At the end of the sampling period recheck the flowrate as before
and record this value. If the difference between initial and final rates is
less than 0.3 1.p.m. the sample is acceptable and the arithmetic average of
the two values is used as. the sampling pater’ If the difference in values is
greater than 0.3 l.p.#. the sample is iditecarded and another taken. in the same

location.

On completion of sampling, carefully remove the filter unit from
the pump, replace the top cover and sealing plugs and place the sample in a
container with the printed surface of the filter in the upright position. In
conveying the samples to the analytical site avoid jarring of the units and
ensure they are maintained in the upright position at all times.

6. CALIBRATION OF PERSONAL SAMPLER

The accuracy of an analysis can be no greater than the accuracy
of the volume of air which is measured. Therefore the accurate calibration
of a sampling device is essential to the correct interpretation of an
instrument's indication. The frequency of calibration is somewhat dependant
on the use, care and handling to which the pump is subjected. Pumps should
be calibrated if they have been subjected to misuse or if they have just been
repaired or received from a manufacturer. If hard usage is given the instrument

more frequent calibration may be necessary.

Ordinarily, pumps should be calibrated in the laboratory both
before and after they have been used as a reference. The choice of calibration
instrument will depend largely upon where the calibration is to be performed.

A rotameter is the most convenient instrument used. The actual set up will be
the same for all of these instruments. The calibration instrument will be
connected in sequence to the filter unit which will be followed by the
personal sampler pump. In this way the calibration instrument will be at
atmospheric pressure.: Connections between units can be made using the same
type of tubing used in the personal sampling unit. Care must be exercised

in assembly procedure to ensure adequate seals at the joints and that the

length of connecting tubing be kept at a minimum. Calibration should be