Monitoring Equipment

As an entrant or attendant, you have the right to witness any testing or monitoring of the confined space to be entered. You may also be responsible for monitoring while in or around the space. 29 CFR §1926, Subpart AA, states that the atmosphere of a confined space must be tested in the following sequence:

1. For oxygen
2. For explosive or flammable gases and vapors
3. For toxic gases and vapors

Any monitoring equipment you use should have an alarm so that if levels ever go above the acceptable short-term exposure level or time-weighted average, you know right away.

The following are types of monitoring equipment you are likely to encounter:

Electronic gas monitors (also known as multi-gas monitors) can test for multiple types of gases at the same time. These have a wide range but are often not accurate enough to determine whether certain gases are above or below the allowed limit.

Electrochemical sensors can measure oxygen levels or the levels of certain other gases depending on the sensor's calibration. These sensors can detect carbon monoxide, hydrogen sulfide, sulfur dioxide, chlorine, and more, all at "parts per million" levels. These measurements are accurate enough to know whether the space complies with OSHA limits on exposure levels.

Catalytic (CAT) oxidation sensors are used specifically to test for combustible gases (gases that can either explode or ignite). These sensors can measure gases in terms of the lower flammable limit (LFL, which is the lowest percentage of that substance present in the air before it becomes combustible). These sensors typically monitor a range of gases within the same general group, rather than specific gases.

Infrared combustible gas sensors are typically more expensive than CAT sensors but are more useful and last longer. Unlike CAT sensors, they do not require any oxygen for them to work. Like CAT sensors, though, they can measure a wide range of materials in terms of the material's LFL.

Photo ionization detector (PID) sensors are used to detect small amounts of organic compounds in the air, such as gasoline or acetone. These sensors are typically used in less dangerous applications to avoid going over the short-term exposure limit or time-weighted average of certain compounds.

Remember, for all types of sensors, calibration and maintenance are key. If a sensor is not properly calibrated, its readings will be inaccurate, making it worthless to workers. All sensors are different, so make sure you read the manual and safety instructions for each sensor you must use.

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