Portable Air Monitor Bump Testing/Calibration
Quick Tips #378
The requirements of the Occupational Safety & Heath Administration (OSHA) General Duty Clause in Section 5(a)(1) state employers are required to provide their employees a workplace free from recognized hazards likely to cause death or serious harm. The use of portable gas detection devices is crucial to complying with this standard by providing employees working in potentially harmful environments the means to monitor their air quality. Harmful environments may include those with high levels of toxic or combustible gases or oxygen deficient or enriched air. Many of these conditions cannot be detected by smell or sight and therefore require instrumental monitoring to view their levels.
Just as a worker is dependent on his personal protective equipment, such as a safety harness/lanyard to ensure protection from a falling hazard, so is a worker dependent on an air monitor to ensure his/her breathing air is within acceptable gas toxicity, combustible and oxygen levels. Neglecting proper maintenance of this piece of equipment is akin to failing to inspect fall protection equipment. Without maintenance and calibration the user is relying on chance to ensure the air-monitoring device will operate correctly when needed in a life-saving application.
Although air monitors/gas detectors from a number of manufacturers are available with a variety of features and can detect a variety of gases, they have one thing in common: they all need to be maintained, bump tested and calibrated properly on a regular basis. The only way to guarantee the instrument will function accurately is to test it by exposing it to a known concentration of test gas which will confirm alarm and sensor functionality. Although OSHA Bulletin SHIB 05-04-2004 confirms that there is no specific standard requiring bump testing, the failure to do so may be cited under the General Duty Clause as it may be seen as failure to provide an environment free from recognized hazards. The International Safety Equipment Association (ISEA) has issued a position statement to further define the practice: “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.”
A bump test, also known as a functional test, should be performed at the start of each day’s use. This procedure tests the alarms and sensors of a gas detector to be sure they are “functional”. The test exposes the detector to a known concentration of gases which exceed the lowest alarm set-point for each sensor. The bump test verifies sensor and alarm functionality but not the accuracy of the instrument (you must rely on calibration for accuracy). To determine which calibration gases are appropriate to your instrument consult the manufacturer’s instructional manual.
If an instrument fails a bump test, it should immediately go through a complete calibration before being put into use. Although a new gas detector most often is calibrated when sent from the manufacturer, it will still require bump testing/calibrating before it is first put into service to ensure it has not been damaged in shipment. Again, consult the manufacturer’s instructional manual to determine which process the manufacturer suggests. .
Calibration is defined by OSHA Bulletin SHIB 05-04-2004 as “an instrument’s measuring accuracy relative to a known concentration of gas. Gas detectors measure the concentration of a gas in an air sample by comparing the sensor’s response to the response generated by a calibration gas of a known concentration. The instrument’s response to the calibration gas serves as the measurement scale or reference point.” As a result of this sensor response, the Bulletin emphasizes the need to calibrate the instrument in the same environmental conditions as the monitor will be used in to ensure accurate gas concentration readings.
An instrument’s sensors will degrade over time and repeated use. The calibration process allows the instrument the opportunity to self-correct so that it will accurately reflect the level of sensor sensitivity. Once a sensor is no longer able to accurately read concentration values, it has then reached the end of its service life and will need to be replaced in order for the instrument to pass calibration. Additionally, any time the detector is dropped or damaged is should be recalibrated.
When purchasing a gas detector/air monitor, several items should also be purchased in order to be prepared to bump test/calibrate including calibration gas, a regulator, tubing, calibration adaptor, cup or cap and a docking/calibration station:
Calibration Gas: The bump test/calibration is only as good as the quality and accuracy of the calibration gas used; therefore it is essential that it is correct. Consult the owner’s manual for the type/mixture of gas needed. Be mindful to not use calibration gas that has surpassed its expiration date as its accuracy cannot be guaranteed. Also, some manufacturers may void their warranty if a gas other than their brand name is used to calibrate the unit.
Many cylinder sizes are available; the most common are 17, 29, 34, 58, 76, 103 and 116 liters. The cylinders are made of either aluminum, which house reactive gases, or stainless-steel, which store non-reactive gases. Aluminum is considered shiny in appearance while stainless steel is seen as dull or milky. Below is a list of common non-reactive and reactive gases used in calibration:
|NON - REACTIVE GASES||REACTIVE GASES|
|CARBON MONOXIDE||ETHYLENE OXIDE|
When replacing your calibration gas or regulator, the following chart lists which calibration cylinders and regulators are compatible with each other:
|Cylinder size||Steel||Aluminum||Cylinder Threading||Regulator Threading||Thread Name|
|17 Liter||X||Male||Female||CGA 600|
Regulator: This item is used to regulate the rate of gas released from the calibration gas cylinder. There are several points to consider when choosing the correct regulator to match the calibration gas cylinder:
- Flow rate: this value is listed in the owner’s manual of your gas detector/air monitor. Using a regulator with the wrong flow rate will decrease the accuracy of bump testing/calibration; it must match the manufacturer’s specifications
- Material type: use brass for non-corrosive/non-reactive gases. Use stainless-steel for corrosive/reactive gases such as ammonia or chlorine
- Cylinder size: the regulator needs to match the size cylinder you are using. Be mindful to read the cylinder label to determine the cylinder size if trying to match it to a regulator. Appearances may be deceiving as a manufacturer can vary the wall thickness and pressure the gas is stored under; not all dimensions of cylinders indicate the same capacity
- Demand or Fixed Flow:
- Demand Flow – pulls gas from the cylinder as needed. Use this type when your instrument has a built-in pump or when performing an automatic calibration using a docking station/calibration station
- Fixed Flow – pulls gas from the cylinder at a fixed rate. Use this type when your instrument does NOT have a built-in pump
Tubing: Used to capture the calibration gas and funnel it towards the air monitor. It typically comes in three foot lengths. If using chlorine or ammonia calibration gas, Teflon/FEP tubing is required. It is suggested to change the tubing annually to ensure it has not been damaged over time.
Calibration adaptor/cup/cap: Used to direct and trap the calibration gas so it flows over the sensors of the instrument. This item most often is included when purchasing the instrument; however, some manufacturers require the item to be purchased separately.
Docking/Calibration Station: Used to house all the calibration equipment in one place and is offered to make the calibration process more convenient. The user docks the monitor and the station runs through a hands free calibration/or a bump test process and the air monitor is ready for use. It varies from manufacturer to manufacturer what items are included in each station. They are not universal and need to be compatible with specific models from one manufacturer. Please note, remember to purchase a demand flow regulator when using a docking/calibration station.
Commonly Asked Questions
- To determine how often to calibrate a gas detector, be sure to consult the manufacturer’s instructions of the specific instrument. Some manufacturers suggest calibrating the instrument every 30 days; other manufacturers leave the calibration decision up to discretion of the end user and/or their safety manager
- If entering permit- required confined space, OSHA mandates calibration of the instrument beforehand in 29 CFR 1910.146(c)(5)(ii)(C)
- The instrument may have a field entitled “number of days since last calibrated” to help keep track of the last time the unit was calibrated
- If a bump test fails, the instrument must go through a full calibration
Find even more information you can use to help make informed decisions about the regulatory issues you face in your workplace every day. View all Quick Tips Technical Resources at www.grainger.com/quicktips.
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