Siemens OCM III, Instruction Manual, Page: 28 / 112

FLOW CALCULATION
Absolute vs. ratiometric
The OCM-3 can be programmed to use either of two methods (P4) for calculating flow
from the head measurement: absolute or ratiometric. The result is the same
regardless of the method used. The principal difference is the information that must
be entered in order for the OCM-3 to carry out the calculation. The user’s choice of
method may ultimately be based upon the information which is at hand. Refer to U
parameters for the primary element selected for a listing of the information required.
For the ratiometric method, it is usually sufficient that the user know the flow rate
(Q
cal
) which occurs at maximum head (h
cal
).
On the other hand, absolute calculations require that the user enter information such
as: the physical dimensions of the primary element and the constant relating to units
of measure for both linear dimensions and flow rates.
e.g.
the general formula for flow through a single exponent primary element is:
Q = KH
x
the specific formula for flow through a 45° V-notch weir is:
cfs = 1.03H
2.5
thus: Q = flow in cubic feet per second
K = constant of 1.03
H = head in feet
The absolute method is not applicable to the following:
Palmer Bowlus flume
H flume
DISPLAY
The normal display during operation is the Flow and Total Display (F2).
status field
flow rate field
totalizer field
relay / "no echo" field
7ML19985AB01 OCM III 28