The nephelometer measures light extinction integrated over a wide range of scattering angles, ~0 to 180 deg, at a single wavelength. Extinction varies with aerosol loading, particle composition and humidity. Two identical instruments are available, and can be used simultaneously. Sample air can be divided so that one unit samples at ambient humidity, and the other measures air that passes through a humidifier controlled at 90% RH. Nephelometers require an air sample inlet, tubing and a pump or other source of suction.
Absorbing gases, including CO2 and R134 refrigerant, are used to provide known extinction