Sampling Kits
Portable HP compressors supply fill air at 2-8 SCFM, large air systems flow air at 13 – 20 SCFM, and LP systems can supply air in excess 100 SCFM. In order to get a true picture of the air at point of use, you must sample under conditions that mimic normal charging conditions. Very few sampling kits can do that.
The two most common compressors are LP (100 - 350 psi) and HP (350 - 4500 psi); both are available in various flow rates. To obtain accurate data, the sample must be taken under normal charging or end use conditions: typically 3 - 16 SCFM. (This is also one of the newest requirements of NFPA.) LP compressors do not usually store large volumes of air, but they can deliver air flow up to 500 SCFM. Sampling gas at high pressure or very high flow rates is inherently dangerous, and this has caused laboratories to rely on two basic sampling kit designs to provide safety..
Most sampling kits use the “ Full Pressure design which uses a tiny hole to reduce the air flow (and therefore the back pressure) within the sampling kit itself. Driven by economics, this design uses a critical orifice inlet (a tiny hole typically 0.008 - 0.030 inches in diameter). That tiny hole allows volatiles and at least some of the oil mist to pass through it for subsequent discovery. However, not all of the large particles or clumps of particles from a dirty air source can reach the test filter. This prevents discovery of wear metal, dirty piping, rust particles and desiccant dust that will enter SCUBA and SCBA flasks. (See Sampling for Oil Mist + Particulate).
Our Air Sampling Kits are the only ones in the U.S. that do not use a critical orifice, (our path is 1/4inch; we can and do find 100% of the particulate and the oil mist being generated by the air system.
We have two basic kits: a simple light weight dedicated Sport/Fire Department kit for sampling HP air systems (5000 psi) for charging at 2 – 20 SCFM, and our Universal kit which allows contractors and the military to sample nearly any air system in the world at very high flow rates (2-200+ SCFM). (The largest air system we have sampled was a 1500 SCFM Rotary Screw compressor which delivered air to a Navy ship in a dry dock.)
All sampling kits determine the volume of air passing through the analytical filter by multiplying the flow rate by the duration of flow. Some designs use a flow meter that attaches to the analytical filter. Measuring the flow with a flow meter while taking a sample is tedious - especially when the compressor is pulsing. Those flow meter kits sample compressed air at a very low flow rate (2 - 3.5 SCFM) and cannot meet the current NFPA criteria for most fire departments. Our experience with flow meters is that they are effective as long as they are closely watched and held steady in a vertical position. Moreover, we have found that flow meters used outside the lab are more prone to sticking because tiny debris collects on the flow meter's interior walls. A sticky flow meter cannot provide accurate flow rate data.
The other more popular way to determine the air flow rate is to place a flow-calibrated pressure gauge in the air path. Our air kits use the pressure gauges which are individually calibrated against a flow meter in the laboratory. Pressure gauges are about 5% less accurate than flow meters, but this is not significant. Flow calibrated pressure gauges are much easier to use, are more rugged and allow a wider range of flow rate use. When the sample kit is received back from the client, we calculate the flow rate used from the pressure and time reported by the client.
Most kits pass 18 – 36 cubic feet of air through their air kit. Where possible, we sample 90 - 160 cubic feet of air because that represents the entire contents of SCUBA flasks, and (given the effect of piping and stagnant air) it is a more realistic test. This is especially true when sampling/testing oxygen compatible air. This is because of the need for greater sensitivity and the goal of more thoroughly discovering particulate in the piping.
A
bulk sample is taken for lab analysis of fixed gases, carbon monoxide, carbon
dioxide and volatile hydrocarbons. The choice of
a container is not trivial, and ours are especially unique; this is discussed in the section Sampling
for Gases and Hydrocarbons.
Determining Oil Mist + Particulate in a gas stream is discovered by passing the gas through an effective filter. If the surface area of the filter is too small, the flow rate will either be seriously reduced, or the velocity of the gas flow will cause the oil to re-atomize and pass through the filter undetected. In either situation, the accuracy and usefulness will preclude a valid sample, and conclusions developed from the data will be invalid. After testing a number of filters, we found the most effective filter was a large diameter (47 mm) thick sub-micron glass fiber filter. Some clients need to look for and count very small particles. In those cases, we use a 0.45micron non-depth filter.
© 2005 Analytical Chemists, Inc