All photometer-based direct-reading aerosol monitors use the principle of the light scattering to determine airborne particle concentration. Most photometers are calibrated in the factory using a „standard test dust” and are adjusted to agree with respirable dust concentration measurements made using reference gravimetric methods. In practice, it is highly unlikely that the „standard” test dust will exhibit the same physical properties (refractive index, density, particle size distribution, shape) as the airborne particles being measured. In order to obtain an accurate measure of airborne particle concentration, the aerosol monitor should always be compared to a reference gravimetric dust sampler placed alongside and adjusted accordingly. This study conducted controlled experiments to compare the measurements of real-time time photometers with gravimetric mass. Particulate matter source tested was wood-dust generated by handheld sanding operation.