Diaphragm meters instrument which is used to measure the volume of gas that passes through it. This is accomplished through the known volume that is displaced for each stroke of the diaphragm. The diaphragm also provides the seal between the measuring chambers of the device. As such the diaphragm meter has proven to be an accurate and reliable means of measurement of gas for many years. This is especially true at low flow rates because of its positive displacement characteristics. This paper includes a brief history of diaphragm meters, an explanation of the operation of the diaphragm meter, a basic review of the function and design of the positive displacement meter, discusses meter ratings and capacity, and introduces temperature compensation.
A diaphragm meter is physically composed of:
1.) A body to contain the gas pressure and form part of the compartments that measure the gas,
2.) Diaphragms that move as gas pressure fluctuates on either side,
3.) Valve covers and seats that control the flow of gas into each side of the diaphragm,
4.) Linkage to connect the diaphragm with the valves and index, and finally
5.) The index which registers the number of revolutions of the entire mechanism.
A diaphragm meter can be compared to a two-piston double-action engine in which the diaphragms correspond to pistons and the meter body to the cylinders. Each stroke of the diaphragm displaces a fixed volume of gas and the diaphragms operate 900 out of phase so that when one is fully stroked, the other is at mid-stroke This provides a smooth flow of gas to the meter outlet and insures the meter will always start regardless of its static position. When a demand for gas is made on the downstream side of the meter, a pressure drop is created across the meter and its diaphragms. This differential, which amounts to 0.1" W.C., provides the force to drive the meter.
Above each diaphragm is a "D" shaped valve .Under the valve are three port openings that direct the flow of gas in and out of the case and diaphragm compartments. As the diaphragm expands, it forces the gas in the case compartment up through the case port. The valve directs the flow of gas into the center port that leads to the meter outlet. A similar process occurs when
the diaphragm contracts. The stroke of the diaphragm is controlled by linkage in the upper port of the meter and a rod (flag rod) that extends down into the diaphragm compartment. The tangent link, as it is called, is attacked to the top of the meter crank and is adjustable in length.
Increasing the tangent length increases the diaphragms stroke which increases the meter proof and vice versa
The crank makes a certain number of turns per cubic foot and transmits this motion to the front counter (index) by means of an axle shaft driven by a worn and wheel. The crank also drives the sliding valves, which are timed to the motion of the diaphragm.
Diaphragm meters should always be shipped, stored, and installed in an upright position. Dust caps on the inlet and outlet connections should be left in place until the meter is installed. Caution should be used with meters that have been removed from service since they may contain gas within the diaphragm chambers.
The meter set should be in a location that is ventilated and readily accessible for examination, reading, replacement,or maintenance. The set should be protected from outside damage and at least three feet from known sources of ignition or air intakes. Electrical isolation for cathodic protection purposes should be maintained.
A diaphragm meter should be installed as close to level as possible. Tests have indicated that tilting a meter will affect its accuracy since, at high angles, the valves tend to come off the valve seat and let the gas bypass the diaphragm chambers. Therefore, the inlet and outlet connections should be within + 1/4" of each other.
The meter should be installed in a manner to avoid undue stress on the connecting piping or meter. The use of a meter bar may be a consideration. If there is foreign material such as sand, rust scale, or welding beads in the gas supply; then a filter or strainer should be provided on the inlet side. The meter should not be installed at the low point since it may act as a trap for liquids. By-pass piping or some other type of testing components, pressure taps, and over-speed protection should be considered.
Avoid having the meter body come in direct contact with soil or concrete walls since alkali in concrete can cause premature corrosion. Under no circumstances should the meter be buried.
Hand-tighten the swivels first and then tighten with a wrench approximately three flats (approximately 20 ft-lb).
Do not over tighten since damage to the rubber gasket inside the swivel cap may occur.
Check all connections for leaks.
Slowly pressurize the system.
MAINTENANCE
The diaphragm meter really does not require much maintenance other than a periodic proof test. Factors that affect meter accuracy include:
2. Maintaining Constant Diaphragm Displacement. A precise and stable diaphragm displacement is
required for each stroke of the meter. Therefore, the effective cross-sectional area of the diaphragm and the diaphragm stroke must remain constant.
3. External Leaks. Any opening, such as cover gaskets, index seal box, or meter connections that
lets gas escape will affect its accuracy.
4. Internal leaks will cause the meter to run slow and are usually found in areas such as the diaphragm
assembly, valves, or flag rod seals.
