One of the most common questions when a project is delayed is how long can the equipment be held before sampling without impact on the samples.
For the Method 25 samples it is always a simple answer, which is for as long as you want. The tanks are shipped under a positive pressure, so any leakage should not cause contamination. The tank is also evacuated before sampling which means that any contamination should be removed from the tank. The traps are sealed after their QC analysis and should be good until needed.
For the Method 25-C or any other procedure which requires a more precise dilution in order to ship as a non-hazardous sample it becomes a little more questionable. We generally perform at least a 24 hour leak check on the tanks, which allows only a 10 mmHg change in that time. In the case of a shorter period for the leak check we allow no more than 1 mmHg change per hour. This is due to the impact of measuring the pressure in the tank having an effect on the pressure within the tank itself. This stringent leak check keeps the questionable tanks from being processed for use. Unfortunately, since these tanks have a double shut off quick connect there is no way to ensure the next time they are connected/disconnected there is no failure.
We have anecdotal information from past instances where roughly 10% of the tanks held for an extended period had a significant change in pressure noted. The same type of information on different shorter term periods indicates few tanks with any significant changes in pressure. However, we do not always receive information on the use or lack thereof for these tanks which affects the accuracy of this data for those with significant changes in pressure. The long term periods are, in my opinion, the better indication of stability as any small leakage over time will be noted. These cases are not common so there is little hard information. We recently had 4 canisters returned some 223 days after they were initially evacuated. For these tanks we had the recorded pressure and temperature for both the shipment out and return recorded to the same levels.
All four of the tanks would have passed the 24 hour leak check requirements and after adjusting the pressures for the changes in temperature, two of tanks showed no change in pressure at all. The calculated changes in pressure were from a low of +0.070% to a high of +0.099% over the 223 days, which is most likely due only to the rounding of the measurement devices and the act of connection to the tanks. This indicates that any extended storage of the tanks for such a project should be good as a last resort unless the connection to the sample console indicates a gross change in pressure.
Triangle Environmental Services, Inc.
P.O. Box 13294 122 US Hwy 70 E
Research Triangle Park, NC 27709 Hillsborough, NC 27278
(919) 361-2890 (800) 367-4862 Fax: (919) 361-3474