Industrial Gas Monitors

 

Background

When it comes to choosing the right gas monitor for any given task it is important to understand both the capabilities and the limitations of the monitors that you are selecting from. Selecting the correct gas monitor could be the difference between going home safely and not going home at all. The most technologically advanced gas monitor could be completely useless to you if its sensors are not capable of detecting the types of gases present in your working environment.

The capabilities of the sensors is the most important factor that should be considered when selecting or purchasing a gas monitor. Too often the decision is made based on other factors such as price, size and other whiz bang features that have nothing to do with the devices detection capabilities.

The various OHS/WHS regulations throughout Australia as well as AS2865 requires that prior to entry and whilst working in confined spaces that the oxygen level remains safe, combustible gases are beneath 5% LEL for entry and beneath 10% LEL when continually monitored and that exposure standards are not exceeded for any other harmful contaminants.

Is the standard 4-Gas monitor up to the task?

It is very common to come across the standard 4-gas monitor (O2, H2S, CO and LEL) being used as the “industry norm” however does the standard 4-gas monitor always meet the requirements?

Is ammonia or perhaps chlorine used in your industry? Or are there other chemical reactions likely to be occurring creating a whole swagger of other gases.

It is important to ensure that the monitors that you select have the correct sensors and sensor technologies for the types of gases that workers may be exposed to.

Could gases in the atmosphere be effecting the operation of the sensors?

 

What does 5% of the LEL actually mean?

 

Each flammable gas has a lower (LEL) and upper (UEL) explosive limit. When we are detecting for the presence of flammable atmospheres we are actually looking for 5% of whatever the flammable gases LEL is.

For instance, methane has an LEL of 5%, we are required to ensure that the flammable gas in the atmosphere is beneath 5% of its LEL. Therefor we are looking for 5% of 5% which would actually be 0.0025% of the flammable gas in the atmosphere.

We are less than 5% of the LEL therefor we are safe to start work

Let’s look at LPG as an example. The propane component of LPG has a Lower Explosive Limit of 2.4 %. A correctly calibrated LEL sensor will be set to alarm at 5% of this meaning that the detector would be alarming at 0.12% which is equal to 1200ppm. The TWA of the propane component of LPG is 1000ppm. A worker could be getting exposed to above the TWA of the propane without ever being alarmed to it.

Is 5% of the LEL accurate for the gases present?

So the gas monitor you are using might be calibrated to methane for LEL. It might be getting used in an ethanol environment.

When the monitor is showing you a result of 5% LEL, you would actually be in around 7.5% LEL.

This is due to the fact that the LEL for each of the two gases being methane and ethanol differ from each other. Therefor we need to make corrections as to what the monitor is showing us.

 

What is the response time of the sensor you are using?

Speed is important when it comes to gas monitors. Response times are referred as t90, which is the time it takes the sensor to display 90% of the actual concentration.

Let’s compare:

A H2S sensor with a t90 time of 15 seconds vs a sensor with a t90 time of 25-40 seconds in a H2S leak.

Both sensors are exposed to the same level of H2S at the same time as each other. After a few seconds the detector with the t90 15 seconds will start detecting and displaying the presence of the H2S. The sensor with a t90 25-40 seconds may not yet be displaying the presence of H2S.

The sensor with a t90 of 15 seconds will continue to display a reading and reach 10ppm meanwhile the t90 25-40 seconds device sits there without displaying a warning.

After 1min and 35 seconds the device with the t90 15 seconds has reached 15ppm and the monitor will go into high alarm alerting the user to evacuate. It could take an additional 42 seconds for the monitor with the t90 25-40 seconds to alarm therefor exposing workers to the presents of a dangerous gas for a longer period of time.

 

This is just scraping the surface of what to know when it comes to industrial gas monitors.