ON MEASURING THE RESPONSE TIME OF CONDENSATION PARTICLE COUNTERS
J.A. ENROTH1, J. KANGASLUOMA1, F. KORHONEN1, M. ATTOUI1,2 and T. PETÄJÄ1
1
Department of Physics, P.O. Box 64, 00014, University of Helsinki, Helsinki, Finland
2
LISA, University Paris Est Creteil, 94010, Creteil, France
Keywords: RESPONSE TIME, PARTICLE COUNTER, SPARK GENERATOR.
INTRODUCTION
Fast particle counting is a desirable attribute for various measurement purposes. Mobile measurements,
rapid size scans or flux measurements are some of the applications where ever faster particle counting is
desired. These requirements are being answered with the introduction of fast water based condensation
particle counters (CPC) by TSI and by introducing alternative designs solutions such as the mixing type
CPC’s of Wang et al. (2002) and Wehner et al. (2011).
These fast response instruments require more precise measurement of their their temporal resolution than
the traditional laminar flow CPCs. The older models, such as the 3010 by TSI have typically slower (> 1
s) rise times (time taken for concentration to change from 10 to 90 %), while the novel faster CPC’s state
to have even sub-100 ms rise times. This acceleration of an order of magnitude makes it important to
understand thoroughly the process of determining a CPCs time response, and the numerous factors that
take part in shaping, and potentially distorting, the observed response. Some of these factors work to slow
the response (diffusion in tubing, slow change in concentration control, size dependent terminal velocities
etc.), while others can have the opposite effect (pressure pulses, coincidence).
The accurate understanding of the response time is not only a technical curiosity, but of importance for
correct data interpretation. Overestimation of the response time is usually less dangerous, and mostly
translates only to poorer measurement resolution, either spatial or temporal. The underestimation,
however, is prone to distort measurement data by the residual effect of the previous measurement points,
and therefore it is vital to know the true response time of the instrument, provided you want to run your
instrumentation near to its maximum temporal resolution.
Various methods have been implemented in determining the response time of CPCs that include valves
(Quant et al. 1992; Hering et al. 2005), spark generators (Wang et al., 2002) or DMAs (Buzorius, 2001;
Shi et al. 2005). None of the methods has gained universal adaptation even in more recent studies. Here
we present results of a comparison of different methods found in the literature for determining a CPC’s
time response. All the experiments described here were using a single fast mixing type CPC built in the
University of Helsinki as the detector.
Like the methodology, the terminology also has not been fully set. The mixing time depicted as τ, is the
most widely adopted common term, which is used to indicate the time constant with respect to a particle
concentration change. As the name implies, this is meant to describe the diffusional movement, or mixing,
of particles within the instrument. The resulting response to a concentration change is thus well described
using a first order exponential function (Eq. 1).
(1)
Many studies report the response of their respective instruments using the τ. However, from this definition
it is not immediately obvious to the end user what is the practical maximum time resolution, and hence
here we adopt the use of ε to indicate the response time, defines as
ε=3*τ
(2)
ε, introduced for the sake of clarity, corresponds to a change in concentration of 95 % (either 0% - 95% or
100 % - 5%). This definition of a response time allows a more useful interpretation of the change, while
still being strictly mathematical in its description.
METHODS
We tested different methods found in the literature for the determination of particle counters response time
in order to find the most suitable method for producing robust and highly reproducible response times.
The most popular methods included using a Differential Mobility Analyzer (DMA), a spark generator or a
valve to produce a rapid change in the particle concentration of the sampled air.
Using rapid control of voltage in a DMA Buzorius (2001) determined the response time for a TSI 3010
CPC. Following this approach, we used a short Vienna type DMA, with 4 lpm aerosol flow and 20 lpm
sheath flow. Additional particle free carrier air was added right after the DMA to ensure turbulent flow,
and hence a plug flow profile.
