• 5 gas analysis as standard (CH4, LEL(CH4) CO2, O2, CO and H2S) - two further optional gas channels
  
• Borehole "ow, borehole pressure and atmospheric pressure measurements
  
• Optional temperature and velocity measurement
  
• All gas and "ow channels MCERTS certi!ed
  
• Display of PID compensation factors for accurate operation of PID sensors in CH4 rich atmospheres
  
• Hexane equivalent measurement of hydrocarbon vapours from common liquid fuel and oil spills
  
• Intrinsically safe EEx ib IIB T1
  

Borehole Flow Measurement

The built-in "ow transducer is very robust. The transducer is calibrated up to 100 l/hr full scale measurement range but is tested at four times this "ow rate. This allows unexpected short bursts of over-range "ows to be tolerated without instrument damage.

PID Compensation Factor

The PID (Photo Ionisation Detector) is the standard instrument used to characterise hydrocarbons other than methane in a borehole. However, the presence of methane attenuates the UV light of the PID quenching its signal and consequently de-sensitising its response. The effect can be catastrophic. Only a few percent of methane can make the indicated PID signal inaccurate by factors of 1/10th or more.

The GFM435 PID compensation factor is a !gure derived from extensive testing of a range of common PID instruments. Its value (1 – 10) must be used to multiply the PID reading to give an accurate measure of the total hydrocarbons in the borehole when methane is present.

With the GFM435 when and if the methane sensor shows an over-range signal or abnormally high value (a good indicator that other hydrocarbons are present in a borehole) the hexane value will remain valid and in range up to 2% allowing standardised monitoring of other hydrocarbon vapours expressed as a percentage of hexane vapour equivalent.