Bump Test and Calibration Issues and Answers

By: Bob Henderson, Contributor

Using small high-pressure cylinders of test gas makes it possible to package complete kits that include the instrument, regulator, gas and accessories in a compact carrying case. Each compact cylinder provides up to 250 daily bump tests. (photo courtesy GfG Instrumentation)

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. The regulatory standards that govern confined space entry and other activities that include the use of direct reading instruments are in agreement with this approach.

The OSHA website has a very useful Safety and Health Information Bulletin (SHIB) on “Calibrating and Testing Direct-Reading Portable Gas Monitors” that is posted at the following link:  https://www.osha.gov/publications/shib093013

The bulletin is not a standard or regulation and creates no new legal obligations for testing and maintaining “DRPGMs”. It is intended to assist employers in conforming with both regulatory and “best practice” guidelines.

Several OSHA standards require the use of gas monitors and include additional 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.

The owner’s manual is part of the materials that are reviewed by third party testing laboratories like CSA^© and UL^©. The review includes verification the manual includes certain cautions and warnings, and that the wording is consistent with the standards to which the instrument is certified. Always follow the instructions in the owner’s manual! 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 Monitors

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 the Safety and Health Information Bulletin.

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.

In the United States and Canada many federal, state, and provincial agencies, including OSHA, have issued instructional best practice guidelines that are largely based on the ISEA protocol. NFPA Standard 350, “Guide for Safe Confined Space Entry and Work” references the ISEA standard as well. In some jurisdictions performing a bump test (functional test) before each’s days use is not yet mandatory, but in all jurisdictions, it is recognized that the safest course of action is to perform a bump test that includes exposure of the sensors in the instrument to test gas before each day’s use.

What causes an instrument to lose accuracy?

Docking Stations are designed to perform bump tests or calibration checks, full calibrations, and automatically store the testing records. (photo courtesy GfG Instrumentation)

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 CO and H₂S sensors may last for years without significant loss of sensitivity, 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.

Similarly, fuel-cell type oxygen sensors near the end of their use-life may develop performance problems such as abnormally slow response. For these reasons performing a daily bump test on oxygen sensors is particularly important.

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.

What do the regulations say?

Instrument users are held accountable to calibrating and / or testing the performance of their instruments in accordance with the manufacturer’s instruction manual. OSHA expects instrument users to be able to document that their procedures match the requirements listed.

The instructions, cautions and warnings listed in the owner’s manual are not by OSHA governed, they are determined by the manufacturer’s assessment of the characteristics of the product, and the external standards to which the instrument is certified by Nationally Recognized Testing Laboratories such as UL® or CSA®.

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. It is important to understand what a qualitative test of this kind does not do. 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. Once again, 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 statement goes on to recommend the frequency for validation of the instrument’s operability:

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, then verification of calibration should be made on a more frequent basis.

A “full calibration” should be conducted as required by the manufacturer. However, as discussed above, a “full calibration” should be conducted 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.

According to the ISEA statement, sometimes even daily testing may not be enough

Certain conditions and events have the potential to adversely affect the performance of the sensors and/or the entire instrument. Sometimes the damage and effect on performance is immediate. Sometimes the underlying damage is chronic in nature and occurs over time. Sometimes, when the instrument stops working properly, it can happen very quickly.

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

Given the requirement for documentation, the capability of instruments to log or automatically retain calibration information is highly desirable. Most data logging confined space instruments automatically update and store dates and other calibration information. Even non-data logging instruments usually include the date, or number of days since the last time the instrument was calibrated.

Most leading manufacturers of confined space gas detectors offer automatic calibration or “docking” stations that automatically tests or calibrates the instrument, then updates and stores the test results.  Docking stations are also able to transparently improve the quality of 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.

Is a manual bump test or calibration check as good as using a docking station?

When you use a docking station the “daily test” usually goes well beyond a minimal qualitative bump test. It not only verifies that the alarms are activated, it verifies the response time of the sensors, and the accuracy of the readings. It can be programmed to notify the user if it is time to perform a full calibration rather than a daily test, and automatically keeps records of your calibration and test results.

However, 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. 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?

The answer depends on whether the sensors are harmed or consumed by exposure to gas, the feasibility and time involved in testing the sensors, and the manufacturer’s guidance. 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.

For instance, cyanide (HCN) sensors are often included in multi-gas instruments used by fire departments. Electrochemical HCN sensors are consumed by exposure to cyanide, and they are not as quick to reach their final response as many other sensors. Cyanide is also a known sensor poison for catalytic type LEL sensors. If you test all of the sensors, including the HCN sensor, it is especially important to use a high enough concentration to complete the bump test in as short a time as possible.

The longer the exposure to HCN gas, the more chance of shortening the life of the HCN sensor or damaging the LEL sensor. Some manufacturers suggest performing a bump test or calibration check on the HCN sensor on a weekly basis, and a full calibration at least once every 30 days.

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.