Another approach has been to use spark generators, for instance Wang et al. (2002) and Shah and Cocker
(2004) in studying fast mixing type CPCs. Here, the spark generator was tested using different levels of
dilution, which all had Reynolds numbers well more than 4000, again ensuring the turbulent flow profile
was maintained until the inlet of the instrument. The spark generator works by applying high voltage to
electrodes which then are brought close enough so that a spark discharge occurs (Schwyn et al. 1988).
This produces a sharp pulse of particles, which then can be used to determine the response time.
The use of valves has been one of the more popular methods for response time determination used for
instance in studies by Quant et al. (1991), Hering et al. (2005), Heim et al. (2004), Held and Klemm
(2006). Unlike in these previous studies, here the valve did not sample from ambient air, but was
connected to an atomizer and an additional carrier/dilution flow was used to ensure turbulent flow in the
tubing. The used set-up is shown in Figure 1.
Figure 1. CPC response time measurement set-up used for the valve based measurements.
CONCLUSIONS
The results of these various response time measurement methods are illustrated in Fig 2. All data in the
figure is from the same fast mixing type CPC, hence the differences should not originate from the
instrument, but rather the different test methods.
Figure 2. Observed responses for various response time measurement set-ups. Concentrations < 4000 # cm-3 were
used for all measurement but the High conc. spark, where concentrations exceeding 50 000 # cm-3 were used. The
solid lines indicate the data fitted to Eq. 1.
The DMA method, even with the added carrier flow and the high aerosol and sheath flow rates, proved to
be the slowest of the tested methods. It is viable, that using high-flow DMA’s and more advanced power
sources, this method can be significantly improved, however the traditional DMA’s appear not to be suited
for the task.
The spark method was found to produce varied results. An attribute of the spark generator is to produce
very high concentrations that can exceed the coincidence limit of most CPCs (Schwyn et al. 1988). The
presence of coincidence was observed to have a significant impact on the measured response time, with
higher concentrations having a more drastic impact. Figure 2 shows the concentration decay as determined
from either high (> 50 000 # cm3) or low (< 4 000 # cm3) concentration spark. The high concentration
spark appears to have a slower response, due to masking effect of coincidence. While the actual
concentration decreases quite drastically, the observed concentration shows a less pronounced decay, thus
resulting in a slower appearing response. It should be mentioned, that the opposite is true if one looks at
the concentration increase. In that case coincidence creates an artificial plateau in the concentration, and
makes the instrument appear to be faster than it is. These problems can easily be avoided by applying
suitable dilution and voltage settings so as not have concentrations in the coincidence range.
When the spark generator was operated using the low concentrations, the decay response appeared to give
more consistent and reasonable results. Unfortunately, the concentration increase could not still be
extracted using this method. The peak of the spark pulse is likely to be too transient to be correctly
captured, and the final plateau value for the concentration cannot be therefore know. This effectively
means that the concentration increase is extremely hard to determine from the spark generator.
In contrast to the spark method, the valve method allows for very fine particle concentration control,
especially if connected to a constant aerosol source, like an atomizer. This makes it easy to avoid
coincidence, while ensuring sufficient signal given the large dilution flows needed to maintain turbulent
condition in the sampling lines. Furthermore, the valve method also allows the determination of the
instruments response to a step change for both concentration increase and decrease.
The main problem found with the valve method is the small pressure pulse that is associated with the
changing of the valve. This can affect the flows of some the sheathed instruments by distorting the internal
flows of the instrument. This however was not found to be a major issue, as shown in Figure 3. Both the
spark and valve methods showed considerable agreement, and are likely suitable for the determination of
the response time of even the faster particle counters.
Figure 3. Normalized concentration decay from spark and valve measurements. The squares depict the median of the
spark measurements, with the dotted line a fit according to Eq.1. The filled circles depict the median of the valve
measurements, with the solid line showing the corresponding fit.
ACKNOWLEDGEMENTS
This work was supported by the Maj and Tor Nessling Foundation (grant 201600249), Academy of
Finland Center of Excellence program (Center of Excellence in Atmospheric Sciences) and European
Commission (ACTRIS-2).
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