The 3786 CPC, with an output of 10Hz, measures particles as small as 2.5 nm and operates on the principle of enlarging small particles using a condensation technique until they are large enough to be optically detected. A sample is drawn through a cooled saturator block and then into a heated condenser, where water vapour diffuses into the sample stream. Supersaturated conditions are created as water vapour diffuse to the centre line of the condenser faster than heat. Particles greater than a critical radius in this region will serve as condensation nuclei for the water vapour and will quickly grow. These droplets are then counted by optical methods. This instrument is routinely located on the FAAM BAe-146.