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How often to calibrate

Posted September 15th, 2010 by Administrator

Improper calibration and irregular service can render your gas monitors useless.
The news reports are all too frequent. Every year several workers die in gas related accidents. Many of these could have been prevented through the use of a properly calibrated and maintained gas detection instrument. Confusion often surrounds gas monitors, which are a technical piece of equipment often operated by non-technical personnel. Operators are often unaware of calibration requirements and many instruments are used until they either don’t function or are in continuous alarm. In addition to the tragic consequences of death and severe injury, are the steep fines and penalties imposed by OSHA, and the subsequent costly lawsuits filed after the accident. For example, in 2002 OSHA fined a paper mill $91,000 after two workers were killed and eight others injured after being overcome by Hydrogen Sulfide gas. A basic understanding of calibration procedures and top-level maintenance issues can save your company from costly fines and lawsuits and most importantly keep your workers safe.

Calibration Frequency

So what is the proper frequency of calibration? This has been a gray area for many years and has helped contribute to the confusion surrounding calibration. Prior to May 2004 OSHA had never made a recommendation on calibration frequency, only stating that a properly calibrated instrument must be used when there is the potential for harmful gas in a working environment. This vague mandate left it up to the manufacturers to decide what properly calibrated meant. Some manufacturers stated every 30 days and others recommended as much as 180 days. This made it all the more confusing if you had monitors from several different manufacturers. However, in 2004 OSHA published a bulletin in which the ISEA(International Safety Equipment Association) issued a position statement clarifying some confusions surrounding calibration.

The ISEA states “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. If the instrument fails a bump test, it must be adjusted through a full calibration before it is used.”
Some clarification should be made between a full calibration and a bump test. A bump test is the act of exposing the instrument to a known concentration of gas. The instrument must respond to within an acceptable tolerance range, and the concentrations should be high enough to activate the instruments’ alarms so that alarm functionality can be verified. If the bump test results are not within the acceptable tolerance range then a full calibration must be performed. A full calibration is defined as adjusting the instruments readings to coincide with a known concentration of gas, which should be traceable to NIST standards.
Bump tests can be performed with a traditional gas cylinder and regulator, or many manufacturers now sell small bump gas cylinders similar to a squirt bottle. The advantage of these cylinders are their small size and that they don’t require the purchase of costly regulators, which is especially helpful if bump checks need to be performed in several different locations with several different cylinders.
Calibration should always be performed using an in date cylinder and using a regulator with the proper flow rate. The gas cylinder is the most crucial element of the calibration, since the instrument is only as accurate as the cylinder. All manufacturers’ guidelines should be followed including regulators, calibration adaptors, tools, and procedures. Certain gases such as chlorine require special tubing and regulators to compensate for their reactive natures. In addition, since sensors can be affected by environmental conditions such as humidity, temperature, and pressure, instruments should be calibrated in the same or similar environmental conditions that the instrument will be used in.
According to the ISEA, less frequent verification may be appropriate if the following criteria are met:
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.
If an instrument fails calibration or exhibits erratic behavior there can be several causes.
Sensor Problems
Most sensors in a gas monitor are either electrochemical or catalytic. Combustible sensors are typically catalytic and the others are electrochemical. Electrochemical cells produce an electrical current when they are exposed and react to the intended gas. Over time the cells gradually decay and drift, and calibration is necessary to compensate for this degradation. The sensor, which is notorious for dying most frequently, is the oxygen sensor. A Typical lifespan for an oxygen sensor is 1-2 years. This short lifespan is attributed to the sensor constantly reacting to oxygen in the atmosphere, hence the sensor is working non stop, much like a battery that is ran continuously until it dies. Other sensors are normally not exposed constantly to their intended gas, and are therefore only working part time, accounting for their much longer lifespan.
In addition to decay, electrochemical sensors can also experience instability over time. Careful attention should be paid to stable readings while under gas, and stable zeros when the instrument is not exposed to gas. Typical tolerance is +/- 2 counts. This stability should also hold while the instrument is moved, as some sensors will develop motion sensitivity over time. Attention should also be paid to reaction times as sensors reactions will slow over time. Consult your users manual on sensor reaction times as these vary by sensor type and manufacturer.
Electrochemical sensors are also prone to leaking and regular maintenance by a qualified service person who can inspect the instruments internals and sensors, will not only detect a problem sensor, but early detection can often identify the problem before the leaking acid destroys the printed circuit boards in the instrument, which can result in costly repair bills.
Combustible sensors are typically of the catalytic type. These are sensors that contain pellements that are reactive to combustible gases. If the pellements become coated with poisons or contaminates these will act as a barrier and prevent the gas from interacting with the pellement. Great care should be taken to prevent catalytic sensors from being exposed to silicone-based substances. Poisoned catalytic sensors typically exhibit low responses to combustible gases. In addition, if the sensor is exposed to extremely high concentrations of combustible gas the pellements can burn out rendering the sensor inoperable. Catalytic sensors showing signs of poisoning should be changed immediately.
If a sensor is not responding to gas, attention should first be paid to the inlet ports to make sure that they are not clogged. If an inlet port becomes clogged this could prevent gas from reaching the sensors rendering the instrument useless. Inlet ports should always be checked after using an instrument in a dusty or dirty environment.
These potential sensor problems should be carefully watched for. If any sensor displays questionable behavior it should be immediately replaced and a full calibration performed. Regular calibration can usually identify a sensor that is questionable allowing preventative maintenance before it becomes a problem out in the field.


