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All gas phase filter tests are of the single-pass format using a particle challenge, but the methods differ widely, as do the particle formulations used in the challenge and the means of analysis to demonstrate performance. The materials used include natural sand/quartz mixtures, alumina dusts, methylene blue aerosols and di-octyl phthalate. A sand-quartz mix is specified for the valuation of air filters for IC engines and compressors, originally in BS 1701, now BS ISO 5011.

Alumina dusts, such as Aloxite 50 and 225, are specified for ventilation filter testing, originally in BS 2831, then BS 6540, and now BS EN 779. The particle size distributions for these two dusts are shown in Figure 1.20 in a form common for size distribution illustration, i.e. the percentage below a particular particle size in the mixture, plotted against particle size.

An aerosol of methylene blue dye is used primarily for the testing of low penetration filters. The particles are extremely fine, with an average size around 0.5 μm and no particles above 1.3m, as shown in Table 1.6. This challenge material would not be used for testing ordinary air-conditioning filters, as these would show a low efficiency with this material. The test technique involves the atomization of a methylene blue dye solution through a spray nozzle at a specified pressure, discharging the spray into a duct that houses the filter under test. Sampling points are located upstream and downstream of the filter, in straight sections of duct.The upstream section is of sufficient length to allow moisture to evaporate from the droplets of dye, leaving a dust cloud of dispersed methylene blue particles.

The performance of the filter is then assessed by comparing stain samples taken on either side of the element.

A technique that is somewhat easier to use is the sodium flame test, which produces an aerosol of sodium chloride particles, as with methylene blue, but assesses the dust content of the air upstream and downstream of the filter by the intensity
of the yellow colour produced by the salt dust in a hydrogen flame (BS 3928). The size distribution of the salt dust is shown in Table 1.7, with a mean size around 0.6 um, and no particle larger than 1.7m. The colour of the flame is observed by a photosensitive cell connected to a meter, which can be calibrated to read the salt concentration directly.

Di-octyl phthalate aerosols are also widely used for penetration tests – they differ by being aerosols of liquid droplets at the test filter, and by being a mono-disperse aerosol (i.e. all of one particle size, 0.3m). Such tests are recognized as suitable
for HEPA filters.

There are three main types of air filter test (in addition to the sodium flame test):
● staining tests (as with methylene blue)
● weight arrestance, and
● particle concentration efficiency.

The determination of atmospheric dust spot efficiency is a staining test, described in detail in BS EN 779, and used for AC and ventilation filters. It is based on the intensity of staining of a target ultrafine filter paper caused by the flow through it of
a quantity of dusty air. The intensity of staining is measured by an opacity meter, and provides an empirical measurement of the concentration of contaminant in the air stream. In the filter test, the downstream air flows constantly through its collection paper, while the upstream air is sampled for only a proportion of the time (because of its much higher dust concentration), and this proportion is controlled to yield an equivalent stain to that from the downstream flow. The flow ratio can then be converted to a filtration efficiency.

The dust weight arrestance test is equivalent to the single-pass flow test described earlier. The test filter is challenged with a weighed quantity of a test dust, the filtrate passing on through a second very fine filter, which captures all of the dust passing
through the test unit. The amount of dust passing is determined by weighing the final filter. The feed dust suspension is created continuously by a suitable combination of a dust feeder and a compressed air venturi ejector, blowing the dust into
the feed air stream. As the test progresses, the weight of dust passing and the corresponding pressure drop are continually measured.

The particle concentration efficiency tests are used for the various grades of high efficiency air filters (HEPA, ULPA), and use the sub-micrometre aerosols described in earlier paragraphs. The procedures are described in detail in BS EN 1822, which
also highlights the Most Penetrating Particle Size (MPPS) as probably the most important performance characteristic.

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