Calibration gas selection is critical in the operation of any combustible gas detector.
A combustible gas detector will respond to a wide variety of ignitable gases and vapours. How the sensor responds is directly dependent upon which gas is selected to calibrate the instrument.
In all cases you should select the calibration gas which best reflects the target gas that you are trying to measure as the best results are obtained when calibration is done using the same gas which will be encountered while using the instrument.
Testing Unknown Gas Mixtures
Often though, the gas mixture to be tested is unknown and a calibration gas must be selected that will best reflect the application.
The choice of calibration gas will change the way the combustible gas sensor responds to other gases. This impacts the readings and alarm indications.
The chart below shows the relative response between methane, propane and pentane.
In this chart you can see the changing responses based upon which of the three gases are selected to calibrate the instrument.
The chart shows that instruments calibrated to methane, will provide a lower response to the other gases whereas instruments calibrated to pentane provide a higher response.
Should You Just Use Pentane to Calibrate All Gas Instruments?
From this chart it could be assumed that pentane would be a better choice to calibrate all instruments because it provides a higher response, therefore safer however, this is not always the case. Catalytic combustible gas sensors can be poisoned by silicone based compounds and airborne lead. Inhibitors such as sulphur compounds and chlorinated compounds can slow the sensor response and in some cases reduce the sensor’s ability to measure some gases. A sensor which has been affected by inhibitors will show a reduction in response to methane before other gases. It is possible that an instrument will respond accurately to pentane, while showing a dangerously reduced response to methane.
Pentane provides a more sensitive response but doesn’t provide a good indication of reduced sensor performance. Methane provides a less sensitive response but does provide a good indication of reduced sensor performance.
What You Should Do?
- Some users calibrate to pentane and then challenge test their instruments to methane once per month or quarter to determine if there is any sensor sensitivity loss.
- Other users continue to use methane which provides the indication of sensor sensitivity loss. With the lower alarms points of today (5%LEL or 10%LEL), methane still provides a safe alarm level for most gas measurement applications.
- A third option has become popular in recent years – pentane equivalent calibration.
What Is Pentane Equivalent Calibration Gas?
The introduction of pentane equivalent calibration gas procedures combines the benefits of methane sensor sensitivity loss with pentane’s faster response to other combustible gases.
This process uses methane as the calibration gas, but adjusts the calibration set point to a value which reflects a pentane response. For example using 1.25% methane as the 50%LEL calibration set point causes the instrument to respond as though calibrated to pentane while still having the methane benefits. This combination seems to provide an interesting solution. Different instrument manufacturers use different gas values but the results are similar.
The selection and use of calibration gas can provide a variety of outcomes and results. Calibration verifies that sensors remain accurate providing an indication of sensor sensitivity loss. Whichever approach is used, the outcome must be to improve the accuracy and safety of the gas detection device.