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THMG141 – Laser Methane Detectors

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New technology for the field is something that has been around in industry for a long while. Bob and Mike discuss a new tool for the tools box found in laser methane detection.

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Thanks for listening and watching!

 

 

History

 

Apparently these meters, like so many of our meters have there roots in industry.

this technology have been around a quite some time but not in the form we are going to be talking about today.

So the birth of the system came about with larger, non portable equipment. it consists of two piece of equipment a source for  the laser and a detector.
The laser source and the detector where spaced out. They were placed in such a way that if a container, like a pipeline or cylinder or rail car would start to leak the rising gas would go between the source and the detector and trigger an alarm.
So kind of like the bells in small stores that ding when someone break thru the doorway.

 

Yes. person walks through the doorway, go between the sensor and the emitter and boom alarm rings.               

 

as with most technology this basic concept became smaller and smaller until we reach what were talking about today with is a portable version of this methane detector.

 

how it works

So the here the generalization on how this works. Each unit is equipped with 2 or 3 lasers.

the difference between two and three really depends on if it has a visible light laser. This laser acts as a visual cue to see the end point. it also acts as a distance calculator.

There are some out here that use a screen similar to thermal imaging camera. So you look through the screen and place a dot on the end target

The other lasers are IR laser. Since methane has a very specific frequency that it will absorb IR radiation one of those lasers is going to be set to that frequency. There are a few but the most popular seems to hover around 3.3um.

The last laser is a reference laser that is not picked up by methane and therefore will pass thru it unchanged.

Just as a side note. There are some that split the one laser and use that as a reference but the idea is still the same. 1 laser for spotting/distance taking, 1 laser as a reference and one to hit and be absorbed by the methane.

Now the key to this whole thing is the the laser must have something to bounce off. If it doesn’t have anything to reflect off then it can’t get IR light back to the sensor. So this is a key to the entire things is finding something to bounce off.

Let’s say where in a room and we shoot out the laser across the room. As the laser travels across the room it encounter a cloud of methane. some of the photons of light are absorbed.

the rest move through the cloud hit a surface and scatter. Now keep in mind there are two lasers doing this. The only difference is one is being absorbed by the methane the other is not. As both lasers scatter off the surface some of the light will make its way back to the meter where the difference between the two are analyzed.

From there we are given a reading say 500 ppm m.

And that brings us to ppm m. which Is one of the first and most confusing parts of this meter.
This is not a point source detector. Lets talk at little about PPM. PPM stands for parts per million. When we speak of ppm think about it in terms of volume. And a point source detector does the same. It takes in a certain volume of air and within that volume.
But on the laser detector the detector is only seeing two things. the total distance the laser travel. and the amount of light absorbed by the methane in its travels.

 

            Let’s close our eyes, unless your driving then don’t close your eyes. imagine a stove leaking gas in a large room. That gas will form a cloud so the closer you get to the leak the higher the concentration.

When we use a point source detector we are examining the ppm in a very specific spot. so i would walk over to the stove and an inch above the stove i am getting 500 ppm. IF i move to the right or left or up or down the ppm can change at the new point.

But the clouds concentration changes as we move around the cloud. take that probe reading just above the stove and move it over a few feet, now we’re getting 300 pmm, or move it to the left and get 100 ppm.

Yes so even within the cloud there are different concentrations. we can see those concentrations when we move the probe to different places. With the probe we are saying at this specific point in space we have x ppm.

With the laser we can’t do that. That laser is going to pass through all different ppm concentrations as it moves through the cloud of gas.

And all the sensor is picking up is the total distance traveled and the amount of lost IR laser in that distance. That lost IR laser correlates to a concentration but not at a specific point in space.

It represents that concentration through all the points in space that the laser traveled. Remember we are in a large room or even outside, the cloud is not everywhere. there are areas with no methane and ares with high methane. it’s going to pick up all methane along the entire route.

So instead of saying PPM we have to add the fact that it is an averaged ppm with the distance traveled.

This seems really confusing at first and that because we used to two things. 1 we used to a point source terminology, 2 were also thinking about using it the same way we would a point source. But we will be operating with in on a totally different way!

 

operational uses

 

We are not using this meter to determine if we are in a flammable range.! I repeat we are not using this meter to determine if we are in a flammable range.

Ok the what are we using this meter for! How do we correlate ppm m with other readings from cat bead or VOC. The answer is we don’t.

We are using this meter in 3 different ways. The first is to just determine if there is methane present in a very quick and safe way.

So think of an apartment building. odor of gas in the hall. Were searching apartments now for a gas leak. We force a door step in and can quickly scan the area to see if there is a higher concentration of gas in that apartment than in the hall. If not we can move on to the next apartment.

 

The same could be done through a window. I can walk up to a window and scan that room without making entry to see if there is methane present.

yes through a window. But there are some caveats that play into this. first the window can’t be an e-glass. do not confuse this with energy efficient windows. energy efficient windows are double or triple pane windows with a low conducting gas in between. They are designed to stop heat loss and gain via conduction.

E-glass is a window coating that is specifically designed to absorb IR radiation and prevent it from going inside the structure. This is because it is the IR frequency that literally carries heat we feel when the sun shines on us.

Since the IR is what the laser is made of it will not go threw the window or the reading will be soooooo messed up that it makes it not reliable.

We have to be very aware of what’s behind the window. If there is curtains or blinds the laser will bounce off the blinds and now the only space your actual sampling from is going to be the small distance between the window and the blind.

Which could have limited air flow and not even indicate the presence of methane.

 

The second is to determine if our actions have in fact produced a difference.

Let’s say we had a house with a substantial natural gas leak in the house. We go to talk in and right away our lel meter brings us to 50%. no good. What do we do?

 we start to vent, we open windows and we evacuate the house. and far enough back to be safe. At this point we take a reading with the detector and report the reading. As we ventilate we should start to see a decrease in the PPM/m over time. this will let us know that are actions are working and we are maintaining a safe distance.

now some caveats to this. Let’s think back to the ppm vs ppm m chat we had earlier. In order to obtain a current idea of whether of not our action are working we need to take a reading from the same spot and shoot the laser in the same location.

now we can have multi spots and multiple locations but each spot must be measured the same. otherwise were not seeing a change in the area we tested before.

so operationaly you want to try to have the same members perform the duplicate readings. In a large operation use of things like cones can help establish the spatial location as to the point the detector was when taking the reading.

 

The third way would be to thy to triangulate where a gas leak was coming from. This is especially useful in an area where you have a leak underground or along a pipeline.

•             The key to this is the get reading from a few different angles. If you drew an imaginary line in the direction of the lasers your leak would be in the area that these lines intersect.

–            So lots of different uses for this meter. none of them is determining if we are in an lel.

 

Pros and cons.

 

Pro

can cover a large open area, reducing time searching for a leak as well as man power

measurements can be made remotely keeping members out of harm

Calibration is min and drift is non existent

These units respond (for the most part) instantaneously. I say for the most part because someone is going to send us a technical note that says no it take .25 seconds to respond so its not instant!

no cross sensitivity. Methane laser will only pick up methane.

Cons

Only pics up methane

we found that corners really screw with the meters readings. so make sure your not shining off a corner

need to have a spot for the laser to reflect back to you. If your in an open field you are not going to get a reading.

unless you have a ceiling of some kind you will not get a reading outside above the ground.

Will not be a useful tool for gauging flammability.

 

our experience so far

 

 

The Hazmat Guys

Author: The Hazmat Guys

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