CleanRooms Mailbox

01/01/2003

Dear Robert Donovan,

I have found your articles in CleanRooms magazine to be very informative. I have two questions on diffusion batteries: Can they be useful for continuous cleanroom monitoring? Where can someone get more data and design criteria for these devices?

Thanks for all the information in your articles past, present and future.

Best Regards,
Morgan Polen
Lighthouse Worldwide Solutions

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Dear Morgan,

A diffusion battery can be used to monitor cleanroom aerosol particles continuously—my December column presented data collected in such a mode. Doing so requires a particle counter, such as a condensation particle counter (CPC), to be located downstream of the diffusion battery to quantify the concentration of particles making it through the diffusion battery.

To simultaneously collect data at different cutoff sizes requires two or more CPCs connected to different taps of the diffusion battery; that is, one CPC measures essentially all aerosol particles greater than one given size. To measure particle concentration greater than a different size, one then has to change the length of the diffusion battery by selecting a different tap or else change the flow rate at which the measurement is made. Having multiple CPCs, each connected to a different tap, allows one to simultaneously collect particle concentrations greater than a multiple number of particle sizes.

I'm told that TSI Inc. in St. Paul, Minn., now markets a diffusion battery with the CPC built in. I suggest checking their Web site for further information and contact numbers. Earl Knudsen's paper (see Ref. 1 of my December column at Cleanrooms.com) is an authoritative review of the principles and history of diffusion batteries.

Robert Donovan
Electronics columnist for CleanRooms magazine