4 gas monitors-how do they work?

For more information about Gas Monitors, please take the Gas Monitor
Training Course
In this informational post, we will briefly describe how a 4 gas monitor works. The name is self explanatory, the instrument is a gas monitor (also called a 4 gas detector), that can measure 4 different types of gases and is mostly used as a portable instrument. Typically a 4 gas monitor will have four different sensors, out of which one would be a combustible gas detector, the second would be an Oxygen sensor and the remaining two would be toxic gas sensors for gases such as Chlorine or H2S (Hydrogen Sulfide). There are different combinations of sensors available of course, with different manufacturers and every one has several different models. So a typical one may be Oxygen plus three toxic gases (say Chlorine, H2S and CO (carbon Monoxide) Or it could be Oxygen, LEL (any explosive gas), H2S and CO. Advantages: A single portable handheld unit can measure four different kinds of gases, which is very useful, for example if one is working in a confined space. It is very handy and popular as the worker needs to only carry a single unit, strapped to his body to monitor the surroundings. Disadvantages: If the unit fails, then suddenly one has nothing to measure any of the gases! So for example if one had four different monitors and a typical confined space entry job did not necessarily ask for H2s monitoring, then one could theoretically use the LEL and O2 meters separately, but if a 4 gas monitor fails (without a spare one) then one has nothing...The second disadvantage is the calibration status of each sensor may be different and labeling becomes complicated. e.g. if the Oxygen sensor is calibrated but the LEL one is overdue then there has to be some way to mark it on the label (assuming the unit itself does not record its status).

Positioning and Placement of Gas Detectors

For more information about Gas Monitors, please download the Gas Monitors Training Course
 here

 Perhaps the most difficult task of any project that involves gas monitors and gas detectors is to decide on the placement of the various detectors and monitors. Even with all the new software( and all the associated bells and whistles) available now, it is more of an art than a rigid science.  Some questions that need answering before you start the job are

1. How do you estimate where the leak would likely take place? From flange joints in piping? Or from valve stems/packings? Or from sampling points? Or from reactor agitator shafts (even with all the mechanical seal stuff)?
2. Once leaked, how would the gas flow? How will the molecules move in the air? We can study gas dispersion models of leaks, but they are not accurate predictions. Would the gas flow just as the dispersion model says it should? Or will it flow in some other pattern?
3. How much would reach the detector?
4. Would the detector sense it? (Would the sensor be "alive" or already poisoned to death a long time back?

Only then can we start siting the detectors. This is of course after the technology has been selected (InfraRed or Catalytic Combustion or Semiconductor or other).