Frequently Asked Questions

Aerosol Spectrometers, Dust Monitors


  • for the continuous measurement of size distributions, dust mass concentrations and other statistical distribution parameters
  • at a minimal temporal resolution of 1 s
  • for the easy handling by a touch display
  • for the integration in process control systems
  • for a stand-alone operation
  • if sensors with optical fiber systems for different concentrations, pressures and temperatures are needed

welas® digital

  • for the measurement of size distributions with a high temporal resolution
  • if a minimal temporal resolution of 10 ms is needed
  • for the automatic filter test with Palas® test rigs and the software FTControl
  • if sensors with optical fiber systems for different concentrations, pressures and temperatures are needed


  • for the continuous measurement of environmentally relevant dust values like PM1, PM2,5 or PM10 
  • for the alignment of PM values with a total filter system
  • at a minimal temporal resolution of 1 s
  • for the easy handling by a touch display
  • for the integration in environmental measurement stations
  • for a stand-alone operation 
  • for long hours of operation without recalibration (LED light source with a high long-term stability)
  • for the recording of meteorological values with weather station

Fidas® supports two kinds of data analysis regarding the mass concentration and PM-values. There are so called classic PM-concentrations  and ambient PM-concentrations.

For the computation of classic PM-concentrations Fidas® evaluates the measured particle size distribution with the same density for all particles. However, you have the possibility to evaluate your data with different densities and you also can change the form factor and the refractive index. Additionally, you have the possibility to enter a gravimetric correction factor, which you can determine with the total filter system of Fidas®.

For atmospheric measurements and workplace measurements so-called ambient PM-concentrations are computed using a special algorithm. The development of this algorithm needed several years and is based on measurement data of several thousand hours with Fidas® and reference devices using gravimetric filter systems to measure PM1, PM2.5 and PM10.  It is optimized for atmospheric and workplace PM-measurements. You can’t change the density or the form factor, everything is preset by the algorithm.The basic idea behind this algorithm is to use a refractive index that matches the composition of the atmospheric aerosol. In general, the correlation accuracy R2 of these PM-values with gravimetrically measured PM-values is better than 0.95.

The so-called IADS (Intelligent Aerosol Drying System) of the Fidas® is using a dynamic heating to compensate moisture effects on particle sizing. At extreme conditions (relative humidity close to 100%) Fidas® can also increase the retention time within the IADS to evaporate water droplets.

The used heating temperature and retention time within the aerosol drying system is a function of the relative humidity and the ambient temperature. The algorithm to compute the temperature and the retention time is based on a thermodynamic model that computes the evaporation of droplets and was validated with experiments.

The IADS has three operating modes:

  • switched off
  • remove volatile / moisture compensation (dynamic heating modus)
  • remove volatile and semi-volatile (heated to 90 °C to evaporate all liquid particles and components)

The signal acquisition electronics and the sensors of both devices are the same. The only difference is that Promo® has - contrary to welas® digital - an Intel Atom Processor for the signal processing. Thus, the data analysis can be effected directly at the device. In addition, Promo® has a touch display which allows an easy handling.


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