Helium & Hydrogen Detection for Vapor Intrustion Equipment Applications
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A growing emphasis in recent years in the environmental industry has been soil gas vapor monitoring/vapor intrusion monitoring. The focus of these studies include vapors settling underneath concrete slabs, traveling along concrete slabs, around pipes, improperly sealed monitoring wells, and around underground storage tanks.
One way to verify the concentrations of harmful compounds that may be traveling this way is by implementing drive points throughout the slabs or area surrounding the monitoring wells. Air samples can be taken from these points by using either sampling pumps like the Gilian Gil Air-5 or Casella Apex Pro to fill tedlar bag samples or using vacuum canisters/bottles. These samples can be tested for contaminant concentrations. Driving the vapor points is simple enough; the problem lies with properly sealing the sampling points at the surface so that ambient air is not introduced into your samples. Ambient air introduced into the samples can cause diluted and inaccurate samples.
Before collecting a sample, determine whether the vapor point is sealed by purging the sample port with a hand vacuum pump and perform a leak test.
The stainless steel sample point can be driven with the help of an AMS Soil Vapor Sampling Kit. Once the point or well has been packed, grouted and allowed to settle for a period of time, the vacuum pump is then connected and a measured volume is extracted. To perform a leak test, a tracer gas such as Ultra High Purity Helium (UHP Helium) can be used in conjunction with a helium leak detector like the Dielectric MGD-2002. A shut in test can be administered by shutting off all valves to the sample point. A shroud, which is any covering allowing empty space above the sample point, is put into place and then sealed around the bottom (essentially creating a dome-like enclosure over top of the sample point) creating a confined space. The tracer gas is then released into the shroud, filling the area that covers the sample point, taking the place of the ambient air. The Dielectric MGD-2002 can be inserted through a port you create in the dome to ensure a high concentration of helium is present. Once the shroud/dome is saturated with the tracer gas, you can then connect a sampling pump to tubing you have connected to your sample port. The tubing runs out of the shroud/dome enclosure to fill a tedlar bag sample from your sample point. Once filled, the bag can be sampled with the Dielectric MGD-2002 to ensure there is not a leak around your sample point. If the tedlar bag was filled with a significant amount of helium, there is a leak present. If no leak is detected, you are ready to begin your sampling event with either the vacuum canister/bottle or with another clean tedlar bag.
If a tedlar bag is used for filling a sample you can also do a field screening for VOCs using a handheld meter like a PID or FID (Suggestions:RAE MiniRAE 2000, RAE MiniRAE 3000, RAE ppbRAE, Thermo TVA-1000). Your other option is to send your sample/s out to a laboratory for analysis.
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