When working in hazardous atmospheres, it's not enough to rely on your senses to verify that the area is safe to enter or to notice that the environmental variables have changed while we were performing the job. That's why fixed and portable gas monitors are of critical importance. These monitors have the ability to detect a toxic and explosive atmosphere at very small concentrations, usually in parts per million (ppm) or even lower. They then notify us before the conditions become immediately dangerous to life and health.
These devices, however, can lose some of their sensitivity. We need to know we can trust our monitors – not only that they are functioning, but that they will be triggered when exposed to the right concentrations of gases. We do this by periodically calibrating and bump testing the monitors.
This article will answer some important questions about bump testing gas monitors and ensuring that the reading you're getting from the accurately reflects the atmosphere you're working in.
What Is an Acceptable Bump Test Result?
A bump test is a quick procedure in which a gas monitor is exposed to a known concentration of a target gas to ensure that the gas triggers the alarm at the set level.
More often than not, the reading on the screen will not perfectly mirror the specs on the bottle of target gas, but as long as the variation is within 10%, we can consider the monitor to function within the acceptable range.
If the reading is not within 10% or if it takes more than 30 seconds from exposure to sounding the alarm, the monitor should pass a full calibration before returning it to field service.
According to OSHA, monitors should be bump tested “before each day’s use in accordance with manufacturer’s instructions.” This can actually send a contradictory message because some manufacturers recommend a bump test no more than once per week.
To be on the safe side, most employers codify in their policy that a gas detecting instrument must be tested daily, before starting work in a potentially hazardous atmosphere. This will surely satisfy OSHA requirements and give the employee peace of mind. More frequent exposure, however, will shorten the lifespan of the gas detecting sensors.
Using a gas mix that is the same brand as the monitor you're calibrating is always indicated. Generic gas mix brands should be acceptable, however, as long as the concentration of the gases is above the low alarm set point.
The calibration gas and bump test gas are often the same, so when buying your bump test gas, don't worry too much about whether it's labelled a bump gas or a calibration gas. It's better to look instead for the proper concentration of gases, your preferred canister size, and delivery method (flow valve or manual trigger).
Although they use the same gas, it's important to always remember that bump testing is not the same as calibration and is not an appropriate substitute when calibration is required.
(Learn more in What You Should Know About Using Calibration Gases)
What About Gas Cylinder Expiry?
Each gas cylinder has a set expiration date. This is because some of the gases in the mix are reactive and will either leach through the cylinder or react with the cylinder material. This is also the reason these canisters are not refilled.
After the period indicated on the canister (usually 24 months, but only 12 in some cases), the concentration of the gases cannot be guaranteed and using it could result in calibration and bump test errors.
It's a good idea, then, not to buy more gas than you need and to use your cylinders in a FIFO (first in, first out) cycle.
Gas canisters come in sizes that range from small 5L bottles to 120L cylinders. The right size canister for your operation will depend on the number of instruments you have to test and how often you need to test them, while ensuring the gas doesn't expire before you get to use it.
5L bottles are more expensive per liter than larger containers, but it is likely the best choice for you if your organization only has two monitors and workers seldom enter hazardous atmospheres. Conversely, an organization with tens or hundreds of monitors all dispatched from the same location could save costs by using large cylinders at their bump testing station.
Manual Test, Flow Valve, or Calibration/Bump Station?
When comparing practicality or time commitment, there is little difference among these methods of administering the gas. All will be fairly quick.
A few bursts of gas on the sensor (after the bump testing sequence is initiated) might be the quickest of them, but this might expose the sensors to more gas than necessary, shortening their life span. Also, because of little control over how much gas releases it might also use more gas. Because of this, this method will be costlier in the long run.
Using a flow valve (with a flow of 0.25 to 1L/minute) will minimize the sensor’s exposure and gas waste, while a bump station will be the most economical from this perspective, with the added benefit of automatically logging the test.
What About Keeping a Log?
Regardless of how often you bump test your gas monitor and what method you use for bump testing, you should keep. Your bump test log should record:
- The serial unit of the gas monitor
- The date and time of the test
- The name of the person performing the test
These days, most units will provide a visual indicator (a check mark, an "OK," or some other symbol), this mark is soon deleted and you will be hard pressed to demonstrate compliance if you have no record of the testing.
A logbook is also a convenient way to verify that all units have been bump tested. If you have five field employees using monitors and only four records in the logbook, you'll know to double check that all monitors have been tested.