When comparing results between different counters, the data can be compared only after the measurement units are matched which may require the particle counter data to be converted. Particle counts can be reported in four different ways:
- Raw-Differential (I)
- Normalized – Differential (II)
- Raw-Cumulative (III)
- Normalized-Cumulative (IV)
Understand Data Conversion of Particle Counters
Convert Raw Counts (I and III) to Normalized Counts (II and IV)
When raw counts are reported and the volumetric flowrate is different between particle counters, the raw counts must be converted to normalized counts by dividing the displayed raw counts by the total volume of air collected during the sample. This is done by dividing the raw counts by the product of the sample flowrate multiplied by the sample collection time. See below.
Normalized Counts (#/vol) = Raw Counts / [Sample Time (Time) X Volumetric Flowrate Volume/Time)]
For example, if a counter , labelled “A”, with a 1 ft3/min volumetric flowrate is used with a 1-minute sample collection interval, and is then compared to another counter, labelled “B”, with a 0.1 ft3/min flowrate with a 5-min sample time, then the raw counts must be divided by time multiplied by flowrate:
▷ Counter A normalized = Raw Counts # / [1 min X 1 ft3/min] = Counts # / ft3
▷ Counter B normalized = Raw Counts # / [5 min x 0.1 ft3/min] = Counts # / ft3
In this example, the units of the result are the number of counts per cubic foot, which can be converted to counts per liter by multiplying the result by a conversion factor of 1 ft3 / 28.32 L, or 0.035 ft3/L.
▷ Counts # / ft3 x [0.035 ft3/L] = Counts # / L
Differential (I and II) vs Cumulative Counts (III and IV)
The reported size channels being compared must be considered when performing comparison analysis between counters. If the counts are cumulative then the particle counts used in the comparison must use the cumulative counts from the same size channels. If the counts are reported in differential counts (i.e., counts from only one size channel) then comparison must also take into account the differential counts from that same size channel and the match between the size channel’s upper and lower bound.
Want to read more about particle count conversion? Download the full paper here. In next week’s blog, we cover the differences between isoaxial and isokinetic sampling in laminar flow. Stay tuned!