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Wetting the Prover
The calibrated accuracy of a prover is determined on its wet measure capacity by its
manufacturer, so the prover must be wetted prior to actual testing.
To wet the prover, follow this procedure. Reset the meter register to zero, and fill the prover to
the zero or 100% marking of the scale. Disregard the meter reading. Drain the prover, and reset
the register. The first meter reading is disregarded as there will be a slight difference between a
wet and a dry prover fill.
After the prover has drained, allow the tank to drain for a set time. Thirty seconds is a typical
drain time, with the count beginning as soon as the fluid empties or is dumped from the prover.
It is important that whatever amount of time is selected, that this same interval be used for all
tests to ensure repeatable test conditions. If a considerable period of time is to elapse between
tests and the prover is emptied, then the prover must be re-wetted prior to subsequent testing.
The re-wetting operation can be eliminated by allowing the prover to remain full until the next
test is to be run.
Making the Tests
Once the prover is wetted, the accuracy tests can begin according to this procedure. Reset the
register to zero, and run the required tests through the meter. Typically, a flow meter will be run
at several flow rates. Do not exceed the maximum recommended rate of flow for the meter. The
difference between the fluid volume as measured in the prover is compared to the reading of the
register on the meter. An error caculation is made, expressed as a per cent. The per cent errors
at each flow rate are compared to arrive at an overall error percentage. The overall error must
fall within certain parameters. In addition, multiple tests at any given flow rate must be
repeatable within a certain percentage.
Accuracy test and repeatability requirements vary from jurisdiction to jurisdiction. Constult your
local weights and measures officials for guidance on the specific requirements for the area in
which the installation is located.
Determining Test Results
Run the meter to deliver a volume of product that corresponds to the prover capacity. Read the
volume of product on the calibrated plate on the neck of the prover. This reading will typically
be in cubic inches. If so, the percentage error can be readily computed with the following
information:
(a) One gallon equals 231cu.in.
(b) A 100-gallon prover holds 23,100cu.in. Thus, 23.1cu.in. represents 0.1%
error.