PD6928 Ex-Proof & I.S. Loop-Powered Flow Rate/Totalizer
Instruction Manual
57
Programmable Exponent Linearization (
EXPONEN
)
The programmable exponent can be used to linearize the signal from level transmitters in open-channel flow
applications using weirs and flumes.
The PD6928, 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 PD6928 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
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
Set upper display for flow rate display and lower display
units/tag for MGD.
