GRIMM Faraday Cup Electrometer

The GRIMM 5.705 Faraday Cup Electrometer (FCE), shown here with its control module, is a relatively simple piece of equipment. It is used to measure the charge carried by a particle and thus can provide an absolute measure of the particle concentration.

Firstly, a bipolar charge is applied to the sampled polydisperse aerosol by diffusion charging (termed 'neutralisation'). This is achieved by passing the aerosol into an electrostatic classifier (EC), which houses a radioactive source. This then subsequently passes into differential mobility analyser (DMA). The DMA allows particles to be classified on the basis of their mobility diameter in a dynamic electrical field. At distinct voltage intervals, size-selected aerosol was passed from the DMA to the FCE. The cage is placed in the path of the collimated particle beam (laminar air flow). The aerosol passes into the isolated filter inside the cup. According to Gauss' law, the charge collected on the Faraday cage is the induced charge, which means that the filter does not need to be a conductor. The filter forms part of the electrometer circuit which measures the current. Note that current is the flow of charge in a conductor: which in this case is directly proportional to the charge carried by the charged aerosol particles. The particle concentration (N) is derived from the measured current (I), surface charge carried by a particle (e) and the volumetric flow rate (Q), as described by the following equation:

The particle number concentration (N) (cm^-3) = I/eQ

where:

I = measured current (amps),

e = elementary charge (1.602 x 10^-19 C

Q = volumetric flow rate (cm^-3).

The advantage of the Faraday cage is its robustness and its ability to provide an absolute measure of the charge carried by an ion beam or stream of electrons. Furthermore the sensitiveness of the FCE is temporally constant and not mass-dependant.

The FCE has several advantages over conventional CPC's:

• Allows measurement of particle concentrations over wide range of concentrations (4-5 orders of magnitude).
• Relative simplicity – no temperature controls inside and no working liquids.
• Operation at different pressure levels (over / under pressure).
• Ultra-fast response (~ 50 ms) => 10-20 Hz.
• No nominal minimum size limit (even below 1 nm), making it ideal for studying particle nucleation processes – such as those associated with atmospheric photochemistry and the role played by ion clusters.

However, the FCE does have its limitations, which are:

• Limited to the measurement of particle concentrations below 1-2 x 10³ cm^-3.
• Sensitive to pressure and temperature fluctuations.
• Sensitive to mechanical stress.
• Only when used in combination with a DMA is it applicable as a particle counter.