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Condensation Particle Counter


HOW IT WORKS

The monodisperse aerosol sample enters the CPC via the sampling tube and travels to the saturator.

Saturator

At the saturator the aerosol particles cross a heated N-butyl-alcohol saturation pipe that is at a temperature of 35C. The butanol is kept in a felt sponge to avoid spillage and damage to the instrument. There is an automatic supply of butanol to the sponge from an integrated alcohol tank. The supply is controlled by an internal sensor. The butanol tank may be refilled by attaching a bottle to the appropriate port outside the instrument (see Care and Maintenance) when the CPC is in standby mode. The cap of the bottle should be removed in order to allow the butanol to flow.

Condensor

The aerosol particles and alcohol vapour then enter the cooling condenser unit. This is at a temperature of 10C. Supersaturated alcohol vapour condenses on aerosol particles whose size is increased to ~10um. Information on the size of the particle is lost here. Every aerosol particle receives a single droplet of alcohol and this is controlled by a carefully managed saturation level. There is no alcohol droplet without an aerosol particle at its centre. Note that condensation of ambient humidity in the condenser can dilute the butanol and reduce the efficiency of the instrument. There is therefore an automatic micropump positioned before the condenser in order to remove this fluid before it runs back into the saturator. The peristaltic pump works automatically when the reservoir is attached to the outside of the instrument, at the relevant port. Only water that runs down the condenser walls is collected and pumped out, since water molecules have a very high diffusion rate and very quickly hit the walls of the condenser.

Optics

The droplets cross a laser beam and light is scattered by them onto a photodiode. The signals are counted continuously, in real time. Signals are counted per second and data is displayed in counts per ccm before being stored.

Filter

An active carbon filter is located above the laser optics, before the outlet in order to reduce alcohol odours and particle emissions to the environment.

Removable Data Card

Data may also be stored onto a removable data card for fieldwork. The internal battery will last for 4-5 hours depending on external conditions such as the temperature and whether the DMA is connected etc. When the memory card is first inserted into the CPC, the instrument will display the amount of data space remaining on the card. Data may be exported as a .txt file or directly into Excel as a spreadsheet.

Earth Observation Data Group, Department of Physics, University of Oxford. Page last updated: @21:11 GMT 13-Apr-2014