Since the beginning of time, back to the caveman days, mankind has always relied on the 5 senses to protect themselves: If something was hot, they wouldn’t touch it. If something tasted bad, they wouldn’t eat it. If something in the air smelt peculiar, they would get away.

Jumping to current day, sometimes that isn’t enough. The world has become more industrialized and more and more different chemicals and compounds have become more common, many of which don’t carry the traditional warning properties.

Take Carbon Monoxide, for example: it’s colorless, tasteless and odorless. However, it is an extremely dangerous and common industrial vapor often found in confined spaces. In times where relying on one’s senses isn’t enough for protection, the importance of air monitoring meters in hazmat response and confined space situations becomes more crucial.

Air monitoring meters are one of the most important pieces of equipment for any activity performed in hazmat or confined space scenarios, former firefighter and current Pearl Engineering Safety Consultant Scott Burkart said.

“Life and death decisions are made with these meters, so it’s so important to understand how to use them.”

4 and 5-Gas Meters

Air monitors typically come in multiple sensor varieties. With up to four or five sensors installed at any given time. These sensors will only detect and alert you to the gases they are designed to detect.

The standard 4-gas meter is equipped to monitor oxygen levels, flammability of the atmosphere and common toxic substances, such as Carbon Monoxide or Hydrogen Sulfide.

Oxygen and flammability sensors come standard while the two other sensors may be swapped out to register other specific gases (Depending on the manufacturer), making these meters extremely adaptable to any scenario. With a 5-gas meter, you have the flexibility of one additional sensor or possible a PID (Photo Ionizing Detector).

The meters measure each chemical or vapor in the air simultaneously. This gives you real-time readings for the sensors that are in your meter. This is an important limitation of the meters that bears weight: the meter will only detect and alert you to chemicals that are detected by the sensors that are in the meter. If you do not have a chlorine sensor, for an example, it will not detect Chlorine. So, heading into every situation it is important to know what gasses may be expected so you can have the appropriate meter.

Measuring Oxygen

A normal, safe atmosphere is comprised of 20.8 oxygen, ~78% nitrogen and the remainder being trace gases. Using that as a base is important to bear in mind when measuring oxygen levels.

Humans need an atmosphere of at least 19.5% oxygen to survive. Below that number victims do not get a sufficient amount of oxygen which will impact their ability to think clearly, lose muscle function and suffocate.

In an environment of 8% oxygen for more than 8 minutes, the situation is almost guaranteed to be fatal. At 6 minutes, chances increase to a 50-50, with those that have cardiac or respiratory issues affected most. If they can be rescued with-in 4 minutes, this gives the victim the best chance of survival.

However, an atmosphere of more than 23.5% oxygen is equally dangerous and is known as an oxygen-enriched environment. Oxygen-enriched atmospheres are highly flammable and, although less common that oxygen-deprived environments in industrial settings, can become disastrous in mere moments.

This was the fate that the Apollo 1 launch experienced in 1967 when a stray spark was introduced to an oxygen-enriched cabin and caused a fatal explosion.

Measuring Flammability

For fire to occur, three things need to be present: fuel, oxygen and a heat or ignition source. This is known as the “Fire Triangle.” The fuel must come in the form of a gas because, contrary to common belief, solids and liquids do not actually burn. Rather, the vapor they give off mixed with oxygen in the air is what is flammable.

The fuel vapors and oxygen must be mixed in the proper proportion for a fire to occur. The proper fuel-air mixture is known as the flammable range and will vary for each chemical. The flammable range’s lower limit is known as the Lower Explosive Limit (LEL) and is what air monitors are actually measuring. In short, an air monitor measures the distance to the point of flammability rather than “flammability” itself. When your meters Reads 100% of the LEL, you are at the point that is now flammable. At 10% of the LEL, you are starting to have flammable gases in the air but not yet enough to ignite.  At 100% of the LEL or higher, there is enough flammable gas in the air to ignite and the only thing missing from the fire triangle is heat. If an ignition source is present, a fire or explosion will occur.

To measure the flammability of an atmosphere, you need an ignition source. However, for safety reasons, you cannot just go around lighting a match, as you can imagine. To detect flammability most meters use what is known as the Wheat Stone Bridge technology.  The flammable gas sensor has two chambers; one is closed as a controlled environment while the other brings in the atmospheric sample. An internal catalytic bead acts as an ignition source within the two chambers – if the atmosphere is flammable, the second chamber will ignite the fuel in the sample causing that chamber to heat up creating more electrical resistance relative to the amount of flammable gas in the air. The sample is cooled before it is discharged.

Measuring Toxins

Toxic gas sensors are much more straightforward than reading oxygen or flammability. Toxins in the atmosphere are measured in parts per million (ppm). The amount of toxin in the air that we can safely work in is determined for us by OSHA, and is known as the permissible exposure limit (PEL). The point at which the ppm reach a level of lethality is known as Immediately Dangerous to Life & Health (IDLH).

Different toxins have different PEL and IDLH levels, so a five-gas meter with additional slot for an additional sensor in your meter may be important. Again, if the correct sensor isn’t in, it won’t be picked up, so it is important to have an idea of what to expect ahead of time.

Preparing the Meter

Prior to each use, your meter needs to be tested to ensure it is in proper working order and you aren’t putting yourself in harm’s way as a result. OSHA recommends four tests before every use:

  1. Calibration: Performed by a qualified person as recommended by the manufacturer. They will check calibration by exposing it to a span gas mixture containing a trace amount of the toxic gases and adjusts the meter’s display to the correct level as listed on the span gas. The end user needs only to confirm the meter is within the calibration time frame.”
  2. Zero Test: Completed by the meter during the startup process when the meter is first turned on. Clean fresh air is run over the sensors and the meter will make adjustments to compensate for the weakening of the sensor. When a sensor will not zero out, is an indication of a worn out sensor and that sensor will need to be replaced.   The end user will need to ensure the meter is in clean fresh air when the meter is turned on to allow it to zero-out.
  3. Pump Test: If the meter has an internal sample pump, or if a pump can be attached. The end user needs to ensure the pump is moving the air sample and there are no leaks in the system.  To test to see if air is being moved through, the end user must cover the inlet of the meter with their finger and the pump should slow down and show a pump alarm.  Many new meters will include this step during the start-up procedures and self-diagnose this issue.
  4. A Bump Test: Is performed by the end user prior to each day’s work. I refer to this test as a confidence test because we expose the meter to the toxic gases it is designed to detect and ensure it can detect them. This test is performed similar to the calibrated test by running a span gas over the sensors and watching for the corresponding bump up of the reading of each sensor.

Scott and Pearl Engineering train and educate safety measures for companies, businesses and organizations in a wide range of areas.

For questions or inquiries, contact Scott at