The measurement of particle size is routinely carried out across a wide range of industries, but the measurement of the size and number of particles in situ, at high flow rates, pressure and accuracy– requires a very complex construction of the measurement system and its calibration procedure. Most conventional methods, like microscopy, laser diffraction, dynamic laser scattering or focused beam reflectance, are very accurate and reliable, but do not meet the above requirements. The chosen method is based on counting the number of photons scattered on single solid particles in a gas flowing at a velocity 15 m/s and under pressure 20 bar. As the detectors, the photomultipliers (PMT) are used, operating in the mode of single photons counting (SPC). Measured particles are up to 3 μm. The developed program for recording the measurement signal from PMT is the result of analysis of the distributions of all its component. How fast the counter is depends not only on the detector pulse-pair resolution, but also on the preparation of the proper shape of the counted signal. For this purpose, the particles were normalized, autocorrelated and fitted to a normal distribution. The program, created in the Labview environment, allows for the acquisition of a 10 s signal with a frequency of up to 3.7 MHz. In experiments we used 50 mW laser and PMT working in single photon mode with pulse pair resolution 2.5⋅10−6 s−1.