PD6720 Field-Mount Loop-Powered Rate/Totalizer
Instruction Manual
21
Programmable Exponent
Linearization (
Prog.E
)
The programmable exponent can be used to linearize
the signal from level transmitters in open-channel flow
applications using weirs and flumes.
The PD6720, 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 PD6720 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 1.10
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.
Input Signal Filter (
FILTER
)
The noise filter is available for unusually noisy signals
that cause an unstable process variable display. The
noise filter averages the input signal over a certain
period. The filter level can be set to low (
LO
), medium
(
med
), high (
HI
), or off (
OFF
). The higher the filter
setting, the longer the averaging time and so the
longer the display may take to find its final value.
The filter contains a noise filter bypass feature so that
while small variations in the signal will be filtered out,
large, abrupt changes to the input signal are
displayed immediately.
Internal Calibration (
ICAL
)
There is
no need to recalibrate
the meter for
milliamps when first received from the factory.
The meter is
factory calibrated
for milliamps prior
to shipment. The calibration equipment is traceable
to NIST standards.
The internal calibration allows the user to scale the
meter without applying a signal. The use of a
calibrated signal source is necessary to perform the
internal calibration of the meter. Check calibration
of the meter at least every 12 months.
Notes:
The signal source must have a full-scale accuracy
of 0.002% or better between 4 and 20 mA in order to
maintain the specified accuracy of the meter.
Allow the meter to warm up for at least 15 minutes before
performing the internal calibration procedure.
The
Internal
calibration
menu is part of the
Advanced
features menu.
Press and hold the
MENU
button for 5 seconds to
enter the
Advanced
features menu. Press the
Up
arrow button to scroll to the
Internal Calibration
menu
(
ICAL
) and press
Enter
.
The meter displays
4.000
mA. Apply a 4.000 mA
signal and press
Enter
. The display flashes for a
moment while the meter is accepting the signal.
After the signal is accepted, the meter displays
20.000
mA. Apply a 20.000 mA signal and press
Enter
. The
display flashes for a moment while the meter is
accepting the signal.
Error Message (
SPAN
ERROR
)
An error message indicates that the calibration process
was not successful. After the error message is
displayed, the meter will revert to input 2 calibration
settings. The error message might be caused by
inadvertently leaving the signal at the previous level or
not maintaining the minimum span. Press the Menu
button to cancel the current calibration process if
necessary.
Information
(
INFO
)
The
Internal
calibration
menu is part of the
Advanced
features menu. It shows software identification
number and version number. To determine the
software version of a meter:
Go to the
Information
menu (
INFO
) and press
Enter
button.
Continue pressing
Enter
to scroll through the
software release number and software version.
Following the information display, the meter will exit
the
Advanced
features menu and return to run mode.
