Porton Counting Reticle

The Porton counting reticle can be purchased here.


Porton Microscope Reticle that meets AIA, NIOSH and Environment Canada Government Standards.


The Porton reticle is used for counting and measuring abrasives and dust particles, the most common of which are asbestos and paint or varnish particles.


In order to determine if dust particles in the air are at a safe level, a sample is collected on a filter that will reflect, realistically, the concentrations of dust at the place and time of sampling. NIOSH Publication 72-10267 details sample collection on page 80.


Once the sample is mounted on a filter, it should be examined using a phase contrast microscope fitted with a Porton Counting Reticle within two days of collecting the sample.


The filter samples collected are evaluated for asbestos fibers that are generally greater than 5um in length.


The half of the Porton reticle with vertical lines serves to define the counting area of the field. Twenty fields located at random on the sample are counted and total asbestos fibers longer than 5um are recorded. Any particle having an aspect ratio of three or greater is considered a fiber. The fiber lengths are determined by comparison with the cirles on the reticle. The square that is divided into six rectangles is the counting field area.


The following formulae are used to determine the number of fibers/ml:


  • Filter Area (mm2) / Field area (mm2) = K
  • Average net count X K / Air Volume Sampled (ml) = fibers/ml


An Example:


Area of filter used was 855 ml2, counting area of one field under the Proton reticle was 0.005 mm2; average net count per field of 20 fields was 10 fibers; and sample was collected at 2 liters per minute for 90 minutes. Then:


855mm2 / 0.005mm = 171,000 K
10 fibers x 171,000 / 2,000ml/min x 90 min = 9.5 fibers/ml.


As with any reticle, you will want to calibrate it with a stage micrometer.


You can order a Porton reticle here.