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Circular sampling probes are still being produced and commonly used to fulfil isokinetic flow conditions. Increasingly though, users recognise the advantages of rectangular shaped sampling probes. Topas Sampling Probes SYS 529 have a rectangular inlet which is gradually reduced to a circular tube connector. The design of the probe is based on the EN ISO 14644, part 3 which defines a maximum width to height ratio of 1:6.
The manufacturing process ensures that all internal surfaces are polished to avoid material deposits and thereby falsifying test results.
The rectangular design enables the scanning of the filter corners too unlike circular probes which inevitably miss some areas.
Rectangular sampling probes are compulsory according to DIN EN ISO 14644, part 3, where test procedures for clean room classification are explained. Many users respond by using them.
The scan rate of rectangular probes is lower compared to circular shaped probes due to the lesser height of the probe. The greater width on the other hand reduces the number of runs over the filter area, which enables a shorter filter testing time.
The circular shape of conventional probes causes a systematic error due to the different scanning times as shown in the figure below. Rectangular shaped sampling probes overcome this problem and lead to an accurate measurement.