Filling a Sample Bag With A Measured Amount of Gas
Every lab that works with gases needs a convenient source of air, nitrogen, or other gas for flushing and dilution. An appropriate setup consists of a large cylinder of gas, say, size 1 or 1A; a one- or two-stage regulator with a range of 50 to 100 psi; a needle valve; a flowmeter with a range of 5 liters/min; and a stopwatch. A set of fittings appropriate to the sample bags should be included. We normally use polypropylene Luer-type fittings wherever possible. Others use quick-connect fittings, which are more expensive but convenient.
A 12" x 12" (30 cm x 30 cm) Tedlar bag of the kind sold by us holds 4 liters; a 24" x 24" (60 cm x 60 cm) bag holds 40 liters.
LEARN THE RULES FOR HANDLING HIGH-PRESSURE CYLINDERS BEFORE USING
1. Evacuate the bag to remove any contained gas, and stopper the inlet tube.
2. Turn on the cylinder and adjust the pressure to about 10 psi. Using the needle valve, set the flow rate to 2.0 liters per minute using the needle valve.
3. Connect the evacuated bag to the gas outlet and start the stopwatch.
4. Check the flow rate every three or four minutes.
5. Make sure the bag does not overinflate. An explosion will ruin the bag and startle everyone in the lab. It's unlikely to do much harm, however, unless the contents are toxic or inflammable.
6. When the appropriate time has elapsed, disconnect the bag and stopper it.
Sample Bag Overinflation
A sample bag explosion may or may not be dangerous, depending on what gas is contained within. But the explosions are always loud and startling. Even a tiny pump can generate the relatively small pressure that's needed to burst a sample bag, even the tough bags made of Tedlar.
