Flow Calculation; Method Of Flow Calculation - Siemens SITRANS L 7ML5400 Operating Instructions Manual

Technical reference
A.7 Flow Calculation
A.7

Flow Calculation

Special emphasis has been placed on providing the most accurate flow calculations possible.
To this end, specific routines have been written to comply with the British Standards
Institute's Specifications BS-3680. These routines calculate correction factors taking into
account second order effects such as approach velocity and boundary layer.
If the PMD does not match any of the eleven preset PMD calculations, or if a PMD is not used,
select a Universal flow calculation (PMD = Universal Head Flow). Use the head/ flow graph or
chart provided by the PMD fabricator (or create one based on the PMD or channel
dimensions).
The SITRANS LUT400 supports Universal curved flow calculation. This calculation creates a
cubic spline approximation of the head/flow curve, providing best results if the curve is non-
linear, and there are no sharp angles.
Select the head versus flow breakpoints to be entered (32 max). Generally, the more
breakpoints entered, the greater the flow calculation accuracy.
Select at least enough breakpoints from the curve to satisfy the following:
• two breakpoints very near the minimum head
• one breakpoint at the tangent points of each arc
• one breakpoint at each arc apex
• two breakpoints very near the maximum head
For combination curves, enter at least 2 breakpoints immediately before and after any sharp
angle (as well as 1 breakpoint exactly at the angle) on the curve.
A.7.1

Method of Flow Calculation

The SITRANS LUT400 can be programmed to use either of two methods for calculating flow
from the head measurement: absolute or ratiometric. The result is the same regardless of the
method used. The main difference is the information that must be entered in order for the
device to carry out the calculation. Refer to Primary Measuring Device (PMD) (2.15.1.)
(Page 234), and PMD Dimensions (2.15.4.) (Page 239) for list of information required.
For the ratiometric method, it is usually sufficient that the user knows the flow rate (Qcal)
which occurs at maximum head (hcal).
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Operating Instructions, 04/2021, A5E33329501-AG
SITRANS LUT400