DATREND Systems venTest, Operating Manual

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venTest OPERATORS MANUAL
Two aspects must be taken into consideration:
• Volume V is calculated, with a short fraction of time dt in which you measure the flow F and
multiply by the fraction of time dt V= F*dt. Then you have calculated the volume which has
flowed through the sensor. A precondition for an exact measurement of the volume is that
flow in the time 'dt' stays constant.
However this can not be ensured as a rule. Therefore the period of time 'dt' has to be set as
small as possible to be able to 'scan' the flow curve as accurately as possible. The total
volume then is the sum of these partial volumes:
V = V1 + V2 + V3 + ....(this can be described as discrete integration)
You have to be aware that this method always contains an error factor because you cannot
choose the time period 'dt' small enough that the flow is really constant within this time segment.
A major criteria is the time resolution dt. The smaller this time interval is the more exact the flow
curve will be scanned. The user generally has no control of this value. You have to count on the
manufacturer of the measuring device to have chosen an adequate time resolution for use and
accuracy.
The other point of view is the criteria when the volume measurement (integration) should begin
and when it ends.
As a rule you use a flow trigger threshold for this. If this trigger is exceeded, the volume measuring
begins. It will be terminated if the flow has dropped below this threshold. The trigger threshold is
generally adjustable. There are other trigger conditions conceivable such as pressure rise via a
pressure trigger threshold or end of pressure rise.
Appendix A
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