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Specifications for the air conditioning of proving rooms should result in a room of substantially constant temperature. This generally results in a modulated system where both warm and cool air are supplied simultaneously to the room-being governed by very sensitive controls. The duct work. should be designed as to evenly distribute the conditioned air to all parts of the room-the location of both the supply ducts and return ducts should consequently be the result of a detailed study. Sometimes a false ceiling of perforated acoustic tile forms an inlet plenum chamber from which the whole room is bathed with the falling blanket of tempered air.

During conditions of satisfactory operation the air temperature within the prover bell should be within 0.5 °F of the ambient temperature. Temperatures at any one point in the room should not vary from a simultaneous temperature at any other point in the proving room proper by more than 1 °F. This usually requires multiple intake and multiple discharge grilles to prevent stratification and the existence of dead zones. The proving room should be under a slight positive pressure to prevent the inrush of untempered air when doors are opened. The circulation should correspond to substantially constant air velocities throughout the room and produce a complete air change every 10 minutes or less.

If an air supply is required for provers this would generally be obtained from a blower system or from a compressed air line. Care should be exercised that the temperature of the air supply to the prover is not different from proving room temperatures. The air should be thoroughly tempered, preferably, at a lower pressure to proving room temperature. Drawing air from the prover room does not insure that the air will be at prover room temperatures.

5.2. Test Preparation

To be assured that the meter will remain as nearly as possible in the same condition as it was when in service, meter screw caps should be put on as soon as the meter is removed from the customer's premises and they should not be removed until the meter is to be acclimated for test. The caps should be replaced immediately upon completion of the

test.

Meters that have been in service are generally full of gas fumes and saturated with mercaptan odor. Provision should be made for exhausting from the room fumes and odors expelled during the test.

Meters received from service should be stored in the room in which they will be tested for a period of at least 16 hours with caps removed to acquire the temperature of the surrounding atmosphere. Meters before being tested should be kept off the floor and distributed so as to allow to provide good air circula

tion and temperature uniformity. They should not be subjected to any sudden changes in temperature, or to any influence tending to raise or lower their temperatures above or below that of the air in the immediate vicinity of the prover.

Meter testing may be performed with the same relative degree of accuracy, at any permissible working temperature between 60 °F and 90 °F, but it is essential that the meter, the air in the prover, and the sealing oil in the prover tank be maintained at the same temperature (within 2 °F) during the test [1].

Separate thermometers should be used for determining the temperatures of the air and of the sealing oil. These thermometers should be checked against each other routinely to determine that their readings do not differ more than one degree through a range from 60 °F to 100 °F.

Adjust the prover to supply air at 1.5 in WC [3] for all tests. The pressure drop across the meter during tests should not exceed an average of 0.5 in WC [1].

The inlet of the meter is attached to the prover hose by means of a suitable connection. Open the valve on the prover connection, place the palm of the hand over the meter outlet and then close the valve. Watch for a drop in pressure in the U-gage connected to the prover connection. If there is any drop, it is an indication of a leak in the connections or in the meter. The leak should be located and eliminated. Soap suds or a commercial liquid leak detector may be used to locate leaks. If the leak is found in the meter, it should be rejected with no further tests made on the device.

The prover valve, hose, and hose connections should be sufficiently large to permit the testing of large meters at the maximum rate at which the gas passes through the meters when in service. The loss of pressure between the bell of the prover and the meter should not exceed 0.5 in WC [3] (do not confuse this drop in pressure with the pressure drop across the meter).

A differential pressure gage connected across the inlet and outlet fittings of the meter under test may be used for measuring the pressure drop across the meter. Connections to the meter under test and the differential pressure gage should be made with adapter fittings as illustrated in figure 7. The adapter fittings [2, 10] should have an inside diameter (d) equal to that of the meter pipe connections within a tolerance of 1/32 in. The length of the adapters should be at least eight times their inside diameter (d). The inside surface should be smooth and free from pits, scales, burrs, or other obstructions. The pressure taps should be located two diameters (d) from the meter inlet or outlet connections. The pressure tap holes should be 8 to 1/4 in inside diameter and bored perpendicular to the adapter wall. The intersection of the pressure tap hole and the inside wall of the adapter should be flush and free from burrs. No connection should be used

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which distorts the inside surface of the adapter fitting or projects into the flow passage.

Since the meters are tested with air as the test medium rather than the product they were designed to meter, the air capacity should be expressed in terms of gas capacity of the meter as follows:

Air Capacity =

Gas Capacity Specific Gravity of Gas

The flow rates used during the prover test should then be based upon the air capacity rate of the meter under test. The Specific Gravity of commercial propane is given as 1.53 and that of commercial butane as 2.00 [11].

The flow of prover air through the meter is controlled by a rate cap orifice plate on the discharge side of the meter. Thus, rate cap orifice plates should be available for all meters that are to be tested. Figure 8 is a cross-sectional view of a rate cap assembly [10]. The rate cap holder and orifice plates can be made from brass stock.

In testing meters removed on request or for high bill complaints, such meters should be tested in the condition "as received" from the customer's premises, and as soon after being removed as possible, giving due attention to securing temperature uniformity. When a number of consecutive tests are made upon a meter which has been in service for some time, in which air is used as the test medium, it is often noticed that the later tests indicate a lower proof than was indicated by the first tests. The difference between successive tests may be as much as several percent. The cause of

this may be the evaporation of condensates that might be inside of the meter, or, in the case of leather diaphragms, it might be from a stiffening of the leather upon the release of condensates which previously saturated the material. the material. As leather diaphragms lose these volatile oils they tend to become stiff, and the measuring chambers formed by them do not have the same capacity as before. For this reason the first test of such a meter with air probably gives results more closely duplicating the conditions of actual use.

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prover outlet valve. Position prover at desired starting point by using circular slide valve. Before making any tests, sufficient air from the prover should be passed through the meter to make at least one complete revolution of the test hand in the proving circle (Fig. 9). At the same time determine if a volume corresponding to that marked on the proving circle passes through the meter for one revolution of the test hand. If the readings do not correspond the index should be examined, as occasionally the wrong index is put in a meter. If the index or index gearing is not correct for the meter, the test should be stopped and the meter rejected. The purging should stop while the test hand is on the up movement, and at a division on the proving circle.

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Step 4. Open the prover outlet valve and pass a sufficient volume from the prover through the meter to cause the proving hand to make at least one full revolution. The test draft should not be less than 2 cu ft or 0.05 cu m. Exercise care to insure that the proving hand is stopped exactly over the mark from which it started at the beginning of the test. Note and record the reading on the prover scale. During the test, note and record the pressure drop across the meter and the temperature of the prover air. The temperature of the air, bell prover

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6. Test Report Form

A suggested Test Report Form is shown in figure 10. The form provides space in the upper portion for owner, customer (if any), and equipment identification. In the body of the form is space for recording the various test runs and for the calculations involved. At the bottom is space for indicating the official action taken as a result of the test, any remarks or instructions, the signature of the inspector, and the "acknowledgment" signature of equipment owner or operator. The upper portion of the report form should be filled in completely before testing of the device is initiated.

Since most of the meters in use at the present time are in cubic feet, the units were indicated in customary units. Where this situation does not prevail, the appropriate metric units should be employed.

7. Reporting a Test

The test data are entered in the appropriate space on the Report Form during and immediately following the test run.

In figure 11, the "Test Data" section of the sample Report Form has been filled out. The column entries are generally self explanatory. The direct prover reading is used in conjunction with the volume measured by the proving circle for calculating the error or proof of nontemperature compensated meters. Examples are as follows:

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