SAMPLING FOR GASES AND HYDROCARBONS
SUMMARY: A grab sample of the air is taken to determine fixed gases and total (volatile) hydrocarbons. Some specifications also require a determination of halogenated compounds. There are 5 types of containers in use: metal cylinders, canisters, Polymeric bags, syringes and glass vials. Some are appropriate, but most are not.
Determining the purity of gases or volatile components (total hydrocarbons, halogenated hydrocarbons, carbon monoxide, carbon dioxide and oxygen), requires a bulk sample in a suitable container. The only published requirements regarding containers are: (a) DOT and IATA require adherence to shipping rules, and (b) the US Navy Diver program requires a minimum of 200 mls sample. There are 4 types of containers used to capture a bulk sample.
Metal cylinders (bombs) are available in many sizes, but they tend to be heavy, take up a lot of room in the air kit, and purge poorly thereby leading to inaccurate data unless a lot of time is spent preparing them. Their best use is in sampling oxygen systems where safety is of greater significance, provided that they are thoroughly purged.
Polymeric bags (Tedlar being the most popular) hold a lot of sample and are inexpensive. Unfortunately, they are too bulky for a multiple sample kit, difficult to purge clean between samples, and can’t be pressurized. Tests on some types of bags reveal relatively rapid loss of certain hydrocarbons as well as helium in HeliOx mixtures.
Glass Septum Vials are used by several labs in the US. These vials are small, lightweight, take up little room, and are very inexpensive. Unfortunately, they suffer from serious disadvantages: they don’t bounce (broken glass is especially dangerous aboard a ship); they require puncturing with needles (which have a history of plugging and bending); the rubber septum can absorb certain hydrocarbons (thereby preventing their discovery); and they hold very little sample. The low sample volume is the most serious because of the following:
1. A slight vacuum forms when the first 2 mL of sample is removed. This vacuum tends to suck in room air which changes the sample composition each time more sample is removed. This goes unnoticed because the contamination is – after all – air.
2. Although the internal volume of the vial is typically 20 mL, only about 3 mL of it is available to the analyst). This precludes complex specification tests which require greater sensitivity and additional techniques (i.e., Federal Specification BB-A-1034, Medical air, Saturation Diving, and NAVSEA criteria for post painting of chambers).
3. The small amount of sample also precludes routine QA repeat analyses.
4. If the sample has hydrocarbons, vials do not provide enough sample to identify the contaminant. If you can’t identify the contaminant, you can’t solve the problem.
Upon discovering the problems with these containers, we were forced to design build and test many versions of a better container. After several years, a lot of field testing, and significant cost, our container has evolved into a light weight metal canister that is self- purging, self-sealing, pressure indicating, and held over 400 mL sample. The container is safe and meets DOT and IATA shipping regulations for transport on passenger and commercial aircraft.