Re: Sampling a fluid process

[alla.linetsky@ca.pwcglobal.com]

The most important characteristic of sampling concentrations of anything,
fluid, solid or powder, is that as sample size increases, the variation
between samples decreases. Therefore, it is essential that all sample sizes
be kept the same.

Determining the optimum sample size is a bit trickier. One must first of
all consider the sensitivity of the instrument measuring the concentration
of impurities (a 'rule of thumb' is that instrument sensitivity must be at
least 10 times greater than the required accuracy of measurement. If the
required accuracy is 1 ppm, the instrument must measure to 100 ppb or
less). This condition will determine the minimum size of your sample, since
instruments will likely have limits for absolute amounts as well as
concentrations.

As for finding the most appropriate larger-than-minimum sample size, you
will have to experiment with different sizes and determine which size gives
you a 'reasonable' amount of variation - not too much, not too little. If
the aim of this sampling procedure is to have a record in some binder to
certify that your company has complied with the environmental regulations
on a particular day, then make the samples as large as you can handle (but
make sure they're well stirred). If, however, the aim is to improve the
process by reducing contaminants as much as possible, then make the samples
closer to the minimum size, so as to get enough variation to enable you to
identify assignable events.

I hope this helps.

Alla Linetsky
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