Gas meter
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A gas meter is used to measure the flow of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consumes fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly affected by temperature and pressure. Gas meters measure a defined volume, regardless of the pressurized quanity or quality of the gas flowing through the meter. Temperature, pressure and heating value compensation must be made to measure actual amount and value of gas moving through a meter.
Three different designs of gas meters are in common use, depending on the total volume of gas to be measured.
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[edit] Diaphragm/bellows meters
These are the most common type of gas meter, seen in almost all residential and small commercial installations. Within the meter there are two or more chambers formed by movable diaphragms. With the gas flow directed by internal valves, the chambers alternatively fill and expell gas, producing a near continous flow through the meter. As the diaphragms expand and contract, levers connected to cranks convert the linear motion of the diaphragms into rotary motion which then drives the counter mechanism.
Diaphragm gas meters are positive displacement meters.
Major manufactures of diaphragm meters in the US include Actaris, American Meter, and Sensus.
[edit] Rotary meters
Rotary meters are highly machined precision instruments capable of handling higher volumes and pressures than diaphragm meters. Within the meter, two figure "8" shaped lobes, the rotors, spin in precise alignment. With each turn, they move a specific quantity of gas through the meter. The operating principle is similar to that of a Roots blower. This rotational movement drives the counter mechanism.
Major manufactures include Instromet, Dresser, and Romet.
[edit] Turbine meters
Turbine gas meters infer gas volume by determining the speed of the gas moving through the meter. A small internal turbine measures the speed of the gas, which is transmitted mechanically to a mechanical or electronic counter. These meters do not impede the flow of gas, but are limited at lower flow rates. Due to this constraint, these meters typically used in large industrial situations and power plants.
[edit] Heating value
The volume of gas flow provide by a gas meter is just that, a reading of volume. Gas volume does not take into account the quality of the gas, the amount of heat available when burned. Utility customers are billed according to the heat available in the gas. The quality of the gas is measured and adjust for in each billing cycle. This is known by several names as the calorific value, heating value, or therm 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. Pressure compensation then occurs in the utility's billing system. Varying temperature cannot be handled as easily, but some meters are designed with built-in temperature compensation to keep them reasonably accurate over their designed temperature range. Others are corrected for temperature electronically.
[edit] 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 (pointer style) or digital readouts similar to an odometer, but remote readouts of various types are also becoming popular.
[edit] Remote Readouts
Remote reading is becoming popular for gas meters. It is often done through and electronic pulse output mounted on the meter. There are different styles available but most common is a contact closure switch.
[edit] Clamp-on gas flow meters
Another type of gas flow meter is the clamp-on gas flow meter which can be used to measure gas in any size of pipe without modification. Such devices are based on two types of technology: (1) time of flight or transit time; and (2) cross correlation. Both technologies involve transducers that are simply clamped on to the pipe and programmed with the pipe size and schedule and can be used to calculate mass flow. Such meters can be used to measure almost any dry gas including natural gas, nitrogen, compressed air and also steam.
