Gas detectors are life saving devices. They are precision instruments that are built solely to measure and monitor potentially lethal gases in the workplace. The only way to ensure that a gas monitor will accurately respond to the hazardous gas or gases it is designed to detect is to calibrate the sensors against a known gas standard.
Every day, workers in a variety of industries face the invisible threat of atmospheric hazards found lurking in the workplace or as they enter confined spaces. Atmospheric hazards have contributed to confined spaces fatalities and have a much higher fatality to injury rate than most other types of workplace accidents. Dangerous conditions involving unsafe oxygen levels, as well as toxic and combustible gases have caused the most accidents in this type of work environment. Atmospheric hazards are not visible, nor accurately detectable through any of the four other human senses.
Atmospheric conditions within confined spaces are also capable of changing rapidly, adding an element of unpredictability and elevating the level of danger. For these reasons, the only way to safely enter and perform work in a confined space is by using a gas monitoring instrument that will alert the worker to potential dangers within the space.
It is imperative that oxygen levels within confined spaces be tested and monitored to determine if the level is either too low, causing asphyxiation, or too high, thereby increasing the explosive potential, for a worker to safely enter. Two of the most common toxic gases, hydrogen sulphide and carbon monoxide can also cause asphyxiation in confined space atmospheres.
These toxic gases are also among the many gases that are combustible when combined with oxygen in certain proportions and an ignition source. Combustible or flammable gases can be naturally occurring or produced during industrial processes, and must be monitored to prevent explosions.
The potential for these hazards to cause harm can be significantly reduced and, many times, virtually eliminated if confined space requirements are carefully followed. Many jurisdictions have put statutory regulations in place to minimize the risks and hazards associated with working in confined spaces. In the United Kingdom, under domestic law (the Health and Safety at Work etc Act 1974) employers are responsible for ensuring the safety of their employees and others. This responsibility is reinforced by regulations.
By order of the HSE, the Confined Spaces Regulation 1997, calls for a safe system of work, when working in confined spaces is unavoidable. One of the components to be included in such a system is equipment for atmospheric testing before, during and after entry and use of personal multi-gas monitoring devices which has been calibrated. As published in the HSE leaflet “Safe Working in Confined Spaces,” this is put as follows:
Testing the air This may be necessary to check that it is free from both toxic and flammable vapours and that it is fit to breathe. Testing should be carried out by a competent person using a suitable gas detector which is correctly calibrated. Where the risk assessment indicates that conditions may change, or as a further precaution, continuous monitoring of the air may be necessary.
In the U.S., OSHA’s Federal Register, 29 CFR 1910.146 states that before an employee enters a permit required confined space “the internal atmosphere shall be tested, with a calibrated direct reading instrument, for the following conditions in the order given: (1) oxygen content, (2) flammable gases and vapours and (3) potential toxic air contaminants.”
Regular instrument calibration must be part of preparing for the potential dangers of confined space work. Only then, can you be certain that the instrument will properly function and accurately respond to hazardous concentrations of gas. Sensor technologies most commonly used for confined space and personal monitoring include catalytic diffusion for combustible gases and electrochemical sensors for oxygen and toxic gases. Each has special characteristics and calibration requirements, but none are immune to the eventual need for verification of the sensor’s response to a known concentration of a target gas.
The instrument’s response to calibration gas exposure is known as its “reference point,” or the point where all atmospheric gases will be measured and compared. When an instrument’s reference point shifts, its readings also shift and become inaccurate. This is known as “calibration drift,” and can be caused by chemical degradation of sensors, drift in electronic components, exposure to extreme environmental conditions, exposure to high concentrations of target gases, or exposure to poisons and inhibitors.
Degradation of sensors
Scientific calculations of sensor degradation and sensitivity reduction show long-lasting performance specifications based on ideal lab conditions. Given this, it should be possible for sensors to last two, three or four years without any significant loss of sensitivity. A lot of sensors do indeed last that long. Technically, the sensors are good enough to withstand just about any variation of calibration procedures. However, the issue really isn’t how good the sensors are, it is how they are used. Unfortunately, the industrial environment that the instruments are used in is not a laboratory and there are too many unknowns that these life-saving devices face each and every day that may affect the accuracy and reliability of the sensor performance. There are a number of reasons why a sensor may unexpectedly lose sensitivity, drift, or fail to respond accurately to the target gas. These reasons may include sensor poisoning, leakage, over-exposure, temperature and humidity extremes, or physical damage due to dropping or immersion.
