Guide to Calibration of Air Monitors
Recognizing the need for gas monitors in your workplace is the first step toward providing a safe environment for your employees. This PE Fact document provides detailed information on when, why, and how to calibrate your portable gas monitors to ensure that they always provide the most accurate readings.
The purpose of a gas monitor is to protect a worker from unseen hazards that may exist in certain environments such as confined spaces. It is imperative that gas detection devices be properly maintained and calibrated in order to function properly and ensure worker safety. Calibration refers to an instrument’s measuring accuracy relative to a known concentration of gas.
Instruments that aren’t calibrated properly can become inaccurate and cause serious accidents. Severe illness and even death can occur when workers are exposed to toxic gases or oxygen-deficient environments. Combustible gas explosions are often catastrophic, injuring or killing personnel and destroying property. That’s why it’s important to follow the International Safety Equipment Association’s (ISEA) recommendation to verify sensor accuracy before each day’s use. The best way to test for accuracy is by exposing the instrument to a known concentration of gas.
Calibrate your way to accurate readings
Since workers cannot rely on their sense of smell to alert them to odorless hazards, gas detectors are needed whenever a worker enters an area with potential atmospheric risks.
A gas detector measures 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 then serves as the point of reference.
Varying environmental conditions affect the responsiveness of electrochemical sensors, as sensor response fluctuates lower or higher depending on existing conditions. For that reason it is recommended that gas monitors be calibrated at environmental conditions that are the same as (or similar to) actual field conditions whenever possible. It’s always best to perform the calibration at the actual location where the device is being used.
A gas detector’s warning alarm alerts the user when an environment is potentially hazardous due to a detectable concentration of gas. If the atmosphere is approaching a hazardous level, the unit’s danger alarm will sound to alert the user that gas levels have reached the programmed “hazard” threshold. The accuracy of the instrument depends on its detection abilities and how well it can translate its findings into an accurate reading.
“Calibration drift” is the term that refers to a shift in the instrument’s point of reference. All instruments are affected by calibration drift over time and unreliable readings are the result. While instruments that have calibration drift are still able to measure the quantity of gas present, they no longer have the ability to convert this information into accurate readings. This can be prevented through regular calibration with a certified standard gas concentration.
Causes of Calibration Drift
A gas detector’s accuracy can be compromised by the following:
- Sensors naturally wear with time, causing a drift in electronic components.
- Frequent exposure to and usage in certain environmental conditions such as extreme 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 including 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 and circuitry.
If a detector has been exposed to one or more of the above conditions it may fail calibration attempts. This means that the sensor(s) has been severely damaged and must be replaced, or that the unit must be professionally serviced.
Calibrate for Safety
The number one reason to keep your gas detectors properly and regularly calibrated is to prevent inaccurate gas concentration readings that could lead to injury or to death. While a unit may look like it’s functioning properly, it may actually be damaged internally. Regular calibration is the only way to be certain that a detector is fully functional. Moreover, implementing a regular calibration policy creates 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.
It’s important that you keep an updated record of each instrument’s calibration. This will allow you and other users to quickly identify an instrument that has a history of excessive maintenance/repair or is prone to erratic readings.
Bump Tests
As an alternative to full calibration, you can often verify an instrument’s accuracy by performing a bump. A bump test verifies calibration by exposing the instrument to a known concentration of test gas. The reading is compared to the actual quantity of gas present (as indicated on the cylinder). A calibration is considered verified if the instrument’s response is within an acceptable tolerance range of the actual concentration. (Note: It is recommended that users check with the detection equipment manufacturer for the acceptable tolerance ranges.) For the most accurate results, instruments should be “zeroed” before a bump test is performed. When performing a bump test, the test gas concentration should be high enough to trigger the instrument alarm.
Full Calibration
If the bump test results are found to be outside of 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.
If bump 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
For the health and safety of those using the instruments, it is important to follow these basic calibration rules.
- Always follow the manufacturer’s instructions. Every gas detection instrument should come with instructions for calibration, including tools and equipment needed to perform it correctly.
- Never use calibration gas after its expiration date. 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.
- Train employees on proper calibration procedures. Proper calibration methods for each unit are outlined in user manuals and training videos or CD-ROMS. Utilize these materials to train any and all staff who will be required to use or calibrate these instruments.
Conclusion
The very nature of many workplaces puts employees at risk for injury and illnesses resulting from oxygen deficiency and exposure to toxic gases. In those situations it is imperative to provide workers with the proper detection technology and personal protective equipment to minimize such risk. To ensure the safety of each worker, be sure to properly verify the accuracy of gas detection equipment before each day’s use.
Please Note:The information contained in this publication is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific questions should refer to the cited regulation or consult with an attorney.
