To calibrate means to compare. During the calibration the measurement devices under test are compared to a reference measurement device. It should be noted that in all device characteristics the reference measurement device should be better than the tested measurement devices. It goes without saying, only calibrated measurement devices provide reliable and comparable results. ”In practice however, users often don’t know how the calibration of their particle counters was performed, so Palas® Managing Director Leander Mölter. A calibration certificate is available for each instrument, but the user often has no knowledge, on which basis the calibration was performed. Therefore, so Mölter, all devices have to be calibrated under the same conditions, if the results should be reliable and comparable. “This has led us to develop a new calibration system on the basis of our recently developed Reference Aerosol distribution System RAS 3000, with which the user can determine on site the counting efficiency differences of its particle counters in relation to a reference measurement device in an easy, fast and inexpensive way.
Measurement principles such as counting that is used in optical particle counters (OPCs) or optical aerosol spectrometers (OAS) offer the advantage, that they can determine the particle size and the particle concentration simultaneously. Today various manufacturers are offering different OPCs and OAS in various technical designs for different applications. OAS for example are used to measure the particle size distributions of aerosols, sprays or for the characterization of test aerosols, for filter testing and for the measurement of atmospheric aerosols. Here, Palas® offers the welas® digital and the Promo® systems, which can determine directly and reliably the particle size distribution in high particle concentrations up to 106 P/cm3 without dilution systems. OPCs have been developed for the characterization of cleanrooms. These devices have to determine reliably rather few incidents in a short time. Therefore OPCs often have a high suction volume flow of 28.3 l/min up to 50 l/min. By comparison, OAS have a lower suction volume flow of approximately 0.28 l/min to 5 l/min. The requirements of an OPCs for the determination of the particle size distribution are less demanding in comparison to an OAS. OPCs have been developed further and optimized for various applications. Various OPCs - with their appropriate device characteristics - are used for the verification or classification of cleanrooms into various cleanroom classes.
ISO 21501-1 defines quality characteristics
For the comparability of various, worldwide available OPCs, objective quality characteristics are necessary. These quality characteristics have been defined approximately three years ago in the standard ISO 21501-1. This standard defines the counting efficiencies, the size resolution capacity, the size classification accuracy, the zero counting rate as well as the maximum measurable particle number concentration, which is necessary for a coincidence-free particle measurement.
The quality of an OPC, such as the lower detection limit, the number of size classes, the exact volume flow and the maximum measurable particle concentration, is decisively influenced by the quality of the optoelectronic components, e.g. the light source, the angle-dependent light scattering and the clearly defined calibration curve. An important characteristic is the counting efficiency, which informs about how many particles are counted in comparison to a reference measurement device.
RAS 3000 for the calibration of the counting efficiencies of OPCs
With the new Palas® RAS 3000 the counting efficiency differences between up to six OPCs and up to five OPCs in comparison to a reference measurement device can be determined fast, reliable and inexpensive. The time for calibration is typically less than one hour.
The RAS 3000 consists of an aerosol generator with very low number concentration output and an aerosol distribution system with six aerosol outlets and six sampling cylinders. At each of the six aerosol outlets, the same number concentration ± 3 % is provided. If no clean, particle free pressurized air with a constant volume flow is available on site, Palas® recommends to use the ACA 1000 (adjustable clean airstation), which delivers a steady and high volume flow of e. g. 180 l/min ± 1 %.
The RAS 3000 provides defined, fast, reproducible and economical calibration for particle counting instruments
The RAS 3000 contains an integrated aerosol generator for the generation of a temporally very stable polydisperse aerosol with Cn,max = 35 P/cm³ = 35 x 106 P/m³. At these concentrations the OPCs can be calibrated without aerosol dilution. With the reference particle measurement device (RPM) the variances in number concentration are determined and recorded at all six aerosol outlets. Then, up to five OPCs can be connected to the RAS 3000 at the same time and the number concentrations can be measured. Each of the measured number concentrations at the respective outlet cylinder (AAZ1-6) is finally corrected with the previously determined values with the RPM.
By its nature of being a relative calibration, this calibration method is only dependent on the stability of the used components like the aerosol generator, the aerosol distribution system and of course the reference particle measurement device. The RAS 3000 provides defined, fast, reproducible and economical calibration for particle counting instruments. With this system the users of OPCs can check the work of calibration service providers. Another field of application of the RAS 3000 is the determination of the protection degree of operation rooms according to SWKI 99-3.