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4.0 Principle of Operation
The HiVolCal®measures volumetric and standard flow rate by utilizing a pressure transducer to
assess the pressure drop caused by air being drawn through an orifice. As the flow rate through the
orifice increases the pressure drop increases as the square root. Twice the flow rate yields a four
times increase in pressure drop. A desirable feature of the orifice is that most of the pressure drop
created by the instrument is recovered downstream of the orifice. Therefore, measurements are
made near the operating conditions of the sampler.
The microprocessor uses an algorithm to combine the signal from the pressure transducer,
barometric pressure sensor, and ambient temperature sensor to provide an instantaneous flow rate.
To eliminate fluttering of the on screen display of flow rate, the first 20 readings are averaged and
then carried on as a rolling average. Barometric pressure and temperature are monitored and
displayed on a continuous basis, when the instrument is switched on. A cutaway diagram of the
measuring head is shown in Figure 2 and a schematic diagram of the system is shown in Figure 3.
5.0 Instrument Set-up
Important notes about using the HiVolCal®
A. The measuring head must have no air flowing through it. Every time the instrument is
switched on, it re-zeros itself. If air is flowing, that flow rate will be set as zero.
B. The control module must be in the position in which it is going to be used, when switching
on (horizontal or vertical). The case houses the pressure transducers, which are subject to
the force of gravity. Positional changes can give rise to minor errors. This effect applies to
all devices containing pressure transducers.
C. In order to perform the most precise measurement audit, it is necessary for the HiVolCal®to
be in thermal equilibrium with the ambient environment in which the sampler is located.
The best procedure is to deploy the HiVolCal®, out of its carrying case, for 10 minutes prior
to the audit. Additionally, if the HiVolCal®is subject to a temperature change of more than
five degrees during use, it should be rebooted.
When the HiVolCal®is switched on
XX
% battery capacity remaining is displayed on the
screen. So long as more than 10% is indicated, it is safe to proceed in that at least one hour
of power is available. The audit is now ready to be performed.
6.0 Using the HiVolCal®
6.1 To perform an audit
Turn off the sampler to be audited. Remove the inlet of the sampler and install the HiVolCal®.
Use adaptor as necessary. Turn on the HiVolCal®, wait for the screen to finish the startup boot,
and then turn on the air sampler. Location of on/off switch shown in Figure 4.
The flow resistance of the HiVolCal®head may cause momentary instability in the air samplers
flow control circuit. Once the air sampler’s main screen flow rate indicator stabilizes, the reading
may be taken. A simple audit data format is shown in Table 1.
6.2 To perform a calibration.
The procedures and calculations for using the HiVolCal®to calibrate a sampler are the same as
an audit, except the flow rate tolerance of is usually +2% rather than +4% as suggested for an
audit.
