A. General. The test of any meter or associated devices, or both, shall consist of a comparison of its accuracy with the accuracy of a working standard or may consist of other methods referenced in these regulations.
B. Test Equipment Capacity. The test equipment shall be of sufficient capacity and ranges to test meters or associated devices, or both, under the conditions specified in Regulation .02 of this chapter.
C. Accepted Good Practice. The Commission will use the applicable provisions of the publications listed in COMAR 20.55.02.02 as criteria of accepted good practice in the testing of meters.
D. Bell Provers. Bell provers may be used to test diaphragm, rotary, and turbine type meters. A prover that is used to test rotary and turbine type meters shall be properly automated and have a satisfactory readout. The test shall consist of passing a specific volume of air through the prover and the meter. The accuracy or proof is either read or calculated from the prover scale or the readout.
E. Working Standards Other Than Bell Provers. Approved working standards other than bell provers may be used to test diaphragm, rotary, and turbine type meters. The meter to be tested is connected to the approved working standard so that the same volume of air, either under pressure or vacuum, passes through both. The uncorrected accuracy is calculated or read directly from the readout on the working standard. Corrections shall be made for pressure and temperature differentials between the meter under test and the working standard to determine the corrected accuracy.
F. Rotary Displacement Meters. Rotary meters on which an original test record of the differential pressures is established as soon as practicable, but not longer than 6 months, after installation may subsequently be tested by comparing the new differential pressures with the original test record. This subsequent test may be in lieu of normal testing with a meter prover or other approved working standard. In establishing the original test record, the pressure differential shall be recorded at two or more load levels, with the minimum being no less than 10 percent of the rated capacity at the operating pressure of the meter. The meters shall be given a differential pressure test at least once every 5 years. When the test differential pressure differs from the original test record by more than 50 percent, the meter may not be used for measurement for revenue billing purposes until, by cleaning or repairing, the differential pressure is not more than 50 percent in excess of the original test record. If the differential pressure cannot be brought within the above limits by cleaning or repairing, the meter shall be removed from service and replaced with an accurate meter. The differential pressure test may not be used as a periodic test on rotary meters on which the results of a differential pressure test would not be conclusive, such as meters connected to loads which are less than 10 percent of the rated capacity of the meters.
G. Pressure Correcting Devices.
(1) The test of a pressure correcting device shall consist of driving the mechanism or by direct comparison of device measured pressure while the pressure element is actuated. A dead weight tester or regulated air or gas pressure may be used to actuate the pressure element. If regulated air or gas pressure is used, a pressure gauge having an accuracy of ±0.25 percent of full scale shall be used to indicate the pressure on the element.
(2) The accuracy is calculated by comparing the correction factor applied by the correcting device with the correction factor that should be applied for the pressure at which the test was made.
H. Temperature Correcting Devices.
(1) The test of a temperature correcting device shall consist of driving the mechanism or by direct comparison of device-measured temperature while the temperature sensor is immersed in a temperature bath containing a precision thermometer that has an accuracy of ±0.5°F.
(2) Specific, reasonably constant temperatures are applied to the temperature sensor. The temperatures used may be the operating (flowing gas) temperature, ambient temperature, and an ice bath (32°F). The device shall be tested at not less than 2 points, preferably with temperatures at least 20°F apart. The accuracy is calculated by comparing the correction factor applied by the correcting device with the correction factor that should be applied for the temperature at which the test was made.
I. Timing Devices. The test of a timing device shall consist of precisely synchronizing the timing device and the working standard at the start of the test time interval. At the end of the test time interval the working standard is stopped and the reading recorded. The accuracy is calculated by comparing the actual time elapsed with the test time interval.