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Manual Zero Set Method
The zero point can be manually entered with the manual zero set method. This method
should be used infrequently. Manual zero set applies a constant offset entered by the user
to all readings.
Table F-2 illustrates an example of a manual zero offset of 10 gallons per minute:
Table F-2
Example of Manual Zero Offset
Uncalibrated Flow Reading
(before manual zero offset)
Manual Zero Offset Calibrated Flow Reading
(after manual zero offset)
250 GPM 10 GPM 240 GPM
To zero the instrument with the manual zero set method:
1. Follow the procedures outlined in Preliminary Steps, above.
2. Minimize the flow occurring in the pipe.
3. From the
FLOW
options group (Section F.4.6), ensure that the minimum flow range
setting is the negative equivalent of the maximum flow rate setting. (For example, if the
maximum flow range setting is 100 gallons per minute, ensure that the minimum flow
range setting is -100 gallons per minute.)
4. From the
FLOW
options group, set the damping so that the flowmeter reads a steady
flow.
5. From the
CALIBRATION
options group, enter 0 (zero) in the text box to the right of the
ZERO FLOW
button.
6. Click on the
SEND
button to send the data to the flowmeter.
7. Allow the flowmeter to settle for ten minutes.
8. Take ten separate flow readings and determine their average.
This average is designated as the variable P (for positive).
9. Disconnect the transducer wiring connections at the flowmeter and swap the upstream
wires with the downstream wires.
The flowmeter should begin to display a negative flow reading.
10. Allow the flowmeter to settle for ten minutes.
11. Take another ten flow readings and determine their average.
This new value is designated as the variable N (for negative).
12. Determine the manual zero point (Zp) by calculating the following formula:
Zp = (P+N)
2
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