Loop Leader PD6620 Series Loop-Powered Flow Rate/Totalizers
Instruction Manual
31
Programmable Exponent
Linearization (
EXPONENT
)
The programmable exponent can be used to linearize the
signal from level transmitters in open-channel flow
applications using weirs and flumes.
The PD6622, in combination with an ultrasonic level
transmitter, makes for an economical way to measure and
display open channel flow rate and total in most weirs and
flumes. A guide such as the ISCO Open Channel Flow
Measurement Handbook can provide the user with all the
information needed: the exponent used in the flow equation
for the desired flow units and the flow rate for any given
head height.
For example, to display the open channel flow rate from a 3"
Parshall flume, the ISCO handbook advises the exponent is
1.547 and at the maximum head height of 3.0 feet, the flow
rate is 3.508 MGD.
3" Parshall Flume Discharge Table
Formula:
CFS
= 0.9920
H
1.547
GPM
= 445.2
H
1.547
MGD
= 0.6411
H
1.547
Where:
H
= Head in feet
Table 12-3
Head (Feet)
CFS
GPM
MGD
3.00
5.428
2436
3.508
ISCO Open Channel Flow Measurement Handbook, 3
rd
edition
With this information the PD6622 should be programmed in
the following fashion. This setup assumes the level
transmitter is programmed to output 20 mA at the maximum
head height of 3.00 feet; but any mA value at a head height
with a known flow rate may be used.
Function
Desired
Programming
Open
Channel
Flow
3" Parshall flume Set Programmable Exponent to
1.547
Flow Rate
Millions of
Gallons per Day
(MGD)
Set 4 mA = 0
20 mA = 3.508
Time base = Day
Total
Millions of
Gallons
Set Totalizer Conversion Factor = 1
(password protect “total” to avoid
accidental reset)
Non-
Resettable
Grand Total
Program meter
so grand total
can never be
reset
Set non-resettable grand total
password
Display
Display Flow
Rate, Total, and
Grand Total
Set upper display for Grand Total
and lower display to toggle
between rate and total.
The Programmable Exponent function is programmed
in the following manner:
SCALE
UNITS
SCALE
INPUT 1
SCALE
DISP
1
UNITS
RATE
RATE
CUSTOM
/SECOND
/MINUTE
/HOUR
/DAY
Rate Time
Bases
GAL/
(T)
L/
(T)
IGAL/
(T)
M3/
(T)
BBL/
(T)
BUSH/
(T)
cuYD/
(T)
cuFt/
(T)
cuIn/
(T)
LiBBL/
(T)
BBBL/
(T)
HECtL/
(T)
AF/
(T)
CUSTOM/
Rate Units
INP 1
04.000
INP 1
mA
Flashes Units
Momentarily
DSP 1
+00,000.00
SCALE
INPUT 2
INP 2
20.000
SCALE
DISP
2
DSP 2
+00,100.00
SAVE?
SETUP
INPUT
RATE
EXPONENT
1.5000
EXPONENT
RATE
SCALE.CAL
SCALE
RATE
TIME
/SECOND
UNIT
GAL/S
