This Safety and Health Information Bulletin is

not a standard or regulation, and it creates no

new legal obligations. The Bulletin is advisory in

nature, informational in content, and is intended

to assist employers in providing a safe and healthful workplace. The Occupational Safety and

Health Act requires employers to comply with

hazard-specific safety and health standards. In

addition, pursuant to Section 5(a)(1), the General Duty Clause of the Act, employers must

provide their employees with a workplace free

from recognized hazards likely to cause death or

serious physical harm. Employers can be cited

for violating the General Duty Clause if there is a

recognized hazard and they do not take reasonable steps to prevent or abate the hazard. However, failure to implement any recommendations

in this Safety and Health Information Bulletin is

not, in itself, a violation of the General Duty

Clause. Citations can only be based on standards,

regulations, and the General Duty Clause.

U. S. Department of Labor

Occupational Safety and Health Administration

Directorate of Science, Technology & Medicine

Office of Science and Technology Assessment

SHIB 05-04-2004

Verification Of Calibration for Direct-Reading Portable Gas Monitors

Safety and Health Information Bulletin

Introduction

Gas monitoring instruments are designed to protect

personnel from unseen hazards that may exist in

workplace environments, including confined

spaces. It is vital to worker safety that these

instruments are maintained and calibrated properly.

Instrument inaccuracy due to improper or irregular

calibration can lead to serious accidents. Exposure

to excessive levels of toxic gas or an oxygendeficient environment can cause workers serious

illness and even death. Combustible gas explosions

are often catastrophic, injuring or killing personnel

and destroying property.

The International Safety Equipment Association

(ISEA), founded in 1933, is a trade association for

manufacturers of protective equipment, including

environmental monitoring instruments. The ISEA

recommends, at a minimum, verification of sensor

accuracy before each day’s use.

The only way to guarantee that an instrument will

detect gas accurately and reliably is to test it with a

known concentration of gas. Exposing the

instrument to a known concentration of test gas

will show whether the sensors respond accurately

and whether the instrument alarms function

properly.

Calibration: The Key to Accurate Readings

Gas detection instruments are used to detect the

presence of toxic and combustible gases, as well as

oxygen deficiency or oxygen enrichment (a fire and

explosion hazard). Workers cannot rely on their

sense of smell to alert them to odorless hazards,

necessitating the use of gas detectors whenever a

worker enters an area with potential atmospheric

hazards.
2

“Calibration” refers to 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.

The responsiveness of electrochemical sensors will

vary with environmental conditions. Sensor

response will be different (lower or higher)

depending on the actual environmental conditions.

Therefore, as much as possible, the monitors

should be calibrated at environmental conditions

that are the same as (or similar to) actual field

conditions. Calibration at locations where the

equipment is to be used is always preferable.

Most instruments are equipped with two levels of

alarms – warning and danger. The warning alarm

alerts the user that the environment has a

detectable concentration of gas and is therefore

potentially hazardous. The danger alarm indicates

that the gas concentration exceeds the programmed

“hazard” threshold, and the area is approaching a

hazardous level. Whether an instrument warns

and/or alarms at the proper time depends on its

detection abilities and its ability to translate its

findings into an accurate reading.

If the instrument’s reference point has shifted, the

reading will shift accordingly and be unreliable.

This is called “calibration drift” and it happens to

all detectors over time. An instrument that

experiences calibration drift can still measure the

quantity of gas present but it cannot convert this

information into an accurate numerical reading.

Regular calibration with a certified standard gas

concentration will update the instrument’s

reference point, ensuring that the instrument will

produce continued, accurate readings.

Causes of Calibration Drift

Over time, the accuracy of gas detection

instruments can diverge from their calibration

settings in several ways:

• Gradual chemical degradation of sensors and

drift in electronic components that occur

naturally over time.

• Chronic exposures to, and use in, extreme

environmental conditions, such as high/low

temperature and humidity, and high levels of

airborne particulates.

• Exposure to high (over-range) concentrations

of the target gases and vapors.

• Chronic or acute exposure of catalytic hot-bead

LEL sensors to poisons and inhibitors. These

include: volatile silicones, hydride gases,

halogenated hydrocarbons, and sulfide gases.

• Chronic or acute exposure of electrochemical

toxic gas sensors to solvent vapors and highly

corrosive gases.

• Harsh storage and operating conditions, such

as when an instrument is dropped onto a hard

surface or submerged in liquid. Normal

handling/jostling of the equipment can create

enough vibration or shock over time to affect

electronic components & circuitry.