Top level maintenance issues
Batteries
Rechargeable batteries should be changed after 2-3 years or when runtime is less than one full 8-hour work shift. Although most instruments will indicate a low battery alarm, the instrument could shut off unexpectedly and leave the worker unprotected. In instruments that use Nicad batteries attention should be paid to preventing a memory condition. Memory conditions are created when the instrument is recharged repeatedly without occasionally allowing the battery to fully drain. Nicad’s should be cycled at least once a month by allowing them to fully discharge and then fully charge. Most new instruments are using NiMh batteries, which don’t suffer from memory related conditions. When using rechargeable instruments the operator should get into a routine schedule of placing the battery on charge after using the instrument, otherwise the operator may find the instrument’s battery dead at the beginning of a work shift. and may have to wait several hours before the instrument is charged and capable of operating. If the instrument uses alkaline batteries, make sure the operators have an adequate supply of batteries on hand for replacements. Often times workers in their haste to complete a task will continue without a monitor if they don’t have replacement batteries readily available.
Alarms
Make sure that both visual and audible alarms are functioning and are activating at the correct OSHA approved limits. Alarm levels should be checked daily to ensure that tampering or electronics problems have not changed the set points. Make sure that the audible alarm is loud enough to be heard in the working environment. Dirt can often clog the buzzer making it almost inaudible. Visual alarm LED’s should also be checked as these can occasionally burn out or be cracked by an impact.
Display
Most modern gas monitors use a LCD display. Make sure that all segments are displaying properly. If segments are missing this could give the operator a false reading. For example an 8 with the center segments missing may look like a 0 to the operator. In addition the missing segments may prevent the operator from missing critical warnings and instrument feedback.
Corrosion
It is a good idea to periodically have your gas monitors inspected by qualified service personnel that can open the instrument and check for corrosion. Instruments are often used in wet environments and liquids can seep into the instrument and begin to corrode the printed circuit boards. This corrosion can lead to shorts and/or component failure. Early detection cannot only save the board but can prevent an unexpected problem out in the field.

Pumps
If the instrument has an integral pump this should be checked to ensure that the proper flow rate is being obtained. A weak pump may not have enough power to draw the gas to the sensors. In addition make sure that the sample tubing is in good shape with no holes, and is clean and unclogged. A filter used at the end of the sample tubing will help prevent water and dust from clogging the pump.
Conclusion
Regular calibration and maintenance are critical aspects of a gas-monitoring program that will ensure that workers are protected from gas hazards in the workplace. It is recommended that a calibration and maintenance log be kept detailing the instrument serial number, date of calibration, who performed the calibration, the cylinder gas values, cylinder lot number, and notes indicating any maintenance performed. This running log will make it easier to identify weakening sensors and will serve as proof that the instruments were calibrated and maintained properly in the case of a gas related event. A calibration sticker should also be placed on the instrument to alert the operator of when the instrument was calibrated. Verifying proper calibration and operation only takes a few minutes and is the only way to determine if an instrument is functioning properly. This is critical because the use of a gas monitor is the only reliable method of detecting gas hazards, many of which are colorless and odorless.

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