26/03/2026
Gas monitors… it’s as simple as turning them on and off you go, right?
Wrong.
We still see it all the time.
-Clip the monitor on.
-Press the button.
-Wait for the numbers.
-Off you go.
It feels like due diligence. It looks like safety.
But it is not the full picture.
A gas monitor does not make you safe.
Understanding what it is telling you does.
It turns on… but is it telling the truth?
A startup sequence does not prove the sensors are working.
It just proves the device has power.
If it has not been bump tested, you do not actually know:
-if the sensors respond to gas
-if the alarms will activate
-how quickly it will react when things change
That last point matters more than most people realise.
Response time matters more than people think
Gas monitors do not detect hazards instantly.
Every sensor has a response time, often referred to as T90. That is the time it takes to reach 90 percent of the actual reading.
That means you could already be in a hazardous atmosphere while your monitor is still catching up.
Now layer in real-world conditions:
-long sampling lines
-blocked or dirty filters
-cold environments
-high humidity
All of these can slow response further.
So when someone drops a probe in, waits a few seconds and says “she’s right”, there is a fair chance the monitor has not even finished responding.
What are you actually looking for?
A monitor is only as good as its target gases.
Most standard units look at:
-oxygen
-flammable gases (LEL)
-carbon monoxide
-hydrogen sulphide
But what if your risk is:
-carbon dioxide
-ammonia
-refrigerants
-solvents
If it is not configured to detect it, it will not alarm. No warning, just false confidence.
LEL readings are not always what you think
Flammable gas sensors are typically calibrated to a reference gas, often methane.
But different gases burn differently.
That means the reading you see may not represent the true percentage of the lower explosive limit for the gas actually present.
This is where correction factors come in.
For example:
-a monitor calibrated to methane may under-read or over-read when exposed to petrol vapours or solvents
-10 percent LEL on the screen might not actually be 10 percent LEL for that substance
Without applying the correct factor or using the right sensor, you are making decisions based on the wrong number.
Ventilation can give you a false sense of security
-Mechanical ventilation is critical, but it can also mask a hazard.
-You might see safe readings at the entry point while hazardous pockets still exist deeper in the space.
-You might also see diluted readings that sit just below alarm thresholds even though the source of the hazard is still present.
-Ventilation does not remove the need for testing. It changes how you interpret the results.
Cross sensitivity is often overlooked
Sensors are not perfect.
Some gases interfere with others and can cause:
-false positives
-false negatives
-misleading readings
For example, a carbon monoxide sensor may respond to hydrogen. Hydrogen sulphide sensors can be affected by other sulphur-based gases.
So the number on the screen might be real, just not for the gas you think it is.
So what does good look like?
Before relying on a gas monitor, you should be confident that:
-it has been bump tested
-it is within calibration
-it is configured for the actual hazards present
-you understand sensor response times
-you are allowing enough time for sampling
-you understand LEL correction factors where applicable
-you are testing in the right locations
-you are interpreting results in context
Want to know more... Check out our Gas Test Atmosphere courses at PARCOR
This course applies to persons who are required to perform gas testing prior to entering or conducting works in a hazardous area or confined space.
