Is Methane making your VOC measurements unusable?

You probably knew that Infrared meters are confused by VOC’s (Volatile-Organic-Compounds). It is far less well known that the accuracy of UV-based Photo Ionisation Detector (PID) VOC readings is also affected by the presence of Methane (CH4).

Gas Data Ltd. has a solution.

PIDs are widely used by the Geoenvironmental sector in remediation and site investigation projects to quantify ‘other’ hydrocarbon gases (VOC’s) present in boreholes. Naturally this is particularly important on sites such as former petrol stations, garages and transport depots, but it does impact landfill sites as well.

If CH4 is present the PID reading can be inaccurate because while the CH4 cannot be seen or analysed by the PID, any CH4 present will absorb the UV light. This reduces ionisation and de-sensitises the response. The outcome is, in some cases, an under-measurement or complete non-detection, of VOC’s.

The safety and compliance implications are wide ranging. If the PID reading is depressed below the level set by the Regulatory Body then impacted material could be left in place.

From an operational perspective current best practice involves using an infra-red analyser to determine if CH4 is present before the PID is used. If CH4 is detected then it is common for a gas sample to be sent off for further analysis, with consequent impact on project timescales.

While Gas Data was aware of this issue, client feedback moved it to the top of our R&D schedule.

We had established an expert panel of customers to identify requirements and beta test prototypes in the development of our GFM436 portable gas analyser. The panel gave us a clear indication of the importance of alleviating the methane problem in site investigations. It highlighted the need for a clear indication of the level of interference that any methane present might give to hydrocarbon readings.

Following extensive research by our engineering team Gas Data was able to determine a non-linear relationship between the PID value (as VOC’s) and CH4 content. In turn, this allowed us to calculate a PID Compensation Factor (CF) value of between 1 and 10.

The CF can be used to multiply the reading obtained with the PID to give a true VOC total. This figure is accurate for the Ionscience Ltd Phocheck range of PIDs up to a CF value of 10 which occurs at approximately 9% methane in the borehole.

landfill managers gas analyserIf the sample being taken contains 9% CH4, with a Phocheck PID the VOC reading could be understated by a factor of 10. This characteristic varies between PID manufacturers.

When in the field, the operator can take a gas sample with their GFM436 gas analyser and then read and record the CF from the LED display. They can then use their PID analyser to obtain a reading of the VOC levels in the borehole. The reading provided by the PID is then multiplied by the CF to give an indication of the true VOC levels present in the borehole.

Operators can now have sufficient confidence in the readings, even when CH4 is present, that lab analysis of samples is no longer the default option.

For more information on how you can use the PID Compensation Factor in your site investigations contact Gas Data today. Call +44 (0) 2476 303311 or email enquiries@gasdata.co.uk