OHS Canada Magazine

Overtime

Gas-Incident Preparation for Firefighters

The fundamentals of using a four-gas monitor


The old saying “Jack of all trades, master of none” nicely sums up what it takes to be a 21st-Century firefighter. Today, fire-service personnel are expected to be competent in myriad skills, but not necessarily outstanding in any particular one.

A skill that is often forgotten is gas monitoring. It is not uncommon for first-due apparatuses to have a four-gas monitor on board. As the name implies, this gas detector has four sensors: a lower-explosive-limit (LEL) sensor, an oxygen (O2) sensor, a carbon-monoxide (CO) sensor and a hydrogen-sulfide (H2S) sensor. The LEL sensor measures gases and vapours that burn, such as propane and natural gas. The O2 sensor measures oxygen enrichment or deficiency. The other sensors measure the toxic gases for which they are named. The technology found inside all brands and models of four-gas monitors is basically the same. As such, the fundamentals of using and maintaining a four-gas monitor are the same for all fire departments.

There is more to using a four-gas monitor than knowing how to turn it on and off. The fundamentals of using the device can be boiled down to three things: maintenance, initial procedures and interpretation of numbers.

Maintenance

Gas-monitor maintenance can be divided into two parts: serviceability check and calibration. A serviceability check is performed daily in station by the firefighters who use the device. The check includes inspecting the gas monitor for physical damage, ensuring that the batteries are fully charged, confirming that the sensors are working by turning on the device and, finally, noting that the device has been calibrated within the timeframe recommend by the manufacture.

Most manufacturers recommend that the gas monitor be calibrated monthly and immediately after exposure to high concentrations of gas. This procedure is vital to ensure accurate sensor readings. Gas monitors that have exceeded the due date for calibration should be taken out of service until calibration can be completed. At Halifax Regional Fire & Emergency, gas monitors are calibrated by personnel at the hazmat station. Each time a monitor is calibrated, a new sticker is placed on the device with the date of calibration. This sticker enables anyone to determine quickly when the meter is next due for calibration.

Initial procedures

Assuming that the daily serviceability check has been completed, there are several steps that must be conducted immediately prior to using a four-gas monitor. These steps are necessary to ensure the accuracy of gas readings. It takes a few minutes to perform these steps. Don’t get frustrated or skip a step, even if someone is urging you to hurry up.

  1. Turn on the device. This can be done en route to the call only if you can safety reach the gas monitor from your seat-belted position.
  2. Zero the device. Two things happen during zeroing. First, the O2 sensor is calibrated to ambient air. During storage, the O2 sensor is constantly exposed to oxygen. This exposure reduces the sensor’s accuracy. Zeroing is necessary to ensure accurate O2 readings. It is vital that this step take place in fresh air. Zeroing in a contaminated environment, such as next to a vehicle’s idling exhaust pipe, will produce inaccurate readings. The second thing that happens during the zeroing process is the removal of any numbers that appear on the display screen after the device is first activated. During storage, all sensors are routinely exposed to miscellaneous gases, such as diesel exhaust. These gases will sometimes react with the sensors to produce a positive or negative number on the display screen. The zeroing process removes these numbers. After zeroing, any numbers that appear on the display screen will be the result of exposure to gases in the current environment and not from past storage or previously zeroing the meter in a contaminated environment.
  3. Bump testing. During bump testing, all sensors are exposed to concentrations of gases that will activate sensor alarms. This step confirms that sensors will detect and react to the gas that the sensor was designed to detect. If your sensors do not respond, a full calibration should be conducted prior to using the monitor. Some departments choose not to bump-test gas monitors. Others choose to bump-test once a week or after each incident. Refer to your manufacturers’ instructions for options.
  4. Clear the peaks. Many gas monitors will record the highest LEL, CO and H2S gas readings. They will also save the lowest O2 reading. These high/low readings, or peaks, should be cleared from the device’s memory before use. This is done to ensure that any recorded numbers are from the current incident, and not from a previous incident or the bump test.

Interpretation of numbers

Firefighter safety and the safety of the public are the main priorities during a gas incident. In order to determine whether conditions are safe or not, firefighters must be able to interpret accurately what the gas monitor is trying to tell them. This means understanding what those numbers on the display screen mean. What numbers are safe or unsafe? What are the department’s benchmark numbers for masking up, evacuating civilians, evacuating firefighters, doffing masks and civilian re-entry?

