Total Control Systems 700-20 Engineering Manual page 18

After the prover has drained, allow a definitive time of a few minutes for drainage from the sides
of the tank. Total drainage time should be the same between all tests to insure inform results. If a
considerable length of time is to elapse between tests, the wetting operation can be eliminated by
allowing the prover to remain full until the next test is to be run.
Making the Tests
The setup is now ready for the accuracy test. Reset the register to zero, and run the required tests
through the meter. Do not exceed the maximum recommended rate of flow for the meter.
Maximum and minimum recommended rates of flow for various sizes and types of 700 series
meters are marked on their identification plates and maintenance manuals.
Determining Test Results
Run the meter to the mark on the dial corresponding to the prover capacity, and read the over or
under delivery in cubic inches or percent on the calibrated plate on the neck of the prover. If the
plate is calibrated in cubic inches, the percentage error can readily be computed on the following
basis:
(a) One gallon equals 231cu.in.
(b) A 100-gallon prover holds 23,100cu.in. Therefore, 23.1cu.in. represents 0.1% error.
The National Institute of Standards and Technology, in its NIST Handbook 44 specifies a
tolerance of plus or minus the following:
Indication of Device
Wholesale
Vehicle
Acceptance tolerances apply to new meters and repaired meters after reconditioning.
Repeatability
When multiple tests are conducted at approximately the same flow rate and draft size, the range
of test results for the flow rate shall not exceed 40% of the absolute value of the
maintenance/normal tolerance and the results of each test shall be within the applicable tolerance.
Changing Meter Calibration
Refer to meter literature for method of changing meter calibration. Any change in the meter
calibration adjustment will change the delivery in the same amount for all rates of flow. That is,
the calibration curve retains its shape, but is moved up or down. Therefore, if a meter tests
satisfactorily at full flow, but drops off too much at low flow, changing the calibration will not
remedy this condition; it will bring the low flow test to 100%, but it will also bring the full flow
above 100% by the same amount that the low flow was raised. A condition of this kind is caused
either by the metering system, need for meter cleaning or repairs, or because of an attempt to
retain accuracy below the minimum recommended rate of flow for the meter.
METER TOLERANCE
Acceptance Test
0.2%
0.15%
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Tolerance
Maintenance Test
0.3%
0.3%
Special Test
0.5%
0.45%