Dust Meters

When selecting a dust meter for monitoring air quality, you should first clarify which variables need to be determined (e.g. air temperature, humidity, particle size, and concentration). If it is necessary to determine particle sizes in μm and to determine the amount or concentration of certain particle sizes in the air, for example for cleanroom classification, then an optical particle counter (OPC) is required.

Fixed measurement or portable solution? Portable particle counters are ideal for intermittent, random testing of cleanroom environments in research, development, and production. Due to their low weight and compact size, portable particle counters offer a convenient solution for monitoring air quality. However, these devices are not suitable for long-term measurement and integrated monitoring. For such needs, fixed particle counters are the right choice. Permanently installed measurement systems can be used for clean indoor area certification as well as for monitoring air quality in operational areas or production areas in the pharmaceutical industry. Determination of filtration classes as well as leak testing is also possible. Which standards are relevant? An important standard is EN ISO 14644-1:2015, which is used to determine cleanroom classes. The standard defines cleanrooms according to detectable particle sizes and particle count. Particle classification is carried out according to fixed size categories, which are between 0.1 ... 5 μm. The measuring devices should cover the required size categories in order to enable an evaluation in accordance with the standard. In addition to EN ISO 14644, ISO 21501-4 describes the requirements and calibration regulations for light-scattering particle counters. Important parameters are the zero count rate, counting efficiency, size resolution, and flow rate.

Which measurement principle is used?

Suspended particles can be detected using various methods with the help of mobile or fixed measurement techniques. In addition to gravimetric methods, known methods include: extinction measurement, laser diffraction, or the scattered light method. The latter is used by manufacturers in many particle measurement devices. Common standards are also often referenced in the specifications for devices with the corresponding measurement technology. In the scattered light method, particles pass individually through the measurement volume and generate scattered-light pulses in the detection zone. Depending on the particle size, these pulses are larger or smaller and therefore provide a measure of particle diameter. Particle concentration can also be determined in combination with the flow rate.

What is the zero count rate? The zero count rate indicates the number of particles measured in a channel when particle-free air flows through the measurement volume. Particle-free air can, for example, be generated using a zero filter. Ideally, the number of particles in the smallest channel should be zero for particle-free air. If the values deviate significantly, this usually means that the optical measurement components are contaminated. In this case, maintenance by the manufacturer should be carried out, if necessary with adjustment and recalibration. What is counting efficiency? Counting efficiency also compares the measured number of particles within a size channel between a particle counter and identical other reference counters. Counting efficiency depends on the channel and, for the smallest channel, cannot be less than (50 +/- 20) % according to ISO 21501-4. Is the flow rate important? In many portable handheld devices, the flow rate is 2.83 l/min or 0.1 cfm or 28.3 l/min or 1 cfm. This value originates from the US American Federal Regulation. The flow rate has a direct effect on the test interval, as the test times and the volumes checked depend on it. The probe inlets are mainly designed as isokinetic sample receivers. The sensor geometry ensures that the airflow is neither accelerated nor slowed down excessively. Airborne particles are thus guided into a uniform laminar flow and “fall” into the measurement chamber.

What is coincidence? In the case of temporal and spatial interaction of multiple light pulses caused by multiple particles in the counter’s measurement volume, two errors generally 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 two particles of half the size. The higher the particle concentration in the air, the greater the potential impact of coincidence errors. With data storage and software? For evaluation and documentation purposes, particle counters with sufficiently large storage capacity and evaluation software are recommended.