PD6600 Series Loop-Powered Process Meters
Instruction Manual
39
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 PD6602, in combination with an ultrasonic level
transmitter, makes for an economical way to measure
and display open channel flow rate 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 PD6602 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
Display
Display Flow
Rate
Set upper display for flow
rate display and lower display
units/tag for MGD.
To display both flow rate and total, consider Precision
Digital’s
Model PD6622 Flow Rate Totalizer
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
FUNCT
EXPONENT
1.5000
EXPONENT
PV
SCALE.CAL
SCALE
pV
TIME
/SECOND
UNIT
GAL/S
