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 pressure (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 equipped 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 down hole sampling point should be checked for leakage as part of the sampling QA/QC procedure.