Milltronics OCM II PL-269, Инструкция по эксплуатации

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EXAMPLE 3: VELOCITY-AREA PRODUCT
This example considers a trapezoidal irrigation channel in which flow is to be calculated by the product of the
cross section of flow with the average flow velocity. The velocity sensor provides 4- 20 mA output with 20 mA
representing a velocity of 400 cm/sec.
The velocity sensor output has been terminated by a 250 ohm resistor in order to suit the OCM II velocity input
which is 1- 5 volts.
NOTE : For velocity sensors with outputs other than 1- 5 V or 4 - 20 mA, refer to the information at the end of
this example to re-scale the OCM II velocity input.
The channel has a base width of 2 metres, and the sides are constructed with a slope of 1:1.
The liquid is 1.0 metre deep at maximum capacity. The liquid velocity is expected to be 85 cm/sec at
maximum capacity.
The transducer is mounted 70 cm (minimum 61 cm) above the highest liquid level.
The transducer to zero-flow level is thus 1.8 metres plus 70 cm which equals 250 cm.
The flow cross section at full capacity is given by:
A = h(b+mh) where h = head (liquid depth)
b = base width
m = side slope (horizontal : vertical)
(n.b.: m = 0 for a rectangular channel)
Hence the maximum area is 1.8(2 + (1 x 1.8)) = 6.84 square metres
The maximum flowrate is thus 6.84 square metres times 0.85 metres per second or 5.814 cubic metres per
second. This is the value to be entered as the calibration flowrate, U0, and is the value for which the OCM II
current output will be 20 mA if the current is to represent flowrate.
The maximum output from the velocity sensor, 20 mA, occurs when the velocity is 400 cm/sec. This is the sensor
calibration velocity, v
sensor
, which must be entered as parameter U3.
The maximum liquid velocity, 85 cm/sec, is entered as parameter U4, v
cal
. This is the velocity for which the OCM
II current output will be 20 mA if it is to represent the liquid velocity.
P0 = 0 Dimensions are in centimetres
P1 = 1 Temperature is Celsius
P2 = 8 Calculate flow by velocity-area method
U0 = 5.814 Cubic metres (per sec) for 20 mA output
U1 = 0 Flowrate time units (per sec)
U2 = 180 Full scale head in centimetres at 5.814 m
3
/S
U3 = 400 Full scale velocity sensor in cm per second
U4 = 85 Maximum liquid velocity in cm per second
U5 = 200 Channel base width in centimetres
U6 = 1 Channel side slope (0 horizontal : 1 vertical)
P3 = 0 Display head in run mode ((F2)
P4 = 250 *Distance from transducer to 0 flowrate head in cm.
P5 = 0 Calculate flowrate for all heads (no low head cut off)
P6 = 2 Fail safe low
P7 = 60 Fail safe after 60 seconds without an echo
P8 = 3 Alarm on low liquid head
P9 = 20 Alarm if liquid level below 20 centimetres
P10 = 5 Cancel alarm if level raises 5 centimetres above 20
PL-269 9 – 4