Can chart recorders be converted to a slower rotation without losing measurement accuracy? In orifice flow measurement, that question has been asked for decades since a slower clock rotation saves money by reducing the number of charts per month, thereby saving labor to change the charts, decreasing chart postage or delivery expenses, and reducing the work required to process the charts to produce a volume statement. If measurement accuracy will not be compromised by doing so, it may be economically prudent to make such changes, especially in light of the current oil & gas market.
Before making a change to slower chart rotation, how can you determine if measurement accuracy will be affected? Starting from a 7-day or 8-day rotation, note the current chart pattern. [A pattern is a temporary flow rate change that routinely occurs more than one or two times per week
.] If there is no clearly discernable pattern that has a period of less than four hours, then you can confidently change these to a 31-day rotation. If there is an identifiable pattern of between four hours and two hours, you can change to a 16-day rotation.
However, if the pattern is between two hours and one hour, you do not want to change the rotation unless the resulting band would be interpreted at the same root-average (consult with your measurement expert for this determination). If the pattern is already less than one hour in duration (and not really discernable), the effect on measurement has already been accounted for and the charts may be converted to a 31-day rotation.
Electronic gas measurement generally has a flow resolution of one second and always provides greater resolution than chart-based measurement. Chart patterns that primarily consist of flow in discrete, closed
(i.e., no daylight between up and down on the differential) "kicks" should not be converted. They already have high measurement uncertainty and consideration should be given to converting meters with this type of flow pattern to electronic measurement to properly quantify the magnitude and duration of the flowing periods.
Example charts that demonstrate the patterns described above are available by clicking here
"If it can’t be expressed in figures, it is not science; it is opinion."
~ Robert Heinlein, American science fiction writer