There are numerous gases, some humanly detectable and others not, leaking around in the places we frequent that could be potentially life threatening to us living, breathing beings if they exist in excess of healthy concentrations. Luckily, there are gas detectors and indoor air quality monitoring systems to detect and alert us when levels become undesirable and potentially lethal. But what good are these systems if their readings are unreliable or inaccurate or alarms aren’t going off when they should be? It isn’t uncommon for us to hear that gas detectors that have been installed have never been serviced, even though government regulations such as Occupational Health and Safety Guidelines and many companies’ operation and safety manuals state they should be serviced on a regular basis.
It is important to realize and remember that a gas detector is a safety device and a properly functioning gas detector could be the difference between life and death. The purpose of a gas detector is to monitor and measure potentially lethal gases (or lack of oxygen) in the workplace and other areas people frequent such as supermarkets, hospitals, schools, enclosed parking facilities, indoor pools, ice arenas, and many more. If the gas detector readings reach a predetermined level, other safety precautions are triggered such as turning on visual and audible alarms, starting ventilation fans, shutting down equipment, calling the fire department, etc.
Making sure the gas detectors are working properly on a regular basis should, without question, be part of a scheduled maintenance program.
To maintain a gas detection system, it is important to inspect each device, conduct a bump test on all the sensors, (unless a full calibration is being done), do a calibration (when required) and record the results in a maintenance log.
Do a physical inspection. Check the unit for wear and tear, tampering, accidental or deliberate damage; for cracks, water damage, loose screws or wires and make sure there isn’t a buildup of dust on the outside or inside of the enclosure.
Decide if you are doing a bump test or a full calibration. Monthly maintenance at minimum requires a bump test, especially for applications involving more dangerous gases and interactions with people, such as Ammonia sensors in ice rinks and Chlorine or Ozone sensors in swimming pools. If the bump test fails or if 6 months have passed since the last calibration, a full calibration should be done.
Keep a maintenance log. All bump tests and calibration functions along with notes about performance, anomalies or otherwise should be noted in a log book. This information could prove useful for troubleshooting or proving due diligence, etc.
BUMP TEST VS CALIBRATION
A bump test is a brief exposure of the sensor to gas. A bump test verifies that the sensor is responding and the alarm is functioning. A bump test takes less time and requires less gas than a full calibration.
Calibration is the exposing of the sensor to a certified concentration of gas for a particular length of time. The calibration process verifies that the gas detector is providing accurate readings. Part of the calibration process requires a process called zeroing. Zeroing sets the sensor to recognize the ambient air as clean air. If the gas detector is in a clean air environment, (meaning there are no other gases present and relevant to that sensor type), then the air in the room can be used to zero the sensor. If the environment is contaminated then measuring and reading that air as zero will provide inaccurate readings. The sensor in this type of environment requires zeroing with a zero air cylinder.
Both types of tests are important. A bump test will let you know if the gas detector is still capable of reading gas levels but it won’t tell you if it is reading the correct levels. For the most part electrochemical sensors will output a zero reading in clean air and when the sensor is dead. Doing a bump test is also a way to check that the sensor is not dead. If a bump tests fails and full calibration should be done. Calibration will correct any degradation or drift that the sensor may have experienced over time and let you know that the readings are accurate. Without regular calibrations, the gas level readings will become less and less true as time passes. If a sensor does not respond as expected after a full calibration, then the sensor probably needs to be replaced.
FACTORS AFFECTING CALIBRATION FREQUENCY
Calibration frequency is one of the most commonly asked questions concerning the use of gas detection instruments. The answer depends on a number of factors, including the type of gas detection system and how it’s being used.
Common factors to consider when deciding how frequently to calibrate are:
- Type of sensor
- Type of gas detector (fixed vs. portable)
- The required use and accuracy of the detector
- Manufacturer’s recommendations
- How confident you are about the functioning of your instrument?
- Did you go through a period of initial use in the intended atmosphere verifying accuracy on a daily basis?
- Do you keep records so you can quickly identify if a detector has a history of excessive maintenance, is prone to erratic readings or needs to be replaced?
- Whether bump testing is part of your maintenance regime
- Whether you suspect there are environmental conditions or potential damage has occurred to the instrument that could affect the instrument’s performance
Considering the factors listed above, it important is to develop a regular maintenance and calibration schedule that is tailored to your specific operating and environmental conditions, and reinforces your confidence in the accuracy of the readings.
For intermittently used detectors and portable monitoring devices we recommend doing a bump test prior to each use, as they are often powered off and on and used in changing environments. If the bump test results are not within the acceptable range, a full calibration must be performed.
Fixed systems may be calibrated quarterly, bi-annually or even annually depending on the application. For continuous monitoring devices we recommend a bump test every 30 days after you have gone through a trial period and verified accurate readings on a daily basis for at least 10 days to ensure it is performing as expected and adapting to its new environment. Any problems such as inappropriate location, interference from other gases or issues with sensitivity can then be corrected and your expectation in its performance can be set with confidence. Once confidence is established a full calibration should be done every 6 months with bump tests every month. If the bump test fails, a full calibration must be performed. All bump tests and calibration functions should be noted in a log book.
The procedure for calibrating the sensors should be simple, repeatable and economical.
- Establish a preventative maintenance schedule and stick to it – whether you do the work in house or hire a reputable technician
- Follow the manufacturer’s instructions on how to properly calibrate your detectors
- Make sure you use the proper calibration adapter that will allow the gas to properly diffuse around the sensor
- Make life easier by buying a calibration kit so you have all the tools you need on hand in a convenient carrying case
- Choose from a wide selection of calibrating gases, including Zero Air, available in 34, 58 & 100 liter size cylinders
Calibration is important because it safeguards against unreliable results; it ensures the sensors are accurately measuring to OSH provincial and state standards and will correctly alert humans of an unsafe environment of toxic or combustible gas buildup. If calibration is not already an element of your business, perhaps it should be. It could save your life!
Note: Calibration and maintenance activities should be done by appropriately qualified and trained personnel. Care and attention must be taken during the process to prevent potential error sources from degrading the results.
For suggestions on gas detection systems, indoor air quality monitors and calibration, please visit www.critical-environment.com.
Wagner , Dave. (2011, July 13) “Bump Test or Calibration Check? General Gas Education” Industrial Scientific The Monitor Blog, accessed 23 September 2016 http://www.askdaveblog.com/general-gas-education/bump-test-or-calibration-check/
Theriot, Francelle. (2011, January 21) “Atmospheric Monitors: Calibration vs. Bump Testing” Roco Rescue OnLine, accessed 23 September 2016 http://rocorescue.wordpress.com/2011/01/21/atmospheric-monitors%E2%80%A6%E2%80%9Ccalibration-vs-bump-testing%E2%80%9D/
MacNeal, James R. (2006, October 1) “Air Monitor Calibration, Bump Testing, and Sensor Challenge” OH&S Occupational Health & Safety, accessed 23 September 2016 http://ohsonline.com/articles/2006/10/air-monitor-calibration-bump-testing-and-sensor-challenge.aspx