sample during each of the seasonal extremes, since the demands on the ventilation
system and hence, the ability of the system to control the contaminant, may vary
h. Closely related to the question of how long to sample, how many samples to
collect, and when to collect the samples is the matter of determining how to sample,
that is, the type of sample to be collected. In this regard, there are four general
categories of samples: spot (grab) samples, partial period samples, full period single
samples, and full period consecutive samples.
(1) Grab samples. This is a method for collecting a sample of gas or vapor.
Grab samples are also called instantaneous samples, as they are collected almost
instantaneously, that is, usually within a few seconds, although some grab samples may
be collected over a period of up to l5 minutes. A grab sample is therefore
representative of the atmospheric conditions at the sampling site at a given point in
time. Normally, at least five to seven samples should be taken, at random, except for
contaminants with a "ceiling" TLV. In this case, the samples must be taken at times
when concentrations are expected to be maximum. Evacuated flask samples, Mylar
bag samples, and detector tube samples are all examples of grab samples. Examples
of grab sample collecting devices are shown in Figures 2-1 and 2-2.
Figure 2-1. Evacuated flask for collecting grab sample.