Dust Measuring Device

An important standard is EN ISO 14644-1:2015, which is used to determine the clean room classes. The standard defines clean rooms according to the detectable particle sizes and number of particles. The classification of the particles takes place according to fixed size classes, which are between 0.1 ... 5 μm.

Measuring devices should represent the required size classes in order to be able to carry out an assessment according to the standard. In addition to EN ISO 14644, ISO 21501-4 describes the requirements and calibration regulations for scattered light particle counters. Important parameters are zero count rate, counting efficiency, size resolution and flow rate.

Airborne particles can be detected by various methods with the help of mobile or stationary measurement techniques. In addition to gravimetric methods, there are such known methods as: the extinction measurement, the laser diffraction or the scattered light method. The latter is used by the manufacturers in many particle-measuring devices. The common standards also frequently refer in the specifications, to the devices with corresponding measuring technology. In the scattered light method, the particles pass through the measuring volume individually and generate scattered light pulses in the detection zone. Depending on the particle size, these pulses are larger or smaller and are thus a measure for the particle diameter. The particle concentration can also be determined in a combination with the flow rate.

The zero-count rate indicates the number of particles measured in a channel when the particle-free air flows through the measuring volume. Particle-free air can, for example, be generated by means of a zero filter. Ideally, the number of particles in the smallest channel should be zero for particle-free air. If the values deviate strongly, this usually means that the measuring optics is contaminated. In this case, maintenance by the manufacturer, if necessary with adjustment and recalibration, should be carried out.

The counting efficiency also compares the measured number of particles within a size channel between a particle counter and identical reference devices. The counting efficiency is channel-dependent and, for the smallest channel, cannot be less than (50 +/- 20) % according to ISO 21501-4.

In many mobile hand-held units, the flow rate is 2.83 l / min or 0.1 cfm or 28.3 l / min or 1 cfm. This magnitude has its origin in the American US Federal Standard Norm. The flow rate has a direct influence on the test interval as the test times and the volumes tested depend on this. The probe openings are designed mostly as isokinetic sample receivers. The probe geometry ensures that the air flow is neither too much accelerated nor too much delayed. The air particles are thereby guided in a uniform laminar flow and "fall" into the measuring chamber.

In case of the temporal and spatial interaction of several light impulses, caused by a plenty of particles in the measuring volume of the meter, as a rule, two errors occur. The concentration is then displayed as too low and the particle size – as too large. The devices cannot distinguish, whether the light scattering is due to one large particle or to two particles of half the size. The higher the particle concentration in the air, the greater the possible influence of coincidence errors.

For the purpose of evaluation and documentation, particle counters with a sufficiently large storage capacity and evaluation software are recommended. With these auxiliary tools, measurement results can be automatically put on paper after each measurement.