Custom Sensor Solutions, Inc.

"Like A Submarine With a Screen Door":

Solving the Permeability Problem of Sample Bags

You say you've worked with gas samples for years? Then you probably have had it up to here with the behavior of some gases in some sample bags. Long ago, I was making a series of measurements using a 40 liter TedlarTM sample bag containing 50 PPM nitric oxide (NO). Hour by hour, the instrument response kept falling off. I blamed the "faulty" sensor -- until I drew a fresh sample of nitric oxide from the cylinder. The signal returned to its initial value! Further tests showed that I was actually measuring the oxidation of NO by atmospheric oxygen, which was slowly permeating through the bag material.

Virtually every organic polymer is permeable to organic solvent vapors, as well as to tiny gas molecules like helium or hydrogen. Even with high-quality materials like TedlarTM, vapors of benzene, carbon tetrachloride, hydrogen sulfide, and ethyl mercaptan all decay rapidly through outward leakage. Air does not permeate very quickly, but there is so much of it outside the bag that its inward leakage can affect other gases (such as nitric oxide).

One solution is to double the thickness of the bag material. For nearly every gas, 4 mil TedlarTM is about the best material you can use (better -- and cheaper -- than TeflonTM), but it has its limits. More importantly, the material is no longer readily available.

Now, Murrell Selden, of Pollution Measurement Corporation, is offering bags made of a new material, 2 mil TedlarTM lined on the outside with a proprietary metalized, impermeable laminated material. These bags price out to a little less than the 4 mil TedlarTM.

Murrell has been testing the bags with different gases and vapors, but the numbers below will serve as a clue as to what you can expect. The experiment began with trace concentrations of four sulfur compounds in the same bag, which were analyzed four times during the following week. At the end of seven days the concentrations of these four, normally unstable, gases were unchanged.

Murrell recently quoted 12x12-inch (4 liter) bags of the 2-mil TedlarTM material, covered with laminate (LAM) for $19.82 list before discounts. You can buy them from Custom Sensor Solutions, Inc., for the same price. Pollution Measurement Corporation can be reached at 708-383-7794.

("Tedlar" and "Teflon" are trademarks of I.E. Dupont de Nemours Co.)

Table I. Initial and final concentrations of four sulfur gases over one week of storage in laminated material. Data from intermediate time periods have been omitted.

Current Feb. 23, 1998

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