My Meter Isn't Going Off, So I'm Safe Right? Well...Not Quite!

Whether you're in the wonderful world of hazardous materials or making an entry into a permit-required confined space, one thing you better be familiar with is how to use a portable air monitor. Regardless of the environment you're in, the operating principles are the same, but just how well do you know your meter? Allow me to elaborate.

1) T90 Response Time

For those of you not familiar with what I'm referring to here, your T90 response time is essentially how long it takes for your meter to display 90% of the actual concentration. The key thing to note here is that every meter's T90 response time is different, so here's where you need to pull out ol' reliable (aka "the user's manual"). Towards the back of the user manual in the technical data section, the manufacturer will list the T90 response times for each applicable sensor (e.g., O2, CO, H2S, etc.). Some may be as quick as 10 seconds, whereas some older ones I've seen in my career are as long as 300 seconds! Yep, that means it take 5 MINUTES to get a fairly accurate reading! Let's be honest...there was probably a lot of "pencil-whipping" going on for people that used that particular meter.

Key Takeaway: Know your meter's T90 response time for every sensor that's installed. Even a 10-second response time can mean you're 10 seconds into a dangerous atmosphere before you realize what just happened (unless you're crawling, you can cover a lot of ground if you're walking for 10 seconds!). Give your meter time to catch up before proceeding!

2) Oxygen Displacement

Normal oxygen (O2) content in the atmosphere is approximately 21% (+/- 0.5%). If you have a RAE meter, your normal background reading will show 20.9% O2, whereas MSA meters will normally show 20.8% O2. Am I worried about a 0.1% difference in oxygen? Usually not - but don't take your eyes off that reading just yet!

Unless you've changed your meter's parameters, your meter should alarm when the O2 gets to 19.5% on the low side and 23.5% on the high side. If my background reading is 20.8% oxygen, my levels can drop by as much as 1.3% before my meter ever decides to buzz, light up, and alert me that something is wrong. Am I worried about a 1.3% difference in oxygen? You bet!

As mentioned above, oxygen is about 21% of the atmosphere, so let's just say it's 1/5 to make things simple. The important thing to remember here is that oxygen doesn't just "disappear"...something has to take it's place. So if my oxygen has dropped from let's say 20.8% to 19.8% (a 1.0% difference), what displaced the oxygen? Unless you have a meter that also monitors for simple asphyxiants, then you won't be able to know WHAT is around you, but you will be able to know HOW MUCH is around you. Don't think that your ordinary 4-gas meter only "reads" O2, CO, LEL, and H2S! It's much smarter than that!

Let's test your knowledge. Given the above situation of the oxygen dropping by 1.0%, how much of the "unknown" asphxyiant is in the atmosphere? 1,000 ppm? 5,000 ppm? 10,000? How about 50,000 ppm? BINGO! Yes, that's correct. It takes 50,000 ppm of an unknown asphyxiant to cause your oxygen levels to drop by 1.0%. But how? Here's where the "1/5 of the atmosphere" part comes in!

For HAZMAT nerds and glow worms like me, you should know that 1.0% volume of air is equal to 10,000 ppm. However, since oxygen is only 1/5 of the atmosphere and asphyxiants don't discriminate against which gases they displace, you have to assume it's displacing ALL atmospheric gases, meaning you have to multiple the original 10,000 ppm by 5. That gives you a grand total of (drum roll, please!).........50,000 ppm of methylethylbadstuff!

You know what's really scary here? If your meter is at 19.8% and has dropped by 1.0% from your normal background reading, your meter isn't going to go off!

Wait...so you're telling me I can have 50,000 ppm of an unknown chemical in the atmosphere and I'm never going to know it? Yes and no. Yes, it's going to be potentially be there, but you DO have the opportunity to know it. It's all a matter of picking up on the context clues that your meter is giving you.

Key Takeaway: It takes 50,000 ppm of an asphyxiant to displace 1.0% of oxygen, so watch your meter closely. Just because it doesn't alarm doesn't mean the atmosphere is safe to enter. Be sure to take into consideration your work environment's altitude when getting a background reading, too!

3) Fresh Air Setup (FAS)

Let's go back to Air Monitoring 101 for a New York minute. One of the first operations you should have learned (after figuring out how to turn it on, of course) is how and when to do a fresh air setup (sometimes called a fresh air calibration). This is where you get the background readings that I've been mentioning throughout this article. The most important aspect of this procedure is the "fresh" part. It may sound simple and straightforward, but trust me, there are people out there that don't fully understand this concept.

When I say fresh air setup , it means you need to be in....you guessed it....FRESH AIR! Don't fire up the meter, hop off the fire engine, then stand right next to the exhaust pipe while performing your FAS. Now you've just (1) contaminated your sensor with pure carbon monoxide (CO) and (2) you've completely screwed up your FAS. What the fresh air setup is doing is getting a stable background reading so that your meter knows what is "normal". Whatever it determines as "normal" will now be the baseline level going forward.

If that exhaust pipe the new probie (that means "probationary firefighter" or "new guy" for you non-FD folks) was standing next to is pumping out 15 ppm of CO while he's performing the FAS, your meter is now going to assume that 15 ppm of CO is "normal" and establish that as the baseline. Now when you go into the house to monitor for a reported CO leak, any actual concentrations of CO up to 15 ppm are going to show up as 0 ppm on your meter. If you hit 35 ppm of actual CO concentration inside the house, your meter is probably only going to display 20 ppm of CO (35ppm - 15ppm = 20 ppm). Considering most meter's CO sensors are programmed to alarm at the NIOSH recommended exposure limit of 35 ppm, this means that once again, you are in a dangerous environment and your meter isn't going to go off due to human error.

Now with that being said, any reasonable person "should" realize that even though it's not displaying 35 ppm, 20 ppm still isn't normal. A normal household should be reading as close to zero as possible, so this should still cause the person to investigate. Your initial readings, however, will just be lower than the actual concentrations in the house. If all else fails, get a second meter (if you have one) and verify your readings. Odds are they have a gas leak from their hot water heater or stove, but needless to say, you don't want to tell the 80-year old lady that "the meter's not going off, so everything is fine", then a few days later, see her name on the nightly news after she died from chronic CO exposure. If that happens, go ahead and get out your computer and start out your letter by saying "Dear Chief,". I'm sure he'll understand, right?

Key Takeaway: Perform your fresh air setup in fresh air each and every time you turn on your meter and get ready to use it. If you ever suspect something is wrong, try performing a second FAS or get a second meter to verify your readings. When in doubt, always assume the worst! Never second guess yourself!

Summary

The next time you get out your gas meter to use, take a few seconds (literally) before rushing in there to save the day. It could make a world of difference! Be sure to look at some of the technical specifications of your meter and practice using it regularly. Just like the old adage goes - "Train like your life depends on it...because it does!"