor is adjusted to read that value.
Also look at how big each adjustment was. A sensor that needs a bigger correction every month is drifting, and drift usually means age. Small, steady corrections are what you want to see.
Mines are rough on sensors, and we mean really rough.
Water spray and high humidity can block the membrane on an electrochemical cell. Rock dust clogs the sintered metal filter that guards the sensor head. A blocked inlet usually makes a sensor sluggish.
Time the response with a bump test. That’s a short shot of gas to confirm the sensor responds and the alarm sounds. It doesn’t change the reading. If the response comes back slow, clean or replace the filter and test again.
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In a hurry? Call us at +1-724-515-4993
Don’t Forget The Other Channels
While you’re at the unit, check every channel, including the ones that didn’t alarm.
Methane channels often use a catalytic bead. Beads can be poisoned by silicone sprays, some lubricants and sulfur compounds that coat the catalyst. Here’s the scary part. A poisoned bead may still pass a zero check while reading far too low in real methane.
That’s why bump tests with actual gas matter more than a glance at the display. Bump the methane channel too.
Temperature can play tricks on readings. When equipment moves from a warm shop into a cold intake airway, readings can jump for a moment before they settle.
Usually it’s harmless. Still, write it down if you notice it, especially if a machine gets brought in from the shop right before a recurring alarm.
Pick a Real Fix
Say you’ve confirmed the CO cell is healthy and well calibrated, and the alarms come from hydrogen drifting downwind from a charging station.
What now?
Raising the alarm point would blunt the monitor’s real job, which is catching a belt or equipment fire early. That’s off the table for us.
There are honest options, though. You can move the charging spot. You can move the monitor upwind of it, using the ventilation plan as your guide. You can fill that channel with a sensor that has less hydrogen response. Or you can add a hydrogen channel so the system can tell the two gases apart.
If you move the monitor, check the new spot against the ventilation plan and the placement rules. Bump it again in its new home and log the change on the mine map.
Then watch the alarm history for a month. A clean history at charging time means the fix worked.
This is the habit that makes gas monitor troubleshooting faster next time.
Put a note in the record that says what you found, what you tested and what you changed. The next technician might be on a different shift and see an odd reading at that same spot. Your note saves them a day.
When Is A Sensor Just Worn Out?
Every sensor has a shelf life, and no amount of cleaning brings back a cell that’s spent.
Electrochemical cells often last a few years. A few signs point to replacement: slow response in bump tests, failure to reach the test gas value during calibration, and readings that wander at zero.
Patterns help here as well. A sensor that fails calibration after every wet season probably needs a better splash guard more than a new cell.
You can see the SmartSense Fixed Monitor, read about reliable gas sensor detector systems and learn how to upgrade mine safety with an underground gas detector.
GET IN TOUCH
In a hurry? Call us at +1-724-515-4993
Need a Second Opinion?
Call Becker Wholesale Mine Supply or reach us online for help with gas monitor troubleshooting.
Frequently Asked Questions
Q: How often should sensors be bump tested?
A: Follow the manufacturer’s instructions and your site rules. Many mines bump test before use or on a set schedule.
Q: When should a sensor be replaced?
A: When it fails a bump test or will no longer hold calibration.
Q: What should I do if a fixed gas monitor gives a false alarm?
A: Follow your mine’s alarm response procedures first. Then review the alarm history, check for possible cross-sensitivity or nearby sources, and verify the reading with a suitable second instrument. Don’t raise the alarm threshold to silence it.
Q: Why might a gas monitor respond slowly?
A: Dust, moisture or a blocked filter can restrict gas flow to the sensor. Check the inlet and filter, then perform a bump test according to the manufacturer’s instructions and your site procedures.
Q: Can I use an expired calibration gas cylinder?
A: No. The gas concentration may no longer match the label, which can lead to inaccurate calibration. Use a current, certified cylinder with the correct gas and concentration for the sensor.
