Appen dix A: Calc ulating Flow Rate
Airmetrics
59
A SAMPLER FLOW RATE CALIBRATION
The MiniVol Portable Air Sampler is designed to operate at 5 lpm at ambient
conditions. At the factory the sampler is calibrated at approximately 21°C at 754
mm Hg, and is adjusted to operate at 5 lpm at these conditions. (See calibration
curve sh ipped w ith samp ler.) In other lo calities, the sa mpler m ust be adju sted to
account for the different ambient temperature and barometric pressure. Adjustment
within a range p reviously established by calibration is usually p erformed be fore
every sampling project. This appendix explains how to use the calibration
informa tion shipp ed with th e MiniV ol to determ ine the flow rate that w ill equal 5
liters/minute at local ambient conditions.
In the calibration procedure used by Airmetrics, the flowmeter is calibrated against
a Laminar Flow Element (LFE) flow measuring device. Six flow rates, ranging
from approximately 4 to 6 liters/minute, are typically measured. The inlet of the
LFE is open to the atmosphere while the outlet is attached to the inlet of the
sampler. (The sampler's filter assembly is not included in the air stream during the
calibration procedu re.)
Figure A.1 shows the results of a typical MiniVol calibration.
The columns in the table in Figure A.1 are defined as:
Qind
Flowrate as ind icated by the rotam eter on the sam pler.
Qact
Flowrate at the actual calibration conditions as determined from the LFE.
Q@s td
The flowrate at standard conditions for the indicated LFE pressure drop.
(Note tha t this is not the s ame as convertin g the actu al flow rate to
standard condition s.) Standa rd conditio ns are de fined as an atmosp heric
pressure (
P
std
) of 760 millimeters of mercury and a temperature (
T
std
) of
298°K.
Qca lc
The calculated flowrate of the sampler that is determined from the linear
regression results.
Airmetrics MiniVol Users Guide
60
Airmetrics
(1)
Diff
The percentage difference in flow rates between the measured and the
calculated flow rates.
For each point in the calibration procedure, the flow rate indicated by the
flowmeter, "Q ind", is recorded, an d the actual flow rate, “ Qact” and “Q@ std” are
calculated from the pressure drop across the LFE.
The flow rate units "lpm" are liters per minute.
The
Linear Reg ression Re sults
in Figure A .1 show s the results of the best fit
line of Qind (independent) to Q@std (depen dent) variables:
where
m
vol
=
slope of the least square line
b
vol
=
intercept the least square line
The variance (
r
2
) is also listed in Figure A.1.
