The location of instruments in the field makes accurate calibration far more difficult than if carried out in a laboratory. This is because the conditions cannot be fixed, for instance gas flow rates in stacks and ducts may vary during the calibration exercise. In a laboratory conditions can be easily controlled. In general on-site calibration can only be carried out under the prevailing conditions: e.g. temperature, humidity, gas flow, particle size, particle shape, particle refractive index, particle density and concentration. As the aforementioned conditions can not generally be altered only a limited part of the instruments range can be calibrated.