Bump Test and Calibration Issues and Answers
By: Bob Henderson, Contributor
Manufacturers and regulatory agencies agree the safest and most conservative approach is to perform a functional bump test or calibration check by exposing your gas detector to gas before each day’s use. But instrument users still have questions.
Oxygen deficiencies, explosive atmospheres, and exposure to toxic gases and vapors injure hundreds of workers every year. The atmospheric conditions that lead to these accidents and fatalities are usually invisible to the workers who are involved. The only way to ensure atmospheric conditions are safe is to use an atmospheric monitor. The only way to know whether an instrument is capable of proper performance is to expose it to test gas. Exposing the instrument to known concentration test gas verifies that gas is properly able to reach and be detected by the sensors. It verifies the proper performance of the instrument’s alarms, and (if the instrument is equipped with a real-time display), that the readings are accurate. Failure to periodically test and document the performance of your atmospheric monitors can leave you open to regulatory citations or fines, as well as increased liability exposure in the event that a worker is injured in an accident.
OSHA recommends developing procedures for calibrating and using DRPGMs that include documentation to verify the proper maintenance and calibration of the instruments.
There has never been a consensus among manufacturers regarding how frequently direct reading portable gas detectors need to be calibrated. However, manufacturers do agree that the safest and most conservative approach is to verify the performance of the instrument by exposing it to test gas before each day’s use. Performing a functional “bump test” or “calibration check” is very simple and takes only a few seconds to accomplish. It is not necessary to make a calibration adjustment unless the readings are found to be inaccurate.
Several OSHA standards require the use of gas monitors and include guidance on testing and maintenance. For instance, OSHA 1910.146 “Permit-required confined spaces” paragraph (c)(5)(ii)(C) explicitly states (in part) that, “Before an employee enters the space, the internal atmosphere shall be tested, with a calibrated direct-reading instrument.” OSHA Compliance Directive CPL 2.100, “Application of the Permit-Required Confined Spaces (PRCS) Standards, 29 CFR 1910.146” explains what is meant by calibrated: “A testing instrument calibrated in accordance with the manufacturer’s recommendations meets this requirement. The best way for an employer to verify calibration is through documentation.”
It is up to the manufacturer to determine the methods, as well as how frequently the instrument should be calibrated. The manufacturer’s requirements are spelled out in the owner’s manual. Instrument users are held accountable to these requirements. If you fail to follow the instructions in the manual it means the instrument is not being properly used and maintained. This could be grounds for an OSHA citation, but even more importantly, it could lead to dangerously inaccurate readings. The cautions and warnings in the manual are there for a reason!
For instance, if the instrument includes a sensor for measuring percent LEL explosive gas, one of the cautions at the very front of the manual is that the LEL sensor should be tested by exposure to known concentration gas before each day’s use. This is to guard against using the instrument if the LEL sensor has been damaged or has lost sensitivity. Why is this important? Standard catalytic type LEL sensors, the most common type of sensor used to measure percent LEL explosive gas, can be easily damaged by exposure to vapor that contains silicones or other LEL sensor poisons and inhibitors. There is no way to tell if the sensor has been affected except by exposing it to gas.
ISEA Statement on Validation of Operation for Direct Reading Portable Gas Monitor
The International Safety Equipment Association (ISEA) is the leading international organization of manufacturers of safety equipment, including environmental monitoring instruments. The ISEA has developed a “Statement on Validation of Operation for Direct Reading Portable Gas Monitors” that provides additional guidance. OSHA has incorporated these recommendations into their own Safety and Health Information Bulletin on “Calibrating and Testing Direct-Reading Portable Gas Monitors.”
The ISEA statement emphasizes the importance of verifying the calibration of instruments used to monitor the atmosphere in potentially hazardous locations, and clarifies the methods used to validate the operational capability of portable gas monitors. The statement defines the differences between a bump test (function check), calibration check, and a full calibration, and clarifies when these validation methods are to be performed. The ISEA statement applies to all of the sensors installed in the direct reading gas detector, not just the combustible sensor, and is designed to be applicable to personal as well as multi-gas instruments. It is not limited to instruments used in confined space entry.
What causes an instrument to lose accuracy?
Single-sensor instruments are designed to focus on a single toxic contaminant or hazardous condition (such as H2S or O2 deficiency), or the presence of a class of atmospheric hazard (such as the presence of combustible gas). “Zero maintenance” single-sensor instruments may or may not include a meter for the display or real time readings, and they may or may not be capable of calibration adjustment while exposed to test gas. Confined space and other types of multi-sensor instruments include several different types of sensors. The type of sensors installed depends on the specific monitoring application.
The atmosphere in which the instrument is used can have profound effect on the sensors. Each type of sensor uses a slightly different detection principle. Sensors may be poisoned or suffer degraded performance if exposed to certain substances. The kinds of conditions that affect the accuracy of sensors vary from one type of sensor to the next.
While the electrochemical sensors used to measure toxic gases like carbon monoxide and hydrogen sulfide are not worn out or consumed by exposure to CO or H2S, they still eventually need to be replaced when they are no longer able to detect gas. Although the sensors may last for years, the loss of sensitivity at the end of life may be sudden. Incidental exposure to other substances may also reduce sensitivity. For instance, many electrochemical sensors can be permanently affected by exposure to organic solvents and alcohols.
