May 2026
 |
Inside the May Issue |
Calibrating and Testing
Direct-Reading Monitors
Operators use direct-reading instruments
(DRIs) to detect the presence and
concentration of toxic and combustible
gases, as well as oxygen deficiency or
oxygen enrichment (which is a fire and explosion
hazard). Workers must not rely
solely on their sense of smell to alert
them to these hazards. Employers should
ensure that workers use these instruments
when working in areas with potential
hazardous atmospheres.
“Calibration” refers to an instrument's
measuring accuracy relative to a known
traceable concentration of test gas. DRMs
compare the sensor's response to a known
concentration of the test gas. To confirm
the validity of this comparison, it is important
to ensure the calibration gas has
not expired (always check the expiration
date of the gas before usage). The instrument's
response to the calibration gas
serves as the reference point.
The responsiveness of sensors will vary with workplace environmental conditions, such as temperature and humidity. Therefore, to the degree possible, operators should calibrate sensors in environmental conditions that are the same as (or similar to) the actual workplace conditions. Follow the manufacturer's guidelines for proper calibration.
Standard procedures for regular calibration that conform to the manufacturer's instructions, internal company policy, and/or the appropriate regulatory agency guidelines will help to ensure that calibration procedures are readily followed by the DRM operators, and that the instruments are operable and accurate when used. Employers should keep calibration records for the life of each instrument. This record enables operators to quickly identify a DRM that has a history of excessive maintenance/ repair, or is prone to erratic readings, and to track drift of the sensors to determine when they need replacement.
The primary reason for proper, regular instrument calibration is to provide accurate gas-concentration readings that could prevent worker illness, injury, or death. Correctly calibrating an instrument helps to ensure that it will respond accurately to the gases it is designed to detect, thereby warning users of hazardous conditions before the conditions reach dangerous levels. Some DRMs have two levels of alarms – a “low” alarm or “warning” level, and a “high” alarm or “danger” level. The warning alarm alerts the operator and workers that the work environment has a detectable elevated concentration of the gas and is, therefore, potentially hazardous. The danger alarm indicates that the gas concentration exceeds the programmed hazard threshold, and that the gas in the work area is above the warning level and approaching a hazardous level. Whether a DRM provides a warning or danger alarm at the proper concentration depends on its detection capabilities, its ability to translate its findings into an accurate reading, and the presence of interfering gases.
Calibration Rules
The following are a few basic calibration
rules for DRMs:
• Follow the manufacturer's guidelines
for proper calibration. Operators cannot
perform any job, including DRM calibration,
properly or safely without the right
tools. The type and concentration of calibration
test gas, sample tubing, flow regulators,
and calibration adapters are key
links in the calibration chain. Operators
should conduct any testing to verify the
operation of the gas monitor in an environment
that is the same as (or similar
to) the working conditions (e.g., temperature,
humidity, atmospheric pressure). Full story »
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