Casella CEL Microdust 880 IS real time dust monitor - Manual (Page 41)

Microdust 880nm & WinDust Application Software
Handbook HB3255-03
Page 41 of 51
Chapter 5 - Calibration.
Calibrating the Microdust 880
All light scattering instruments exhibit some sensitivity to a particulates refractive index, size, shape
and colour. The Microdust’s use of a narrow forward scattering angle minimises these effects but for
optimum calibration to a specific dust type, gravimetric calibration and adjustment of the instruments
span control may be required.
The optional gravimetric and respirable adapters incorporate 25 or 37mm filter cassettes and provide
a convenient method of gravimetrically calibrating Microdust instruments. Standard gravimetric
techniques can be used with the modules to give a cross check between the readings of the Microdust
and a weighed sample. If errors are found between the instruments reading and the gravimetric value,
then the ‘Span’ control can be adjusted to give a closer correlation of the two readings.
Calibration procedure.
Before performing a calibration run, the Microdust should be correctly zeroed in accordance with the
procedure detailed on page 15.
During the calibration run it is necessary to determine the average real time dust concentration which
has been measured by the Microdust. With models incorporating the internal logger option, the
recording and analysis of the data is a simple task. For non logger models then it will be necessary to
keep a record of the T.W.A (Time weighted average) value corresponding to the calibration period.
The following procedure is used to perform a calibration run;-
1. Adjust the ‘Zero’ control and confirm that the ‘Span’ is set to factory calibration point.
2. Fit the Gravimetric or Respirable adapter onto the probe and load a per weighed filter into
the cassette holder.
3. Adjust the sampling pump to provide the appropriate flow rate;-
•
The respirable adapter requires a flow rate of 2.2L/min.
•
For isokinetic sampling, pump flow rate is calculated from the sample velocity and
cross sectional area of gravimetric inlet tube (2cm 2 )
•
For general TSP measurements actual flow rate is not critical but will affect mass of
collected sample.
4. Clear the Microdust T.W.A value and (where applicable) start the internal logger. Start the
gravimetric sampling.