Combustible sensors are prone to damage due to exposure to poisons or substances that inhibit the sensor’s response to combustible gas. Combustible sensors may be affected by exposure to volatile silicones, chlorinated solvents (such as methylene chloride), sulfides (including H2S), hydrides (such as phosphine and arsine), or even exposure to high concentrations of combustible gas. Sensors may also suffer loss of sensitivity due to aging, mechanical damage due to dropping or immersion, or loss of sensitivity due to other causes.
Even if a sensor is internally healthy, if gas is not capable of reaching and diffusing into the sensor because of blockage or leakage in the pump or sampling system, or because the external filter has become clogged or contaminated, the sensor cannot properly respond. Thus even “zero maintenance” single-sensor instruments should be periodically exposed to gas to ensure that the instrument is capable of proper response. Even if the sensor response and readings are correct, if the alarms are not properly activated, or if the instrument fails to operate properly in other ways when exposed to gas, the instrument must be serviced to restore proper function before it can be used.
Validation of Operability
The ISEA protocol begins by clarifying the differences between a “bump test”, a “calibration check” and a “full calibration”.
A “bump test” (function check) is defined as a qualitative check in which the sensors are exposed to challenge gas for a time and at a concentration sufficient to activate at least the lower gas measurement alarms. The test confirms that the gas is capable of reaching the sensors, that when they are exposed to gas the sensors respond, the response time (time to alarm) after gas is applied is within normal limits, and that the alarms are activated and function properly. However, a qualitative function test does not verify the accuracy of the readings or output of the sensors when exposed to gas.
A “calibration check” is a quantitative test using a traceable source of known concentration test gas to verify that the response of the sensors is within the manufacturer’s acceptable limits. For instance, a manufacturer might specify that readings in a properly calibrated instrument should be within ±10% of the value of the gas applied. If this is the pass / fail criterion, when 20 ppm H2S is applied to the instrument, the readings must stabilize between 18 ppm and 22 ppm in order to pass the test. It should be stressed that these pass / fail criteria are manufacturer guidelines. Different manufacturers are free to publish different requirements.
A “full calibration” is defined as the adjustment of an instrument’s response to match a desired value compared to a known traceable concentration of calibration gas. The calibration procedure, including the concentration of gas applied, method used to apply gas, and method used to adjust the readings are determined by the manufacturer.
The ISEA statement recommends that a “bump test” (functional test) or “calibration check” of direct reading portable gas monitors should be made before each day’s use in accordance with the manufacturer’s instructions using an appropriate test gas. Any instrument that fails the test must be adjusted by means of a “full calibration” procedure before further use or taken out of service. If environmental conditions that could affect instrument performance are suspected to be present, such as sensor poisons, verification of calibration should be made on a more frequent basis.
A “full calibration” should be conducted as required by the manufacturer, or whenever testing indicates that adjustment is required. Even if the instrument is not yet “due” for a “full calibration”, if the instrument fails a “bump test” or “calibration check” it must be calibration adjusted before further use.
The ISEA Protocol provides a list of conditions that can adversely affect the sensors and trigger a need for more frequent validation. Make sure to test the instrument after any event like dropping or submerging the instrument, or if the instrument changes hands during the course of the workday.
Docking stations make CS instruments even easier to use and maintain.
Most data logging direct reading instruments automatically update and store dates as well as monitoring and calibration information. Even non-data logging instruments usually store the date, or number of days since the last time the instrument was calibrated.
Most gas detector manufacturers offer automatic calibration or “docking” stations that automatically tests or calibrates the instrument, and updates and stores the test results. Docking stations usually go beyond the minimum requirements for bump test and calibration checks. Most docking stations verify that the alarms are activated, the time it takes for the sensor to reach the alarm after exposure to gas, and the accuracy of the reading. Docking stations can also alert users to overdue maintenance procedures and may be able to communicate directly with the factory or service center.
While there are strong advantages, there is no requirement to use a docking station. Performing a manual bump test is fine! But make sure you keep good records. Many users keep a notebook or journal in the same case as the instrument and test gas, or periodically download the test and exposure results from the instrument.
What is the best concentration of gas to use for the daily bump test?
Use a concentration of gas for each sensor that is at least 50% higher than the lower alarm setting that is being tested. Be careful not to use a concentration of gas that is close to the concentration needed to trigger the alarm. Most common sensors, like the ones used to measure LEL, O2, CO and H2S reach at least 50% of their final reading (T50) within the first 10 seconds after exposure to gas. But it can take over a minute for the same sensors to reach 100% of their final stable reading (T100). If the low alarm for your CO sensor is set at 50 ppm, and you use 50 ppm to test the sensor, it may take 80 – 90 seconds for the reading to finally reach and trigger the alarm – if it ever does. On the other hand, if you use 200 ppm CO to test the sensor, it will probably take less than 5 seconds to trigger the alarm.
Does the bump test have to include all of the sensors?
OSHA recommends developing standard procedures for calibrating and using instruments that include documentation to verify the proper maintenance and calibration, but the guidance does not include the requirement to test all of the sensors during the bump test.
In general, with 4-gas O2 / LEL / CO / H2S instruments it is so easy and convenient to test the sensors that there is nearly universal acceptance that all of the sensors should be tested during a bump test or calibration check. But when the instrument includes certain other types of sensors, or when the exposure to gas has negative consequences for some of the sensors, it may be necessary to take a different approach.
Direct reading gas detectors are designed to help keep workers safe in potentially life-threatening environments. Verifying the proper performance of your gas detectors is a mandatory part of every program that requires their use. But more importantly, it’s an essential part of keeping your workers safe.