Gas Meter Calibration Requirements - OSHA and Technical Considerations

Typically, it is thought that OSHA requirements drive the push to frequently calibrate gas meters. In fact, however, it is the cutting edge gas sensor technology that necessitates the need to calibrate your gas detectors. They have their limitations.

Calibration Drift - Why Gas Monitor Calibration is Necessary

Manufacturers and OSHA require that gas meters regularly be calibrated because of something call calibration drift. Generally speaking, calibration drift occurs when the communication (electrical signal) between the sensor and the monitor changes or drifts from what it was at the time of its last calibration. This change or drift is unavoidable.

Calibration Drift - The Causes

A number of factors cause calibration drift:

• AGE - Gradual chemical degradation of sensors and drift in electronic components that occur naturally over time.
• ENVIRONMENT - Chronic exposures to, and use in, extreme environmental conditions, such as high/low temperature and humidity, and high levels of airborne particulates.
• SENSOR POISONING - Exposure to high (over-range) concentrations of the target gases and vapors.
• MONITOR ABUSE - Harsh storage and operating conditions, such as when an instrument is dropped onto a hard surface or submerged in liquid. Normal handling/jostling of the equipment can create enough vibration or shock over time to affect electronic components and circuitry.

 It is true that oxygen sensors generally suffer the most calibration drift. However, all four sensors in a typical confined space gas detector - Oxygen, Combustible, Hydrogen Sulfide, and Carbon Monoxide - are subject to the calibration drift problem. And this includes H2S/CO dual-tox sensors.

Calibration drift even occurs if the monitor has never been used or has been stored away. Neither of these prevent calibration drift. This bears repeating. It does not matter if you’ve haven’t used the gas monitor. The sensors still age and drift and calibration will be needed.

Gas Meter Calibration and Frequency

OSHA does not give any detailed recommendation concerning gas monitor calibration frequency. What OSHA does say is "follow the manufacturer's recommendations with regard to calibrating the instruments."

Helpfully, RKI Instruments outlines two extremes of calibration frequency:

• Example 1 - Users who require the readings to hold up in court as data for certain legal applications must calibrate both before and after each test or each series of tests, in order to remove all doubt of the proper functioning of the instrument.
• Example 2 - The other extreme is someone who only uses the instrument a couple times a year for non-critical applications. This type of user should calibrate their instrument before each use.

 Most users will be in the middle of these two extremes. RKI, for example concludes that "typical calibration frequencies for most applications are between 3 and 6 months, but can be required more often or less often based on your usage". Honeywell BW recommends calibration at a minimum of every 6 months.

GFG, a German manufacturer of confined space gas detectors says this: “Any incidents or exposure to contaminants that might adversely affect calibration should trigger a functional (bump) test or calibration check before further use.” Incidents that might adversely affect calibration are those cited under the calibration drift section of this article.

Calibration Frequency is ultimately determined by the end user based on OSHA's requirement to follow the manufacturer's guidelines. Neither the manufacturer nor a distributor can make a specific recommendation. Nor can our service center (or any other) claim that a calibration performed in our lab is “good” for six months. Again, this is due to the calibration drift problem already discussed.

Do I Really Need to Calibrate?

The answer to this should be clear. Yes. OSHA says, "Proper maintenance and calibration of the instruments ensures their accuracy in detecting worker exposure to harmful gases in the workplace."

As we said earlier, OSHA says "follow the manufacturer's recommendations with regard to calibrating the instruments."

In effect, OSHA's position makes the manufacturer's requirements about calibration "the OSHA standard" for that particular gas monitor. The manufacturer's requirements are, of course, usually found in a particular monitor's instruction manual or a manufacturer's bulletin. The OSHA position is that you follow their recommendations. So, yes, you have to calibrate your gas monitors.

What a Gas Monitor Calibration Does

When a gas monitor is calibrated a few important things occur electronically. First, the monitor determines if its sensors can actually detect the gases they are supposed to detect. If a sensor cannot do so, it will fail calibration. Manufacturers handle this in different ways, but all will notify you that a sensor has failed a calibration.

The second thing a calibration does is that it aligns the sensors current electronic signal or voltage with the concentrations of gas in the gas calibration cylinder.

In other words, the gas monitor now knows what a sensor’s voltage or electronic signal is for the concentration of gas it is being exposed to in the calibration gas cylinder.

For example, say the correct calibration gas used to calibrate your gas monitor contains something like 100 ppm CO, 25 ppm H2S, 50% LEL, and 18% Oxygen. When the sensors are exposed to these gases during the calibration process, the electronics in the gas detector will align a sensors current signal/voltage with the this known concentration of gas it is exposed to during calibration. It now knows that the sensor’s current signal output equals 100 ppm CO, for example. When this relationship changes, you get calibration drift.

When a sensor’s signal or voltage output can no longer be aligned to the calibration gas during calibration at all, the sensor will fail calibration. Generally, this means that the sensor will need to be replaced. On occasion this might also mean your calibration gas is bad.

Calibration vs. Bump Test

A bump test is NOT a calibration. We’ve written at length about bump testing in this article: Bump Test - What It Is and How to Do It. It is easy and inexpensive to do and should be a part of your safety protocol. A bump test should be performed before each use. However, bump testing your monitor does not do what a calibration does, as described earlier in this article.

Bump testing can be a crucial indicator of when a calibration is required (often even before the manufacturer’s recommendations). A failed bump test means, at a minimum, that your confined space gas sniffer needs to be calibrated.

Importantly, even if your meter passes a bump test every day, you still need to abide by the manufacturer’s minimum calibration schedule. This, as we’ve said, is usually at most going to be every six months.

Calibration and OSHA Technical Considerations

OSHA, in their Calibrating and Testing Direct-Reading Portable Gas Monitors Bulletin, is very clear why they require gas meter calibration. They rightly point out that, "exposure to hazardous levels of toxic gases or to an oxygen-deficient atmosphere...can cause workers to suffer serious injuries or illness, and even death. Flammable gas explosions are often catastrophic, resulting in worker injuries and death, or destruction of property."

Calibrations prevent injury and death. This is a legitimate and serious concern. Lives are lost every year due to gas monitors that are not giving accurate readings.

OSHA's explanation for why inaccuracies in meter readings occur has nothing to do with a desire to implement heavy handed rules and regulations. It has everything to do with technical considerations.

All sensors lose accuracy over time. The amount they lose and how long it takes varies. This loss of accuracy causes calibration drift. As we stated earlier in this article, this means that the reference points for the measured gases established at the prior calibration are no longer valid. Therefore, the readout is no longer accurate. Calibration is needed. There is no way around this fact.

Conclusion

So the requirement to calibrate frequently, is wholly due to the limitations of the technology. It's not heavy handed regulation. Such requirements will and do save lives.

Calibration can be done by you or in our lab. There is no prohibition against you calibrating your own meters. In light of this fact, there are two ways to get your gas sniffer calibrated.  (1) You can buy the correct calibration kit for your meter. Calibration kits are usually around $425. It would allow you to both calibrate and bump as often as you wish. Only the gas would need replacing. (2) You can send the monitor back to us. We currently charge $85 for a calibration. You get a calibration, a calibration sticker, and calibration certificate.

If you bought a monitor from us (or buy from us), we send out 6 month email notifications reminding you to calibrate your meter.

Hopefully, you've now been convinced that you really do need to calibrate your gas monitor/s. Whichever way you choose to do so, we are here for assistance and guidance.

Helpful Downloads

• RKI Calibration Frequency FAQ
• OSHA Calibration Frequency Bulletin
• ISEA Statement on Calibration Frequency