Because they are the most convenient way of handling small to medium volumes of gas, from 100 milliliters to 100 liters, sample bags are heavily used in gas sensor research. But surprisingly little information is available on their proper use. In this article, we want to discuss some of our experience in the choice of sample bags and their proper care.
Why Use Sample Bags?
When handling limited volumes of gases at a given concentration, you have several choices. No single solution fits all circumstances. You can draw a standard gas directly from a prepared cylinder, dilute it online using rotameters or mass flow controllers, generate it as it is to be used, or make and store dilutions in a gas sample bag.
Sample bags have several advantages over other handling methods:
The bags, and the equipment to use them, are inexpensive. A contaminated or damaged bag can be discarded without bitter tears.
Liquid-in-gas mixtures can be made quickly, accurately, and inexpensively.
They are, for the most part, resistant to chemical attack, and conversely, do not generally affect the sample.
The sample is at atmospheric pressure, making calculations straightforward.
Gas-in-gas, as well as liquid-in-gas, mixtures can be made using gas sample bags.
The Custom Sensor Solutions Precision Gas Diluter requires the use of sample bags, since the samples mixed must be at atmospheric pressure.
Stock samples, eg, gas samples made accurately at high concentration and intended for later dilution, can often be stored for days and weeks between uses.
Choice of a Sample Bag Material
Bags come in several materials: polyethylene, Mylar, Teflon, Tedlar, and various laminates. We're accustomed to thinking of Teflon as a material of choice for most things because of its chemical inertness. But Teflon, like polyethylene, is also very porous to small molecular weight gases, and also absorbs most solvent vapors. For most purposes, Tedlar comes out on top in terms of inertness and low permeability.
Tedlar is made from sheets of the polymer poly(vinylidene difluoride) or pVDF. Chemically, pVDF is halfway between Teflon and polyethylene. On treatment in a high voltage field, however, the polymer strands line up in an orderly crystalline formation with very few molecule-sized pores for gas molecules to slip through. Tedlar is also mechanically tough.
Very small gases, such as hydrogen, methane, and helium, can diffuse through Tedlar nonetheless, so we choose a metalized laminate material for such bags. This laminate uses a thin layer of aluminum metal on polyethylene to block movement of small gas molecules across the membrane.
We sell bags in two sizes, 12" x 12" (30 x 30 cm), which holds up to 4 liters of gas, and 24" x 24" (60 x 60 cm) which holds up to 40 liters. We can have bags of custom sizes and shapes made to your order.
Auxiliary Equipment to Use With Sample Bags
High volume oilless pump for evacuating waste gases from bags.
Cylinder of high-purity air and/or nitrogen for making dilutions.
Regulator for above.
Rotameter calibrated to 2 - 4 L/min for controlling gas flow into the bag.
Timer, to prevent bag explosions due to overfilling.
A selection of suitable tubing and fittings. We have standardized on ¼" OD - 1/8" ID Bev-A-Line tubing (vinyl lined with polyethylene) and polycarbonate Luer fittings for making connections.
A variety of syringes, from 60 cc disposables to 250 microliter microsyringes.
Preparing Your Lab to Use Sample Bags
The five basic operations you want to be able to perform with sample bags are
Using the gas inside the bag for testing or analysis.
Evacuating unneeded gas from the bag, and disposal.
For most of these, you need a means of dispensing known volumes of gas. A cylinder of a suitable flushing gas, such as high-purity air or nitrogen, plus a needle valve and floating-ball flowmeter (rotameter) are essential for most of this work. Finally, and crucially, a timer is needed to determine when enough gas has entered the sample bag and to prevent explosion of the bag.
We recommend the use of two timers, one, to measure the length of time the gas has been flowing, and a second, to be carried on the person during the 15 or 20 minutes it may take to fill a bag. The purpose of the second timer is to make certain you return in time to stop the flow at the correct time, and also to prevent a bag explosion.