A gas monitor/detector looses sensitivity over time due to aging, exposure to high concentrations of gases, sensor poisoning etc and so the accuracy of the gas monitor must be verified on a regular basis. This can be done by performing a calibration that will ensure the gas monitor is operating correctly
Calibration Frequency :
The frequency of span calibration is best determined by local standards, company policies and industry best practices.
The calibration frequency should be determined by the level of risk. A high risk situation will require a higher frequency of calibration in comparison to a low risk situation. Best practice is to test the gas monitor with a known concentration of gas before each use (bump test/challenge test).
Another way to determine the calibration frequency is to check the response of the gas monitor daily in the intended atmosphere for a pre-determined time (example being 10 days). If there is nothing in the atmosphere that is causing a loss of sensitivity to the installed sensors then these tests indicate that there is no need to make any adjustments and the time between checks can be lengthened.
Calibration Procedure :
The calibration procedure includes both a zero calibration (fresh air calibration) and sensor calibration with known concentrations of gas (span calibration).
- Zero calibration is performed to establish baseline readings of atmospheres that are known to be free of toxic and combustible gases.
- Baseline readings are zero for carbon monoxide, hydrogen sulphide, combustibles and 20.9% for oxygen.
- Span calibration is the adjustment of the gas monitors response to match that of a known concentration of gas. Sensors can lose sensitivity through normal degradation, exposure to high gas concentrations or sensor poisoning.
- The Span calibration is performed to ensure the gas monitor detects the target gases within specific operating parameters.
Equipment required to perform a manual span calibration:
- Calibration gas with the following concentrations (concentrations may vary depends on make/model)
• 50 ppm CO
• 25 ppm H2S
• 1.62 Vol% CH4 (32% LEL)
• 19 Vol% O2
- Tygon tubing
- Gas flow regulator able to provide 0.5 LPM
- Calibration adapter
ZERO calibration procedure (also known as fresh air calibration)
- Verify the ambient air is free of toxic or combustible gases. If air is not free or cannot be verified free, obtain Zero Air gas cylinder.
- Power on the device
- Press and hold the Right Operating Button until PLEASE WAIT displays.
- The zero calibration is now in progress
- If display indicates “IS AIR CLEAR” verify the atmosphere is clean and press the Right Operating Button to start the Zero Calibration.
- Wait for device to complete Zero Calibration.
If passed, the LCD displays APPLY GAS to being the SPAN calibration
SPAN calibration procedure
When APPLY GAS is displayed :
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- Attach the flow regulator to the gas cylinder and verify cylinder pressure
- Connect the tygon tubing to both the regulator and calibration adapter
- If the password screen appears enter the 4-digit password using the left operation button to scroll through the numeric characters and the right operation button to accept the entry.
- Attach the calibration adaptor to the gas monitor and apply the gas from the regulator.
- Once each sensor is calibrated, the device displays CAL PASSED then REMOVE GAS.
- Remove Calibration Adapter and wait for the device’s main display to return before turning off the device.