Electrochemical sensors, commonly used for carbon monoxide, hydrogen sulphide and oxygen monitoring, are normally stable and degradation is slow. However, normal degradation of electrochemically-based toxic and oxygen sensors is accelerated by low humidity and high temperatures due to the chemical reactions and consumption of the electrolyte. Catalytic sensors are susceptible to poisoning when exposed to substances containing silicon, halogenated hydrocarbons, and high concentrations of hydrogen sulphide. Also, any sensor may be rendered useless if hit with extremely high concentrations of the target gas. Some monitors combat this with an “over-range” feature which will kill the power to the sensor at a certain limit so that it has limited exposure to the gas to preserve the life of the sensor.
When calibration drift occurs, as it does in all instruments over time, the device is still capable of measuring gas concentrations. The problem, however, is in the accuracy of the numeric reading. Performing a full calibration resets the instrument’s reference point and ensures accurate readings, which are vital to worker safety.
Contrary to what you may have been led to believe, there is no electronic method for compensation or self calibration of sensors that will correct the effects of drops, shocks, or extreme exposures to gas or temperatures. When you think about a typical industrial environment and the multiple workers that carry the monitors into confined spaces, you realize that there is a good chance that they will be bumped or dropped, stored in a hot truck, or hit with a strong blast of gas. These factors affect the sensor’s ability to react to gas at the maximum accuracy possible. Think twice about a claim that there is a gas detector that requires no calibration – it’s simply unreasonable.
Bump or functional test
With so many reasons why a sensor can lose sensitivity and fail to respond in a gas hazard situation, frequent confirmation of the sensor’s performance is justified. There are two methods of verifying instrument calibration. A “bump” or functional test is defined as the brief exposure of the monitor to a concentration of gas(es) in excess of the lowest alarm set-point for each sensor. The instrument reading is compared to the actual concentration of gas and if it is within an acceptable range of the actual concentration (usually within 10%), then its calibration is verified. A bump test will ensure that the sensors are working properly. When performing a bump test, the test gas concentration should be high enough to trigger the instrument alarm for each sensor. If a functional test fails, then the instrument must be adjusted through a full calibration before it is used.
A full calibration goes a step further than a functional test. A full calibration ensures maximum accuracy of the instrument if performed successfully. Again, using a known concentration of test gas, the instrument reading is compared to the actual concentration of the gas and then adjustments are made to the readings if they do not match. Today, most direct-reading instruments offer quick, push-button calibration with electronic corrections in place of older potentiometer adjustments. If a sensor fails calibration it should be replaced and the instrument must be recalibrated.
Recently, OSHA posted a Safety and Health Information Bulletin referenced by many multi-national companies, which underscores the criticality of atmospheric monitoring and specifically addressing the need for regular calibration of direct-reading portable gas monitors. The Information Bulletin states that inaccurate gas concentration readings could lead to injury or death and that avoiding this is the primary reason for calibration.
Although it is not a standard or regulation, the Bulletin is a clear recommendation to follow the guidelines put forth in the position statement released by the International Safety Equipment Association (ISEA) on instrument calibration for gas monitors used in confined spaces. The ISEA statement says, “A bump test or full calibration of direct-reading portable gas monitors should be made before each day’s use in accordance with manufacturer’s instructions, using an appropriate test gas.”
Probably the biggest point of contention about calibration is the frequency between full calibrations and functional or bump testing the instruments. ISEA recommends the following, if conditions do not permit daily testing:
- During a period of initial use of at least 10 days in the intended atmosphere, calibration is verified daily to ensure there is nothing in the atmosphere to poison the sensor(s). The period of initial use must be of sufficient duration to ensure that the sensors are exposed to all conditions that might adversely affect the sensors
- If the tests demonstrate that no adjustments are necessary, the interval between checks may be lengthened, but it should not exceed 30 days
ISEA recommends more frequent testing if environmental conditions that could affect instrument performance are suspected, such as sensor poisons.
The most important tool for accurate calibrations is the test gas itself. Always ensure that the gas cylinder has not reached its expiration date before calibration. The type and concentration of the gas, sample tubing, regulators, and calibration adapters must be appropriate for the instrument and sensors. Combustible gas sensors are non-specific and can be calibrated to any number of different gases. Choose the calibration gas that most closely matches the gas that will be encountered. If not known, or if a mix of gases is suspected, then calibrating with pentane gas is recommended. Using pentane as a calibration standard will allow the sensor to detect a larger group of hydrocarbons commonly found in confined spaces. Today, many multi-blend cylinders of gas are manufactured to simplify the task of calibrating multi-gas monitors. Match the cylinder contents and to the sensors installed and make sure the concentrations will handle the instrument alarm set points.
