A typical landfill or contaminated land site under investigation will comprise of a suitable number of gas monitoring wells, with a planned series of monitoring visits. The monitoring will include gas flow measurements, which together with methane and carbon dioxide concentrations will be used to calculate gas screening values (GSV) and hence allow the determination of whether any gas protective measures are required. Gas flow is a measure of the rate at which gas is being produced by decomposing organic matter below the surface which passes into a borehole. If the site is capped, i.e. a non-permeable cover has been used to retain all (or at least the vast majority) gases produced within the ground, this can lead to a pressure build up within the borehole, which can provide inflated gas flow results prior to a stable value being reached, which should indicate gas production below the surface.
Gas screening values are the product of gas concentration values and the flow measurement taken from the borehole being monitored. The technologies used for gas measurement are well known and accepted as fit for purpose, however this is not the case for borehole flow measurement. With various means of flow measurement available, preferences for one method over another have left councils and consultancies split over which is the most suitable.
This series of lab based testing compared two methods of low flow measurement to determine which provides the most accurate and quickest, stable response when placed under the same conditions. This was achieved using an externally calibrated Brooks Mass Flow Device to provide known values of gas flow which the different methods of measurement were tested against. This included specific targeting of the typical range of gas flow values seen on contaminated land sites, including landfill perimeters, former petrol forecourts and brownfield land.
In addition, the reaction time of each technology to reach a stable measurement of the flow rate was also taken. This is of interest as site technicians use these two methods of measurement as they are often required to perform a wide range of different types of monitoring over a large number of sample points in one day, making measurement time sensitive.
What is flow?
A capped site, with sealed boreholes, will build up a static pressure, Ps. Although this pressure is related to the rate of gas generation it is also heavily influenced by other parameters, such as the gas permeability and thickness of the cover, and of course any gas escape routes that may exist through the cover.
The equilibrium situation within a gassing site is shown in Fig 1. Here the gas generation within the site is balanced against the loss of gas. Under these conditions there will be no gas flow in or out of the borehole and the gas pressure in the borehole, PB, will equal the gas pressure in the surrounding area, PL. When a flow measuring device is connected to the borehole and the tap opened the situation changes. If the flow device has no restriction to flow, such as when the tap is opened, the pressure in the borehole will force the gas out until the borehole is at or near atmospheric pressure. In removing the gas, the pressure equilibrium of the site has been upset.
With the tap open and the system open to atmosphere, via analysis equipment, we have a borehole at near atmospheric pressure within a site at higher pressure. The low pressure borehole will act as a sink for the gas in the ground of the site which is still at the elevated pressure surrounding it. We will thus get a flow of gas into the borehole from the surrounding area, as shown in the Fig 2.
Geotech uses a similar technology to that of orifice plates for measurement of flow within the GA5000. This comprises of two pressure transducers separated by a known restrictor. This restrictor is a small opening which the gas is allowed to flow through. By measuring the difference between the two transducer readings located pre and post restrictor a flow rate through the restrictor can be determined. This provides a highly accurate flow measurement. However, due to the size of the restrictor used, the reaction time will be increased as it will take longer for the gas mixture to pass thought the instrument.
Hot wire anemometer
The hot wire anemometer works by passing a current through a very thin wire, typically made of tungsten. This current in turn heats the wire. As the gas flow passes over the wire it is cooled, which causes the resistance of the wire to be increased, a variable which can be measured by monitoring the potential difference passing through the circuit. This technology is very sensitive, providing quick responses when the wire is at the required temperature. However due to its sensitivity, variance in the temperature of the gas could also affect the flow reading detected by the anemometer.
Maximum variances and average accuracies of both technologies were measured and compared, with the GA5000's dual pressure transducer method proving to have a much greater level of accuracy in comparison to the hot wire anemometer. As expected the, GA5000 also took a longer time to stabilise between target values due to the nature of the technology used. The Hot Wire Anemometer however varied in its reaction times, while reacting quickly and proving it to be sensitive to changes it took only a matter of a few seconds to stabilise between close targets. However, on a number of occasions within the testing it failed to settle on a fixed flow rate and in fact continued to drift far beyond the target value.
Overall the GA5000, while taking a few seconds longer to stabilise on a flow rate, provided consistently more accurate results and the flow logging feature graphical display allowed the user to see this stabilisation of results in a clear form. While outperforming the hot wire anemometer in the accuracy tests consistently and only being a short distance behind in average reaction time testing, areas for improvement within the GA5000 flow measurement have been identified and further testing has been scheduled to determine whether lower range (0 - 2 l/hr) flow rate measurement accuracy can be improved further.