The Problems with Calibrating to Pentane

Two units respond to alarm bells at a school after a caretaker reports a suspected gas leak. The local gas company is on the scene and refuses to enter the building due to elevated lower-explosive-limit (LEL) readings. The ladder crew begins to investigate with a multi-gas detector.

There is no perceptible odour, and the multi-gas detector indicates zero per cent LEL. However, the gas company is still getting elevated readings.

If you’re a firefighter, is this the type of call to which your department might respond? Which detector do you trust? What do you do next?

The crews on scene bring in a second detector from the rescue unit. This solid-state combustible-gas-leak detector uses different technology. This detector immediately begins to detect gas, and as readings elevate, the detector auto-scales from parts-per-million to several per cent LEL.

Yet the multi-gas detector still indicates zero per cent LEL. Using the solid-state sensor, crews trace the source of the leak to an old boiler room; they discover an uncapped pipe that runs directly from the sewer, which is allowing methane gas to escape.

Methane is naturally an odourless gas. Gas utility companies add mercaptan, a harmless gas that has a rotten-egg odour, to natural gas to warn residents of leaks.

This incident prompted further investigation. Why did the multi-gas detector show zero per cent LEL, while the solid-state detector and the gas company’s detector indicated elevated LEL readings?

The answer lies in how an LEL sensor operates and how calibration gas can affect its performance.

LEL sensors use the principle of catalytic oxidation, which generates heat. Essentially, the catalyst inside the LEL sensor “burns” the gas being sampled. As heat is generated, resistance across the sensor increases, which causes a change in voltage across the sensor. The change in voltage is proportional to the gas concentration.

Why is this important? Different gases burn at different temperatures. Methane ignites at 537 degrees Celsius, while pentane needs a temperature of only 260 degrees Celsius to burn.

The sensor needs almost double the amount of energy to detect methane as it does for pentane.

Think of sensors like a battery: they gradually lose power over time. Therefore, it is possible that an LEL sensor with a degraded catalyst would respond to pentane during calibration and bump testing, yet might not respond to methane. The detector will essentially be blind to methane and possibly other high-ignition temperature gases such as hydrogen and propane, which burn at 500 and 455 degrees Celsius respectively, while appearing to operate normally.

There are three main reasons why an LEL sensor may lose sensitivity to higher-ignition temperature gases: an aging sensor; the sensor may be physically damaged; or the sensor may have been exposed to poisons. Some poisons that may affect sensors are silicon compounds such as Armor All, lead compounds, halogenated hydrocarbons (Freon), sulphur compounds, acids and pesticides.

The multi-gas detector used by fire crews on scene at the school was calibrated to pentane. However, the solid-state detector and the gas company’s detector were calibrated to methane.

Despite the fact that fire crews had successfully “bumped” their detector that day, they were operating with a sensor that had been poisoned. Essentially, the crews could have been standing in 100 per cent LEL of methane, and their detector would not have indicated a hazard; this is the danger of a poisoned sensor.

Methane is the main component in natural gas. According to the National Fire Protection Association, between 2007 and 2011, municipal fire departments in the United States responded to an estimated average of 51,600 gas fires each year. Almost all gas fires involved natural gas or liquefied-petroleum (LP) gas.

So why do fire services calibrate to pentane? To put it simply: safety (or perceived safety). Pentane’s correction factors are lower than those of methane, so pentane calibration is more accurate than other calibrations to a wide range of flammable gases. Methane’s factors are accurate to a large number of gases, but are very inaccurate for others.

Almost all detector manufacturers recommend bumping and calibrating to the target gas, meaning the gas most likely to be encountered in the field; for firefighters, that is methane or natural gas.

According to Jason Morton, a product support manager at Dräger in Mississauga, Ontario, a catalytic sensor is the most suitable for the fire service. However, firefighters should be vigilant when it comes to the age and accuracy of the sensor.

“For safety monitoring applications, a catalytic sensor is suitable for use because it will respond to a very wide range of combustible gases,” Morton said. “However, as the sensor ages, its response characteristics to different gases will change.

“Sensitivity to gases with higher ignition temperatures, such as methane, will decrease to the point at which accurate measurements are no longer possible. But sensitivity to gases with a lower ignition temperature, such as pentane, may still be sufficient.”

The best way to ensure that a monitor is working to its full capacity is to test the sensor with the target gas; doing so will prove the sensor’s ability to perceive even the most challenging flammable gases.

What is your target gas? Think about this: How many pentane calls has your department responded to in the last year? The past 20 years? How many natural-gas calls has your team responded to? The choice for safety is obvious; the need for change isn’t always as obvious.

Steve Clark is an acting captain with the City of Hamilton Fire Department in Hamilton, Ontario and a member of the department’s hazardous-materials team. Connor Hadaway is a former firefighter with the City of Hamilton Fire Department; he is currently employed with the Ontario Provincial Police.