Fact:

The only way that you can be certain that a portable gas monitor will respond appropriately to a potentially threatening gas hazard is either to perform a full calibration, or perform a “bump” test with a known concentration of calibration gas, approved by, and usually supplied by the manufacturer of the gas monitor.

What exactly is a “bump” test anyway?

Very simply, it is a brief exposure — or “bump” — of the monitor to a known concentration of calibration gas. The test is used to verify that a gas monitor’s sensors respond appropriately, and that the instrument functions properly. In many cases, it’s an alternative to full calibration.

With the many technological advances in gas detection equipment, why is it still necessary to bump test instruments before using them? There are a number of answers to this question, but we’ll stick to the technical one first. The signal produced by most gas sensors in a clean air environment is the same as the signal produced by the sensors when hey have lost their sensitivity to gas: It is ZERO.

Now for the practical reasons . . .

Gas monitors are used in tough environments. They are usually clipped to a belt, a hard hat, or a pair of coveralls. They can get jarred against ladders, machinery, and other equipment. They might be dropped from high places onto concrete, or into mud and water. They are exposed to rain, dirt, oil, grease, and anything else the environment has to offer. Under these conditions, damage to these instruments can, and does, occur. Some damage is visible, but some is not. As examples:
• The sensors’ protective membranes can become clogged with dirt or oil, thus blocking the flow of gas to the sensors.
• The sensors may have broken electrodes or contacts.

Either of these conditions take us back to the fact that the sensors’ signals without gas present is the same as the signal that exists if the sensors cannot respond to gas for any reason: It is ZERO. So, the only way to identify whether the monitor is not functioning properly is to test it with a known gas.

Suppose this: When you left for work today you knew beyond a shadow of a doubt that the gas monitor you were carrying would be called on to save your life. Would you be concerned about what happened to the instrument yesterday? Or the shift before you picked it up? Maybe something happened that rendered it inoperable. Would you want to test it before you went out to the job site, just to make sure it functioned safely? Of course you would. That gas monitor you’re carrying today could be called on at any time. You never know when.

An Analogy

Several years ago, an electrician in a TrenchSafety Confined Space Entry class explained a common practice when checking an unknown electrical voltage. The first step is to use a voltmeter to check a known voltage. If the voltmeter indicates the correct voltage, the second step is to use the same voltmeter to check the unknown voltage. The third step is to repeat the first step (check the known voltage) to make certain the voltmeter is still working correctly

Performing the “bump” test is a similar process.

To Repeat Our Opening Fact . . .

The only way you can be certain that a portable gas monitor will respond appropriately to a potentially threatening gas hazard is either to do a full calibration, or to test it with a known concentration of calibration gas before each use. There simply is no other option to ensure your safety.

P. S. Here’s a link to an OSHA document entitled “Verification of Calibration for Direct-Reading Portable Gas Monitors” that discusses calibrations and “bump” tests.

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