For verification of accuracy, calibration gas should be gravimetrically produced and traceable to the U.S. National Institute for Standards and Technology (NIST). NIST traceability comes from certified, traceable weights used in the gravimetric filling process. This means the gases are produced using a calculation of gas weight based on the molecular weight of each gas component in the mixture. The gravimetric process is the most accurate method for producing calibration gases, and is not affected by temperature or pressure. Other methods, including dynamic blending and pressure blending, are dependent on temperature and pressure. Supporting documentation and certificates of analysis should be available from the calibration gas manufacturer to verify the process used and the results of the gas mixture as proof of accuracy.
The importance of regular instrument calibration is critical to prevent inaccurate readings. There is no global standard or universal procedure written to direct companies on the specifics, mainly because many types of instruments are used in various environments and use conditions. OSHA instructions are to follow the manufacturers’ recommendations, which shift the responsibility to both the user and the manufacturer of the monitor.
In addition to the reference to calibration of gas detection equipment, there has been legislation enacted in Canada requiring a daily bump test of instruments in some provinces. Even so, the number of users enquiring about manufacturer’s recommendations and searching for recommendations that are in conflict with the legislation continues to increase, stating that they feel no need to comply with the bump test rule if the manufacturer’s policy says otherwise.
Automated testing systems
The best way to ensure regular instrument calibration is to develop a procedure that includes a schedule for bump testing and full calibration for all gas detectors in a company’s fleet. Numerous manufacturers have designed and provided systems that are built to automate the test, calibration and documentation requirements that industry does impose on the use of gas monitoring instruments. These systems are designed to automatically maintain the instruments by bump testing, calibrating, downloading hygiene data, testing integrity, and charging the instruments automatically with no need for end user intervention.
What makes these systems so popular? Many companies have discovered the convenience and cost savings of automated calibration stations or full function instrument management systems which can be programmed to bump test or calibrate instruments on schedule or on demand so the instruments are correctly calibrated and ready at a moment’s notice. The fact that the end users do not need to do anything but set the instrument into a cradle and walk away is appealing to everyone. With companies downsizing, instrument maintenance sometimes suffers. There are fewer instrument specialists available to maintain fleets.
These automated systems are ideal solutions for such a situation. One person does not have to manually maintain all the instruments and documentation. The system can be set up to automatically manage and maintain the fleet with little or no user intervention. With automatic record keeping built into the systems, all the required documentation is safely stored on computers and can be printed at any time.Databases manage the data storage and allow for easy searches on calibration histories and hygiene data.
In this era of “doing more with less,” these automated, intelligent systems reduce human intervention and are not only appealing from a resource standpoint, but they minimize the time spent on routine processes while increasing consistency and reliability. In addition to the regulatory requirements of properly calibrating them, gas monitors are like automobiles in that routine maintenance may help to extend the repair cycle, but eventually the battery, sensors and general mechanical repairs are inevitably required with this type of instrumentation.
Manufacturers may offer extended warranty plans, instrument exchange programs, or planned repair service options to support the ongoing maintenance tasks, but often the repair resources and budget dollars are not planned at the onset of the initial purchase. It is not unusual that repair and maintenance costs of gas monitors are unknown due to sporadic repair schedules, assimilation into operating expenses or lack of historical records. Without pre-planned repair and maintenance services, a company may face worker downtime if instruments required for safe working conditions are in the repair shop and replacement instruments are not available.
The most critical part of a gas monitor is the sensor, which has a shelf-life and therefore should not be purchased too far in advance prior to its use. Parts procurement can become a ‘Catch-22’ between pre-planning and shelf life dilemmas, which imply micromanagement tactics, must be used to achieve an economical balance. Consideration should be given to programs that offer on-call or immediate delivery of age-sensitive parts to avoid inventory problems.
Regulatory compliance is just the start of the obligation required to own and operate these life-saving devices. Company procedures and best practices should be driven by the usage patterns, operating conditions and environmental conditions present. Don’t take chances – calibrate with knowledge – calibrate with confidence. Calibration is the key to accurate gas detection.
Published: 10th Jan 2009 in Health and Safety International