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 for various applications and for a wide range of working conditions over the past almost 5 decades. At the same time, butanol has several disadvantages, including its strong and unpleasant odour, negative effects when inhaled over prolonged periods and flammability, making it troublesome to use in places with strict safety regulations. In this work, we propose addressing these negative issues by replacing butanol with dimethyl sulfoxide (DMSO), a substance that has not been used as a CPC working fluid to date. DMSO is an odourless, non-flammable and non-toxic substance that is easily accessible and inexpensive. During thorough experiments, this new substitute working fluid has proven to be stable with respect to its performance for CPCs and storage. We show that DMSO behaves in an equivalent manner to butanol when used to operate a CPC in terms of the instrument's counting efficiency, D50 cut-off diameter (the diameter at which 50 % of the particles are counted with respect to a reference) and concentration linearity. In addition, we have demonstrated this for operating pressures ranging from ambient pressure 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 remains a liquid, even in work environments with temperatures ≤ −10 ∘C, without any impact on the CPC's counting efficiency performance.