Hydrogen in the gas — what it does to your detection

Most flammable gas detection in the UK was selected, calibrated and positioned on one assumption: the gas is natural gas. Hydrogen breaks that assumption. Not in the place most people look, but in several places they don't.

Hydrogen is already in the gas you're testing

This is not a future problem. Coke oven gas in steelmaking is more than half hydrogen. Refinery and petrochemical process streams carry it. Battery rooms release it during charging. Many power station generators are hydrogen-cooled. And blending hydrogen into the gas network is under active government consultation, with transmission trials already run at 2%, 5% and 20%.

If your people test atmospheres on any of these sites, hydrogen is a gas their instruments need to see.

The LEL isn't where the risk is

The usual reassurance goes like this. Hydrogen's lower flammable limit is 4.0% by volume. Methane's is 4.4%. A 20% hydrogen blend sits at roughly 4.3%. So nothing much changes.

That's true, and it's the wrong question. The differences that matter are elsewhere:

• Upper limit. Hydrogen stays flammable up to 77% by volume. Methane stops at 17%.
• Ignition energy. Hydrogen ignites with more than ten times less energy than methane. Sources that would never light natural gas can light hydrogen.
• Density. Hydrogen is around 14 times lighter than air. It rises and disperses fast, and it collects at high points.
• Equipment group. Hydrogen is Group IIC. Methane is Group IIA. Equipment certified only for IIA or IIB is not suitable.

Infrared detectors cannot see hydrogen

This is the one that catches people out. Infrared (IR) sensors work by measuring how much infrared light a gas absorbs. Hydrocarbons absorb strongly. Hydrogen, a molecule of two identical atoms, absorbs none.

An IR sensor will read zero in a flammable hydrogen atmosphere. In a 20% blend, a methane-calibrated IR sensor only sees the methane, so it reads roughly a fifth low.

IR has been widely adopted because it doesn't suffer catalytic poisoning and keeps working in low oxygen. Those advantages are real. But on any site where hydrogen is present, an IR-only flammable channel has a blind spot.

Catalytic sensors can see it

The catalytic sensor oxidises flammable gas on a heated element and measures the heat released. Hydrogen oxidises readily, so a catalytic sensor responds to it.

Its strength has always been that it responds to almost any flammable gas. The caveat is that its response to hydrogen is not the same as its response to methane. A methane-calibrated catalytic sensor reading hydrogen needs the manufacturer's correction factor, and that factor differs between instruments.

Your toxic channels may be reading it too

Many electrochemical carbon monoxide cells respond to hydrogen. On a hydrogen-bearing site, the CO channel can alarm with no CO present. The size of the response depends on the cell. Some are built to compensate for it, many are not.

The real risk is behavioural. A CO channel that alarms for no apparent reason soon gets ignored. Photoionisation detectors (PIDs) don't help either: hydrogen's ionisation energy is beyond the reach of standard PID lamps.

Fixed detection: height and ignition

Fixed heads positioned for natural gas may be in the wrong place for hydrogen. A hydrogen release rises quickly and pools at the highest point of an enclosure, often above the heads. Combined with the low ignition energy, a gas that rises past your detectors to an ignition source is the scenario to design out.

What to check

1. Does any atmosphere you test contain hydrogen, now or under planned changes?
2. Which sensing technology is in each portable instrument and each fixed head?
3. What does the manufacturer say about hydrogen response and cross-sensitivity for each sensor?
4. Are fixed heads positioned and certified for a lighter-than-air, Group IIC gas?
5. Do the people wearing the monitors know which channels will and won't respond to hydrogen?

The last question is the one most sites can't answer. Instruments don't fail in these situations. People act on readings they don't understand.

JMS gas detection assessments review exactly this: the gases on your site against the technology you've deployed. Our Gas Monitor and AGT courses teach operatives and authorised gas testers what each sensor type can and cannot see. Talk to us about either.