SAMPLING KITS – A COMMON SENSE VIEW
SUMMARY: Sampling must mimic normal charging conditions. Small compressors charge at 1-5 SCFM, but large air systems charge SCUBA and SCBA tanks at an average flow rate of 13 – 20 SCFM. Sampling at this rate is referred to as Full Flow sampling, and represents the biggest challenge. Most kits use a limiting orifice to control flow, but this design has serious problems.
THE CHALLENGE
There are three categories of compressors: LP, MP, and HP, and all are available in various flow rates. To obtain accurate data, the sample must be taken under normal flow conditions. The average flow rate for SCUBA charging is 13 – 20 SCFM. Sampling gas under pressure is inherently dangerous, and the means used to control the flow is very important.
AIR KIT DESIGN TO CONTROL PRESSURE
Most sampling kits use the “Full Pressure” design to allow the air to flow into the kit at high pressure, but pass through the kit at low pressure. Driven by economics, the inlet fitting in most of these kits contains a critical orifice (a tiny hole about 0.010 inches in diameter) which causes the flow into the sampler to be only 1 – 5 SCFM. One laboratory uses a slightly larger orifice (a 0.025 inch diameter hole) which allows sampling at flow rates up to 20 SCFM. Although that hole might allow air and oil mist to pass through it, that orifice is still too small to allow all of the particulate from a dirty air source to reach the test filter. (See sampling for oil mist + particulate)
Our Air Sampling Kits are the only ones in the US that do not use a critical orifice. Instead, the flow control is accomplished by pressure reduction. Everything that is present in the air is detected. Our kits have many advantages and allow sampling at 1 – 200+ SCFM.
DETERMINING AIR VOLUME PASSING THROUGH THE FILTER
All sampling kits determine the volume of air passing through the analytical filter by multiplying the flow rate by the duration of flow. One design uses a flow meter attached to the analytical filter outlet to determine the flow rate. Although tedious, our investigation shows that this works as long as the flow meters are routinely cleaned and checked to be sure that no debris or sticky film is on the flow meter interior walls.
The other way to determine the airflow rate is to place a flow/calibrated pressure gauge in the air path. The analyst calculates the flow rate from the pressure reported by the client. Gauges also need to be checked every now and then because compressor pulse, and system vibration can permanently alter the gauge calibration.
Our air kits use the pressure gauge approach. Each gauge is independently calibrated and frequently maintained.
HOW MUCH AIR SHOULD BE SAMPLED
Most kits pass 18 – 36 cubic feet of air through their air kit. We sample 80+ cubic feet of air because that represents the entire contents of most SCUBA and Nitrox tanks.
OBTAINING A BULK SAMPLE FOR GASES AND HYDROCARBONS
A bulk sample is taken for lab analysis of oxygen, carbon oxides, and hydrocarbons. The choice of container is very important. See Sampling for Gases and Hydrocarbons.