Landtec Tab > Landtec_GA5000 Machines > Sampling from Two Tap Boreholes

Where boreholes have been installed for gas monitoring and constructed using a single perforated pipe over significant depths the British Standard BS8576 recommends that two taps are installed to enable recirculation of gas within the borehole. This involves connecting a tube to one tap and suspending it at depth, leaving the other tap open to gas in the top of the well. The following is a description of the method to be used to sample from wells with this construction.

The taps are labelled S for short (i.e. open to the top of the borehole) and L for long with the tap connected to the tube suspended at depth.

This means that attached to the underside of the L one - is a piece of tubing which extends down towards the bottom of the borehole.

The idea behind this is from the British Standard BS8576 - which requires in single pipe installation boreholes that the gas is recirculated around the borehole during monitoring to overcome any stratification of gases within the borehole - especially deep ones. This means that whilst taking the gas concentration readings the exhaust gases from the machine are not vented to the atmosphere as normal, but they are recirculated back down the borehole via tubing that needs to be connected to the S gas tap.

This requires a completely different methodology of gas sampling as follows: -

On arrival at the borehole - you will take the readings in the following way: -

1) DIFFERENTIAL PRESSURE reading from the gas tap marked S to the WHITE port on the gas machine - with the L gas tap SHUT .

2) GAS FLOW reading from the gas tap marked S to the BLUE port on the gas machine - with the L gas tap SHUT .

3) GAS CONCENTRATION reading - with the INLET (WHITE) port connected to the L gas tap, and the OUTLET (exhaust) from the gas machine connected to the S gas tap. OPEN UP BOTH GAS TAPS and switch on the gas machine pump and wait until the readings have stabilised as you would normally do. Please take a note of how long it is taking to stabilise in each borehole, and write a comment in Other Issues in the Activities. I would expect the deeper boreholes to take longer to stabilise, and it may take longer than the 300 seconds that your machines are set to pump.

4) REMEMBER TO CLOSE BOTH GAS TAPS WHEN YOU HAVE FINISHED