Are all test dusts created equal....?

In an earlier blog on aerosols and test dusts, I described how the test dusts are standardised in order to ensure that the testing of filters should be the same across all test locations. In some cases, however, the use of a standard formulation does not mean that all test dusts are created equal. In the case of soot, this is because the ISO standard still doesn't exist but in the case of AHRAE 52.1 test dust, the standard is documented and standardised but there are a number of suppliers in the market and this can lead to a significant difference in performance. 

ASHRAE 52.1 test dust is specified in the ASHRAE 52.2 test standarg as comprising the following components by weight:

• SAE ISO Fine Test dust   72%
• Powdered carbon             23%
• Milled cotton linters            5%

There are two principle commercial sources for the test dusts, the US based Powder Technology Inc. (also known as PTI) and the frighteningly similar Particle Technology Ltd in the UK. 

During alignment work earlier this year we confirmed what is well understood in the market, that although both labs manufactured their dusts to the same specification, the performance of the test dusts on elements tested to EN779 was significantly different.   

A range of F8 and F9 synthetic bag elements were tested with both PTI and Particle Technology sourced ASHRAE 52.1 test dusts. Data was taken at 250Pa, 350Pa and 450Pa for DHC and fractional efficiency. The efficiencies of the media were within experimental error with each other (<2%).

0.4 micron Fractional Efficiency Offset between F8 and F9 filter elements tested to EN779 due to test dust source

However the impact of the different dusts on DHC was significant.

DHC offset between F8 and F9 filter elements tested to EN779 due to test dust source

The DHC of European sourced dust was close to 30% lower than that of US sourced dust. This confirmed other comments from major manufacturers of elements such as Camfil and AAF. 

Why? The reason is that whilst the specification is correct for both sources of test dusts, the US test dust appears to have longer cotton fibres in the blend. This causes lumps to form and these lumps do not load so evenly creating a looser filter cake on the surface of the media. This looser filter cake results in gaps that allow air to flow through the filter creating a lower pressure drop and longer lifetime. 

The lack of homogeneity of the US dust has a further knock on effect. The long fibres act to create lumps and these lumps can jam up the dust feeder from time to time. 

The inconsistency of this test dust has led to some labs to not use the ASHRAE test dust but to solely use the SAE ISO fine test dust (which in itself has a significantly higher lifetime than the ASHRAE test dust) and the forthcoming ISO 16890 will certainly be based on SAE ISO fine test dust as a consequence. 

So is this phenomenon limited to this dust? No even with single batches of SAE ISO test dusts there are often significant batch to batch variations seen for multipass. We saw this when we changed test dust batches of SAE ISO Medium Test Dust earlier this year. There was a sudden change in media lifetime for fuel media (ISO19438). 

In short ,batch changes or source of test dust do significantly impact the result so you always need to be aware of the impact of test dust source or even test dust batches.  

 Not all test dusts are therefore equal! So take care when analysing data side by side. 

Have fun. 

Tony