PD6700 Field-Mount Loop-Powered Process Meter
Instruction Manual
18
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 PD6700, 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 PD6700 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
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 PD6720 Flow Rate Totalizer
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
.
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.
