A New Working Fluid for Condensation Particle Counters for Use in Sensitive Working Environments

Abstract. The working fluid of a condensation particle counter (CPC) is one of its essential characteristics. Butanol is commonly used as the working fluid in alcohol-based CPCs due to its proven performance in various applications and a wide range of working conditions over the past almost five decades. At the same time, butanol has several disadvantages including its strong, unpleasant odour, negative effects when inhaled over prolonged periods, and flammability, making it troublesome to use in all places with strict safety regulations. We are proposing to address these negative issues by replacing it with dimethyl-sulfoxide (DMSO), a substance that has not been used as the working fluid in a CPC up to now. DMSO is an odourless, non-flammable, non-toxic substance that is easily accessible and inexpensive. During thorough experiments, this new substitute working fluid has proven to be stable in its performance in CPC and storage. We could show that DMSO behaves equivalent to butanol when used to operate a CPC in terms of the instrument's counting efficiency, D50 cut-off diameter and concentration linearity. In addition, we have demonstrated this for operating pressures ranging from ambient down to 200 hPa without the need for any safety precautions. Our new working fluid was tested on three CPC units representing two different models, almost always in side-by-side measurements. So far, we have achieved the best results with operating temperatures of 40 °C for the CPC’s saturator and 5 °C for its condenser. To address a less desirable property of DMSO, it could be mixed with a slight amount of water to decrease its freezing point significantly. When mixed accordingly, the substance stays liquid even in work environments with temperatures ≤ −10 °C and without any impact on the CPC's counting efficiency performance.