Article: Calibration: Who Needs It? by James Moore -- Occupational Health and Safety Magazine

Posted by James Moore on Oct 14th 2010

It's time we laid down the law on what, where, and how often to calibrate portable gas detectors.

"We calibrate once a year whether we need it or not." "We were told to calibrate every 180 days." "Calibration? We send it out to have that done every quarter." "Bump test? Yeah, my buddy Jim holds the monitor up to his mouth and breathes on it; if it goes off, we know it works, or maybe that he had onions last night for dinner."

I may have invented the last quote, but the first three are directly quoted guides on how often to calibrate your gas monitor from people in the field. There's a lot of confusion out there about bump testing and calibration, and it's time we laid down the law on what, where, and how often to calibrate portable gas detectors. Let's first dispel a few myths, then get right down to creating a simple and effective calibration program for your monitors.

Myth 1: Gas detectors need to be calibrated only to make up for sensor drift. If I know it's going to drift only 1 ppm to 2 ppm, there's no need to bump test.

Answer: There's no knowing what that gas detector has been subjected to since it was last calibrated unless you check it with gas yourself. Who knows where the last guy had it, what chemicals it was around, or what buttons he pressed. Unless you check it with gas, you could be putting yourself at risk.

Myth 2: My instructional manual says the unit performs a self bump test, so I don't need to actually expose it to gas.

Answer: These types of bump tests verify only that the sensor itself is responding to gas. This means if the sensor was splashed with any kind of compound that will cling to the membrane, the self bump won't be able to tell and will give you the OK, but the gas monitor still won't see gas. I've seen plenty of units with working sensors but with membranes coated in gunk, goo, or oils that block gas from getting to the sensors.

Myth 3: I got this cylinder of calibration gas on eBay for $100 cheaper to save my budget. It's still good to use, right?

Answer: Most likely not. All calibration gas manufacturers attach an expiration date to their cylinders. This means your cheap, half-used cylinder of 4 gas most likely has expired. Always check manufacturing dates and expiration dates when using a calibration gas cylinder. Reactive gases such as hydrogen sulfide, ammonia, and chlorine react with the cylinder walls over time. I've seen hydrogen sulfide drop from 20 ppm to 6 ppm over the course of a year. Always get your calibration gas from a reputable supplier.

Myth 4: I don't use this monitor except once or twice a year, so I don't need to buy calibration gas or set up a bump test procedure.

Answer: Wrong. To regulating bodies, if it's not documented, it didn't happen. If you don't have documentation of when and where the unit was calibrated, with what gas lot, and with what calibration gas, it's as if you'd never done anything.

Always maintain a bump test schedule for your monitor. If you need to pick up your monitor and go immediately on emergency calls, which come in about once a month, this means bump testing daily in the mornings to ensure the unit is functioning properly for that day. If you know in advance which days you'll be using the monitor, you can wait to bump test/calibrate the day you'll be using it, but always maintain a written or computerized log of your tests.

Myth 5: I have a two-year disposable single-gas detector. It's disposable, so that means maintenance free, right?

Answer: It means everything else I've said in this article still applies. That two-year disposable unit still needs to be bumped daily and calibrated monthly, or you don't know how well it will respond to real gas. Nothing replaces the hard test of actually exposing the detector to calibration gas.

Setting Up Your Program
With those out of the way, it clears the path to set up a simple, reliable program for gas monitor calibration and testing. I'm going to be using a free form we have up at, but you can use any similar form you have created yourself. Keep in mind that many of the steps I'm listing below can be removed if you have purchased a calibration docking station for your unit. Most of these will keep records automatically for you, but be sure to check with the manufacturer about how to retrieve these records in the case of an accident.

You may not be familiar with a couple of terms used below. A "bump" test involves holding a gas detector up to a level of gas just long enough to bring it into alarm and ensure sensor functionality. A "calibration" is done using an internal menu within the gas detector and requires an exact blend of gas to re-establish accuracy of the sensor. To put it simply, a bump test checks that it works at all, and a calibration takes it another step and ensures the unit works accurately.

1. Check your calibration gas. What you need to do first is to check the expiration date. If you're within a month or two, it is time to buy a new cylinder! Watch out for some of the reactive gases; nitrogen dioxide and chlorine are known to have shorter accuracy expiration dates. When you have your cylinder in your hand, be sure to write down the date it expires so you can order a new cylinder a few weeks before. Also check to make sure your calibration gas fits what the monitor needs. Call your gas detection manufacturer to ask exactly what concentrations are necessary for its monitor by default and check to make sure no one has changed the expected values in the monitor itself.

2. With your calibration gas verified, you should record the date and then zero and calibrate the unit. Make certain you are in a room with clean air or have a Zero Air cylinder hooked up to the unit. Zeroing before calibration in a room next to a furnace with 200 ppm carbon monoxide floating around will not give you an accurate reading. Your calibration will hold for 30 days, so make sure you fill in all of the applicable boxes on your spreadsheet, including the date.

Every shift following this one where the monitor is used, it should be zeroed and then bumped. Notice here, I've said "shift" as opposed to "day." Many times, a monitor will change hands mid-day and run a second shift. The user who is getting the unit always should bump the detector and check the alarms to be certain the unit is still functional. You never know what the last guy did to the unit, so I always recommend bump testing the detector first thing.

3. If the unit fails a bump, try recalibrating it. Electrochemical sensors drift over time, and in most cases this is all that is necessary to re-establish accurate readings. If your unit fails calibration, contact your local gas detection repair center or call the manufacturer to order a new sensor. Many gas monitors will give you a "span" reading, which will tell you how much life is left in a sensor and allow you to preorder a sensor before it technically dies.

The forth step may be the most important, but it's not really a step at all, it's a rule: Adhere to this program like glue. Once you've started it, keep the habit and make sure all others follow the rules and record their data. A great gas monitor does you absolutely no good if it's inaccurate or doesn't respond at all; unless you're there every second of every day with every gas monitor, you never know what trials and tribulations they're being put through. Calibration and bump testing procedures are the first line of defense in gas safety.

For some extra reading on good calibration procedures, see this guide on OSHA’s Web site:

You'll hear complaints about the budget, about the time wasted, and about the paperwork, but in the end, if you've saved even one person's life, it will be worth it. My father nearly went down to a carbon monoxide leak back in the days before safe calibration procedures were in place. The monitor did not even register. Thankfully, a friend pulled him out to save his life. With the technology we have today, there is really no excuse to lose even one worker, friend, or father to a gas leak. Stay safe, stay accurate, stay alive.

Author: James Moore is President of Ideal Calibrations, LLC, a Southgate, Mich.-based company dedicated to the repair, calibration, and testing of gas detection technology. He is a contributor to The Incident Commander Blog, For more information on this and any other topic regarding gas detection, please contact him at [email protected] or visit

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