HyQuest Solutions Pty Ltd
Point Velocity Display - Model PVD100
©
Copyright
40
Issue 1.00 4 Apr, 2007
CAT
62
57
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Appendix B Measuring Meter Dwell
The meter dwell measurement function performs a measure of the contact time in relation to a
single rotation of the meter. For the cat whisker contact, it is the measure of the cat whisker
contact time on the cam lobe. The resulting measurement is called “dwell”. Dwell is
expressed in terms of degrees and for a cat whisker contact; the contact time should be
adjusted to be the width of a single cup. This adjustment results in the dwell angle of 60º for
a meter with 6 cups and represents optimum performance. The magnetic contact meter (Mag
Head) requires no adjustment and therefore measuring the meter dwell angle does not apply
but can be performed. When a meter dwell measurement is performed, the
PVD100
takes
three measurements and displays the result on the LCD after each measurement, as shown.
The “Dwell Angle” can be computed by using the following simple formula:
Dwell Angle = (Contact time) x 360
(Total Time)
The contact time and total time shown above in the formula are expressed in terms of the
number of samples measured at a sample rate of 1200 Hz as it is measured by the
PVD100
.
A high sample rate is used to reflect a more accurate measure of the contact time and total
time measured. The repeatability and variability in measuring the dwell is directly related to
mechanical factors such as the condition of the pivot bearings, and the cat whisker wire
contact surface and cam lobe. Wide variations in readings reflect conditions that may require
further inspection of the meter and its mechanical parts. Some variation in readings is
acceptable and represents proper adjustment; however there should be no more than a 10%
difference between all the readings. Optimal setting is near 60º (a single meter cup width).
High conductivity of >25,000 micro Siemens can affect the operating performance of the
PVD100
and should a current meter be required to operate in high conductivity water, a
lower dwell angle setting should be used. A setting of not less than 20º for higher velocities
(>0.65 m/sec or 2 ft/sec) is suggested.
