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A gas meter is used to measure the flow of Fuel Gas es such as Natural Gas and Propane . Gas meters are usually used at every residence and commercial building that consumes fuel gas supplied by a gas Utility . Three different designs of gas meters are in common use, depending on the total volume of gas to be measured. DIAPHRAGM METERS These are the most common type seen in most installations and almost all residential installations. Within the meter are two chambers formed by movable Diaphragm s. With the gas flow directed by Valve s, the two chambers fill from the gas source and empty into the gas load, alternating their roles so that gas delivery to the load is essentially continuous. As the diaphragms expand and contract, levers connected to Crank s convert the linear motion of the diaphragms into rotary motion which then drives the Counter mechanism. These are Positive Displacement meters. ROTARY METERS This is also a positive displacement meter but uses what is essentially a Roots Blower rather than a pair of diaphragms. In this way, the meter is capable of handling both higher volumes and much higher pressures than a diaphragm meter. To provide a leakproof seal, the indicating mechanism is sometimes Magnetically coupled to the measuring rotors through the pressure case of the meter. TURBINE METERS This type of meter uses a turbine contained within the gas pipe. This is not a positive displacement mechanism. These meters allow the highest gas flow, but they have the lowest ''rangability''. The positive displacement meters can accurately measure (at ±1%) gas flows over a very wide range from their maximum rated flow rate all the way down to 1/100 of that flow rate or less, turbine meters are only accurate down to between 1/12 and 1/33 of their maximum rating (depending on the Absolute Pressure of the gas). CALORIFIC VALUE The measurement provided by positive displacement meters is an absolute measurement of the gas volume flowing through the meter; the measurement provided by a turbine meter is an approximate measurement of gas volume. Unfortunately, gas volume (whether absolute or approximate) isn't the measurement actually desired by the gas utility. The measurement actually desired is the thermal energy contained in the fuel gas (which is proportional to the number of Molecule s of fuel gas delivered to the customer), this is known as the Calorific Value . The calorific value of Natural Gas can be obtained using a process gas Chromatograph , which measures the amount of each constituent of the gas, namely: Additionally, to convert from volume to thermal energy, the Pressure and Temperature of the gas must be taken into consideration. Pressure is generally not a problem; the meter is simply installed immediately downstream of a Pressure Regulator and is calibrated to read accurately at that pressure. Varying temperature can not be handled as easily, but some meters are designed with built-in temperature compensation to keep them reasonably accurate over their designed temperature range. INDICATING DEVICES All three types of gas meters can be obtained with a wide variety of indicators. The most common are indicators that use multiple clock hands or digital readouts similar to an Odometer , but remote readouts of various types are also becoming popular. |
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