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Wednesday, 23 Aug 2017

Direct Sampling Systems for Soil Gas

An alternative method of soil gas sampling is to collect soil gasses directly through probe rods. Using this method cleanliness of the rod bore is an imperative. If more than one rod length is used, it is also necessary to use the proper size O-rings at all rod connections. As with any soil gas sampling, it is desirable to eliminate any annular space when inserting the probe rods, thus preventing the possible flow of atmospheric air down around the rod and into the sampling train. AT14/AT14AL Expendable Drive Points normally work well for this purpose for both 1.0 in. and 1.25 in. rods. Remember, any of the PRT point holders can also be used for through -the-rod soil gas sampling. When syringe sampling is to be performed, a short length of silicone tubing is often used as a hose connection at the gas sampling cap. This allows a syringe needle to be inserted through the silicone tube section and directly into the rod bore for sampling.

Vacuum/Volume Systems

The Vacuum/Volume System is a combined pump and tank system designed for soil gas sampling. In typical operation this tank is pumped down with the system vacuum pump. The vacuum pump is then shut off and the tank is opened to the soil gas sampling train. By measuring the change in pressure in the tank, the soil gas sampling to be performed at the same pressure at each sampling point, improving the consistency of the soil gas sampling point. Vacuum/Volume systems are equipped with an "oversize" 4 in. (102 mm) vacuum gauge on the tank which is also marked in standard temperature and presuure (STP) volume. A ball valve is used to separate the vacuum tank from the soil gas sampling train.

Vacuum/Volume Systems are available in both mounted and fixed configurations. Standard models are equpped with 12 VDC vacuum pumps.

A big plus to this system is that measurement of a change in pressure in the vacuum tank serves as confirmation that soil gas has indeed been extracted from the soil. As with all soil gas sampling, the sampling train between the Vacuum/Volume system and the downhole sampling point should be checked for leakage as part of the sampling QA/QC procedure.

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