Below are some benchmark numbers used by Halifax Regional Fire & Emergency. The sources of these numbers include manufacturers’ instructions, internal operating guidelines and provincial legislation. Your numbers may be different for a variety of reasons, such as using another brand of gas monitor, being mandated to follow different regulations or because your department approaches gas incidents in another manner. Always follow your fire department’s internal operating procedures, policies and guidelines. If your department has not established any benchmark numbers, now would be a good time to raise the issue with the appropriate officer.

LEL sensor:

— Unit of measurement: per cent of lower explosive limit
— Measurement range: zero to 100 per cent of LEL in increments of one per cent of LEL
— Response time after exposure to gas: greater than 35 seconds
— Don respiratory protection at one per cent of LEL
— Evacuate civilians at one per cent of LEL
— Evacuate firefighters at 10 per cent of LEL
— Less than 10 per cent O2 by volume will result in LEL readings less than actual gas concentrations
— If O2 readings are below 10 per cent by volume, determine LEL levels by dilution sampling
— Flashing display: OR stands for over range, meaning that flammable-gas concentrations are above 100 per cent of LEL
— Blank display: flammable-gas concentrations are above 100 per cent of LEL, or the LEL sensor is dead
— Doff respiratory protection and/or allow civilians to reoccupy a structure at zero per cent of LEL
— Oxygen-enriched readings greater than 23 per cent by volume will result in LEL readings that are higher than actual concentrations of gas

O2 sensor:

— Unit of measurement: per cent by volume
— Measurement range: zero to 30 per cent by volume, in increments of 0.1 per cent by volume
— Flashing display: OR stands for over range, meaning that O2 levels are above 30 per cent by volume
— Response time after exposure: greater than 10 seconds
— Normal O2 readings: 20.8 to 21 per cent by volume
— O2 deficient reading: less than 19.5 per cent by volume
— O2 enriched reading: greater than 23 per cent by volume
— Don respiratory protection at 19.5 per cent by volume
— Evacuate civilians at less than 19.5 per cent or greater than 23 per cent by volume
— Evacuate firefighters at 23 per cent by volume or greater
— High humidity or the presence of an oxygen-displacing gas can cause O2 readings to drop

CO sensor:

— Unit of measurement: parts per million (ppm)
— Measurement range: zero to 1,500 ppm, in increments of one ppm
— Flashing display: OR stands for over range, meaning that CO levels are above 1,500 ppm
— Response time after exposure: greater than 50 seconds
— Don respiratory protection at 25 ppm
— Evacuate civilians at 25 ppm
— Allow civilians to reoccupy a structure at zero ppm
— Recharging batteries, welding, home brewing, dry cleaning chemicals and diesel exhaust will cause false CO readings

H2S sensor:

— Unit of measurement: parts per million (ppm)
— Measurement range: zero to 500 ppm, in increments of 0.1 ppm
— Flashing display: OR stands for over range, meaning that H2S levels are above 500 ppm
— Response time after exposure: greater than 50 seconds
— Don respiratory protection at 10 ppm
— Evacuate civilians at less than 10 ppm
— Allow civilians to reoccupy a structure at zero ppm
— Gas sources that will cause false H2S reading: water treatment facility, combustion of coal or petroleum, incomplete combustion, and pulp mills

I have by no means exhausted all the topics you may wish to cover during a review of gas monitoring. Additional topics may include vapour density, sampling pumps, tubing, accessories and correction factors.

Seldom-used skills will eventually become dull and fade away. Firefighters and officers need to review the fundamentals of using a four-gas monitor periodically. Take responsibility and prove your worth to your crew. Confirm that a serviceability check is performed on your shift. If the device is overdue for calibration, take it off the truck and send it for service. Read the manual and the appropriate policies and procedures. These references will contain your initial procedures and benchmarks for gas incidents. Develop cheat sheets that outline key information in point form. Attach a small laminated cheat sheet to your gas monitor or leave a copy in the device’s storage container. Review the cheat sheet every time you inspect the monitor. Quiz your fellow firefighters on your department’s benchmarks until there is no hesitation or mistake. This investment of time will improve your department’s effectiveness with the four-gas monitor and reduce the odds of equipment malfunction or a fatal firefighter mistake.

Bruce Lake is a training officer with Halifax Regional Fire & Emergency. He is a veteran hazardous-materials technician and is a graduate of the fire-service management program at Dalhousie University. Prior to joining the fire service, Lake served in the Canadian Forces as a nuclear, biological and chemical defence officer.