Dwyer Series PUF - Manual (Page 25)

3: Operating Procedures
21
3.5.2 Adjusting the set zero flow offset
The PUF instrument operates by comparing the time taken to send an ultrasonic signal between two transducers
in either direction. A
Set zero flow offset adjustment is provided to compensate for any inherent differences
between the two sensors, noise pick-up, internal pipe conditions etc. It can be used to ‘zero’ the flow indication
under no-flow conditions.
1. Stop the liquid flow.
2. With the instrument in FLOW READING mode, press the Velocity function key and observe the reading
(m/s). Any reading other than 0.000 indicates an offset error and in practice this will typically be in the range
±0.005 m/s (0.016 ft/s) – possibly higher on smaller diameter pipes. If a greater figure is shown, it is worth
calibrating the offset to obtain a more accurate result. Continue as follows:
3.5.3 Adjusting the calibration factor
With the system running:
1. Stop ( Stall) the PUF’s totalizer facility and zero it (Paragraph 3.6.1).
2. Run the PUF’s totalizer to measure the total flow over a 30 to 60 minute period, and note the total flow
indicated by the reference flow meter over the same period.
3. Calculate the % error between the PUF and reference meters. If the error is greater than ±1% calibrate the
PUF as detailed below.
If you have adjusted the Zero Cutoff point to anywhere above ‘0’ you must reset it to
‘0’ before you can observe and adjust the Set zero flow offset, as its value is very
small. Once the Set zero flow offset has been calibrated you can then reapply the Zero
Cutoff if required.
3. Press the Options key to access the FLOW
READING OPTION screen shown.
4. Select Set zero flow (m/s) and press the
ENTER key.
5. Press ENTER on the subsequent screen to
accept the change, which will return you to the
screen shown.
6. Scroll down to select Exit and press the ENTER
key to return to the FLOW READING screen.
FLOW READING OPTION DD-MM-YY HH:MM:SS
Data review
Zero Cutoff (m/s) : 0.00
Set zero flow (m/s) : 0.00
Damping (secs) : 10
Totalizer : Run
Reset +Total
Key Point: In order to cancel any applied offset, you must either read flow via Quick
Start or switch the PUF instrument OFF & ON. Any value that you trim-out using the
offset adjustment will be added/subtracted from the flow reading across the whole range.
Key Point: USE THIS FACILITY WITH CARE & ONLY WHERE NECESSARY
The PUF instrument is fully calibrated before leaving the factory and under normal circum-
stances does not require further calibration when used on site.
This facility can be used to correct the flow indication where unavoidable errors occur due
to the lack of a straight pipe or where the sensors are forced to be fitted close to the pipe-
end, valve, junction etc.
Any adjustment must be made using a reference flowmeter fitted in the system.