Often, after exposure to the more extreme

conditions above, when calibration is attempted,

the detector will either display a failure message or

it will not allow the user to fully adjust the display

reading. At this point, the severely damaged

sensor must be replaced and/or the detector

serviced by qualified personnel.
Worker Safety: The Number One Reason for

Proper and Regular Calibration

The primary reason for proper, regular instrument

calibration is to prevent inaccurate gas

concentration readings that could lead to injury or

to death. Correctly calibrating an instrument helps

to ensure that the instrument will accurately

respond to the gases that it is designed to detect,

warning users of hazardous conditions before they

reach dangerous levels. In addition to detecting

and correcting for calibration drift, regular

calibration assures the user that the instrument is

functional. Gas detection instruments are often

subjected to harsh operating and storage conditions

where they can be damaged. Both of these factors

can affect instrument performance, leading to

inaccurate readings or even instrument failure.

While a unit may appear to be sound during a

visual inspection, it actually could be damaged

internally. Regular calibration is the only way to be

certain that a detector is fully functional.

Moreover, a standing policy for regular calibration

sets the tone for a safety-conscious work

environment and indicates to workers that safety is

a priority. As a result, workers may be more likely

to keep safety principles in mind throughout the

workday.

A written record of calibration should be kept for

the life of each instrument. This record allows

users to quickly identify an instrument that has a

history of excessive maintenance/repair or is prone

to erratic readings.

Bump Tests vs. Full Calibration

There are two methods of verifying instrument

accuracy: a functional or bump test and a full

calibration, each appropriate under certain

conditions. A bump test verifies calibration by

exposing the instrument to a known concentration

of test gas. The instrument reading is compared to

the actual quantity of gas present (as indicated on

the cylinder). If the instrument’s response is within

an acceptable tolerance range of the actual

concentration, then its calibration is verified.

(Note: It is recommended that

users check with the detection equipment

manufacturer for the acceptable tolerance ranges.)

Instruments should be “zeroed” before the bump

test in order to give a more accurate picture of the

bump test results. When performing a bump test,

the test gas concentration should be high enough to

trigger the instrument alarm.

If the bump test results are not within the

acceptable range, a full calibration must be

performed. A full calibration is the adjustment of

the instrument’s reading to coincide with a known

concentration (generally a certified standard) of

test gas. For verification of accuracy, calibration

gas should always be certified by and traceable to

the National Institute of Standards and Technology

(NIST). In most cases, a full calibration is only

necessary when an instrument fails a bump test or

after it has been serviced. The full calibration and

bump test should be conducted in a clean fresh air

environment.

When to Bump Test and When to Calibrate

In the past, there often has been confusion

regarding proper calibration procedures and

frequency. To clarify this issue, the International

Safety Equipment Association (ISEA) issued a

position statement on instrument calibration that

states, “A bump test or full calibration of directreading 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.

ISEA recommends more frequent testing if

environmental conditions that could affect

instrument performance are suspected, such as

sensor poisons. The ISEA allows for less frequent

calibration verification under certain conditions

(see below), but the interval between testing should

never exceed 30 days.
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.

• When calibrating an instrument, always

follow the instrument user’s manual for the

manufacturer’s recommended calibration

frequency and procedure.

Calibration Rules

The following are a few basic instrument

calibration rules to ensure a clear path to health

and safety.

• Follow the manufacturer’s guidelines for

proper calibration. No job, including

instrument calibration, can be performed

properly or safely without the right tools. The

type and concentration of calibration gas,

sample tubing, flow regulators and calibration

adapters are key links in the calibration chain.

Using equipment provided by the original

manufacturer should ensure a proper start to

every calibration.

• Only use certified calibration gas before its

expiration date. The most important tool used

in calibration is the gas itself. The instrument

can only be as accurate as the gas used to

calibrate it. Be certain your supplier can

provide a traceable certificate of analysis for

every calibration gas cylinder. The

concentration of calibration gas, particularly

the concentration of reactive gases such as

hydrogen sulfide or chlorine, will only remain

stable for a finite period of time. Never use

calibration gas after its expiration date.

• Train workers on the proper methods of

calibration. Most instruments are designed to

be field calibrated with instructions detailed in

user manuals, training videos or computerbased training modules. Everyone responsible

for performing instrument calibration should be

trained and tested accordingly.

Conclusion

There is an inherent risk in many workplaces of

injury or illness from respiratory hazards such as

oxygen deficiency and the presence of toxic gases.

Detection technology and products exist to

minimize such risk. Properly verifying the

accuracy of gas detection equipment before each

day’s use will help to ensure that each worker

finishes the job safely.

For More Information Contact OSHA at:

www.osha.gov

On-Line Resources:

International Safety Equipment Association

1901 North Moore Street

Arlington, VA 22209-1762

(703) 525-1695

www.safetyequipment.org/calibration.PDF