Determining frequency for gas detector calibration
Answered by Bob Pellissier, president, RKI Instruments Inc., Union City, CA.
Calibration frequency is one of the most common subjects of questions regarding the use of gas detection instruments. Regulatory agencies typically refer users to follow manufacturers' recommended protocols for calibration.
The calibration frequency for a gas detection instrument depends on how the customer will use it. For example, some 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 – to remove all doubt of the proper functioning of the instrument. The other extreme is someone who uses the instrument only a couple of times each year for noncritical applications. This type of user should calibrate the instrument before each use.
We generally recommend users develop a frequency of calibration tailored to their application and usage. Initially, the user may begin by calibrating once per week, and noting any necessary changes or adjustments. If, week after week, very little or no adjustment is needed, the calibration frequency can increase to the point that only a small adjustment will be needed when calibrating. In general, for most users, this frequency ends up being somewhere between one and three months. For users who do not wish to develop their own frequency, we recommend they calibrate once a month.
For those who "bump test" their instrument prior to each use, the calibration cycle can be extended to three to six months for instruments that successfully pass the bump gas test. There is no universal standard for pass/fail tolerance on a bump test. The user must determine the tolerance based on frequency and usage. A typical tolerance could be +/- 20 percent or +/- 30 percent, or a simple triggering of the instrument's alarm.
Some newer instruments feature autocalibration, which makes calibration quick and painless. Using a four-gas cylinder, a four-gas portable monitor can calibrate all four channels together within just a couple of minutes. With this simplification of the calibration task, we encourage users to calibrate their instruments more frequently than they may have done in the past.
Note: A bump test or calibration generally is recommended if it is suspected that the instrument has been subjected to any condition that could have an adverse effect on the unit (sensor poisons, high gas concentrations, extreme temperature, mechanical shock or stress, etc.).