Purge Test Calculator
Click on the link above to download our purge test calculator.
Using the Jones Environmental Purge Test Calculator
Our purge test calculator takes into consideration the entire “dead space volume” including the void space of the sand pack around the probe tip, the annular space occupied by the dry bentonite, and the internal volume of the tubing used for probe installation. The individual parameters for calculating the sand/pack and dry bentonite include width of the borehole, height of the sand pack, and porosity of the soil. The internal volume of the tubing considers the inner diameter of the specific tubing and depth at which the probe was set. Of note, the calculator includes an extra foot of above surface tubing to tabulate the final purge volume.
The width of the borehole equates to the size of the rod/auger used for installing the vapor probe. Typically, direct push rigs use a 2-inch diameter drill rod and hollow stem drill rigs use 8-inch augers.
The height of the sand pack is the sampling range of the probe. The height of the sand pack is variable but a 1-foot sand pack is most common (e.g., the probe tip is set at 5 ft. bgs and the sand pack will occupy the space 6-inches above the probe tip and 6-inches below the probe tip. In effect, the sand pack is set at the interval 4′6″ to 5′6″).
Porosity or pore space is the amount of air space or void space between soil particles. The soil porosity parameter in our purge calculator is the amount of void space in the soil around the probe. Typically, soil porosity is between 20-30%. It is important to highlight that porosity is the ratio of void space to total volume and consequently the diameter size of the grain does not solely affect porosity. For example, uniform sand and uniform clay will both have “high” porosity yet sand granules are larger than clay granules. On the other hand, heterogeneous mixtures such as well-sorted sand or sand/gravel mixtures will have low porosity.
The inner diameter of the tubing should always be used to calculate the volume of the tubing. Typical inner diameter values for common tubing are 0.188 inches for PTFE Teflon and 0.190 inches for nylon based tubing (e.g., Nylaflow). We strongly encourage all vapor probes to be installed using only PTFE Teflon or Nylaflow. The adverse effects of polyethylene tubing severely compromise the analytical results from soil vapor samples collected using polyethylene tubing.
The depth of the tubing is the depth at which the tubing is set. For example, a probe set at 15 feet bgs will have a tubing depth of 15 feet. Of note, an additional foot of tubing is automatically added to the purge calculator to account for above surface tubing.
There is also a parameter function for inserting the desired sampling rate. Typical sampling rates are 200 mL/min or less. Here, maintaining a consistent sampling rate for all samples during a soil vapor investigation is key for generating consistent analytical data. That being said site specific conditions (e.g., low-flow situations and large diameter tubing) may necessitate deviation from a consistent sampling rate.
If you have any questions or need help determining the purge volume give us a call at 714-449-9937 or send us an email Purge Test Help
