Appen dix A: Calc ulating Flow Rate
Airmetrics
61
Figure A.1.
Portable Sampler Calibration Output
Airmetrics MiniVol Users Guide
62
Airmetrics
(2)
Sampling at Ambient Conditions
The MiniVol's size-selective inlet is an impactor whose particle size selection
characte ristic is depe ndent up on the ve locity with which th e air stream impacts
upon the im paction plate. The impactor is design ed to have a no minal 10µm (part
206-000) or 2.5µm (part 206-100) cut off at an actual air flow rate of 5.0 liters per
minute. To maintain this cut off size, the sampler’s flow rate must be adjusted
(flowmeter set p oint -
I
sp
) so that the flo w rate thro ugh the s ize-selectiv e inlet is
maintain ed at 5.0 lp m at
ambient
conditions.
To calculate the flowmeter set point, you need:
•
the sampler’s ca libration slope,
m
vol
, and intercept,
b
vol
. This inform ation is
supplied to you on the “MiniVol Portable Sampler NIST Traceable Flow
Calibration” that ca me with yo ur sampler (Fig ure A.1);
•
the expected am bient temperatu re,
T
act
, in K°, and pres sure,
P
act
, in mm Hg,
expected during the sampling study. This data may be estimated from local
weather service data or from other reported historical data. If the U.S Weather
Service a tmosph eric pressu re is used, b e sure that th e “station p ressure” is
used. That is, atmospheric pressure
not
corrected for the repo rting site’s
elevation above s ea level.
If the local “station pressure ”,
P
act
is not readily available, it c an be rea sonably
estimated by using Equation 2.
Where
P
act
= ambient atm ospheric pressu re
P
sea
= sea level atmospheric pressure (nominally 760 mm Hg)
E
=
site elevation in feet
The Flow meter Set Poin t,
I
sp
, is calculated using Equation 3.
