International Safety Equipment Association (ISEA), a trade association for manufacturers of protective equipment, including environmental monitoring instruments. The ISEA recommends, at a minimum, verification of sensor accuracy before each day’s use. This does not necessarily include calibration, but would require bump testing - the process of subjecting the gas monitor to the gas(es) they are supposed to detect and making sure they register - each day. The ISEA provides information about the differences between bump testing and full calibrations, and I’ll include it here because it does supply a good description of the process of when bump testing is helpful and when it isn’t enough:
Bump Tests vs. Full Calibration There are two methods of verifying instrument accuracy: a functional or bump test and a full calibration, each appropriate under certain conditions. A bump test verifies calibration by exposing the instrument to a known concentration of test gas. The instrument reading is compared to the actual quantity of gas present (as indicated on the cylinder). If the instrument’s response is within an acceptable tolerance range of the actual concentration, then its calibration is verified. (Note: It is recommended that users check with the detection equipment manufacturer for the acceptable tolerance ranges.) Instruments should be "zeroed" before the bump test in order to give a more accurate picture of the bump test results. When performing a bump test, the test gas concentration should be high enough to trigger the instrument alarm. If the bump test results are not within the acceptable range, a full calibration must be performed. A full calibration is the adjustment of the instrument’s reading to coincide with a known concentration (generally a certified standard) of test gas. For verification of accuracy, calibration gas should always be certified by and traceable to the National Institute of Standards and Technology (NIST). In most cases, a full calibration is only necessary when an instrument fails a bump test or after it has been serviced. The full calibration and bump test should be conducted in a clean fresh air environment.In practice we have heard that companies develop their own intervals of when to bump test and when to calibrate. Our clients inform us that if they use their monitors infrequently, say every 30 days or less, then they do calibrate each time they monitor so they have confidence the monitor is responding properly. On the other hand, companies that use their devices all the time, say daily, have told us they calibrate far less frequently, perhaps every 4-6 months, but do bump testing more often (every couple of weeks) to make sure the devices are registering. Since this is already confusing, let’s go ahead and mix in the legal implications of the data provided by the gas monitors. For readings to hold up in court as incontrovertible your gas detector must be calibrated both before and immediately after each use. This is the only way to be completely sure the monitor is functioning properly. What we say is that it’s important to create an environment where safety is important; where all equipment – not just monitors, but harnesses, lanyards, SRLs, winches and other PPE – are checked regularly. As for the monitors, the best information we have comes from the president of RKI Instruments, a gas monitor manufacturer who wrote an article for the National Safety Council. He explained that folks who “bump test” before each use can extend the calibration cycle to “three to six months for instruments that successfully pass the bump gas test”. It’s also important to remember that bump testing or calibration is especially necessary when a gas monitor has been potentially damaged. The monitors are made to be durable and work for extended periods on tough work sites, but they are also sensitive instruments and severe conditions such as high gas concentrations, extreme temperatures, falls or electrical shocks cannot be assumed to have left the device fully operational. If you want to be sure your monitor is working correctly, you must bump test. If your device fails bump testing, you must calibrate to make sure the sensors and the monitor are able to provide their life saving alerts.