Gas Meters - PBworks

Gas Meters

Topics Covered: • Introduction to Gas Meters

• Bellows Gas Meter

• Operation of the Bellows Gas Meter • Meter Dials • Reading the Gas Meter • Clocking Burner Input • Test Meter and Gas Leaks • Temperature Compensated Meters • Classification of Meters by Capacity

Gas Meters

Topics Covered: • Rotary Displacement Gas Meters

• Operation of the Rotary Gas Meter • Reading the Rotary Gas Meter • Test Dial Arrangement for Rotary Meter

• Correction Factor For Meter Capacity at

Elevated Pressures

• Prohibited Meter Locations

Introduction to Gas Meters

• Gas meters are used to measure and record gas flow in both domestic and commercial applications.

• The operation of the gas meter is based on the principle of positive volumetric displacement of the gas at conditions existing in the gas line.


• There are basically two types of positive displacement gas meters the bellows or diaphragm meter, and the rotary meter.

• The Bellows or Diaphragm Meters are used for both domestic and commercial applications.

• The Rotary Meter is used only for medium and large commercial applications.


• The Bellows meter can be further divided into two categories, the tin case meters, and hard case meters.

• The tin case meter, because of its soft metal construction can only be installed inside the building.

• The tin case meter is being replaced by the hard case meter, which can be installed outside.


• Rotary meter installations which are used to supply commercial and industrial customers, usually have a by-pass arrangement.

• The by-pass will facilitate continuous gas supply to the customer during meter removal due to service or replacement.

Bellows Gas Meter Operation

• When any appliance are is turned in a gas system the pressure drops slightly in the gas outlet line from the meter to the appliance.

• The outlet line has gone from static to a working pressure, however gas pressure in the inlet line going to the meter stays high.

• This causes an unbalanced pressure in the meter, the unbalanced pressure pushes on a diaphragm.


• The unbalanced pressure on the bellows, makes the bellows expand like an accordion creating a gas flow through the meter.

• The flow of gas into and out of the separate chambers in the meter is controlled by sliding valves so that the gas output is smooth and steady.

• The meter dials track the number of times the meter chambers are filled and emptied.


• The volume of gas that passes through the meter is recorded, as long as a gas appliance is on there is an unbalanced pressure in the gas meter and the meter operates.

• There are three types of meter dials found on standard bellows or diaphragm types of meters.

• Consumptions Dials, Indicating Dials and Test Dials

Gas Meter Consumption Dials

• Consumption Dials record the gas consumption over a period of time for billing purposes.

• On small domestic meters there are four consumption dials rated at 1000 ft3; 10,000 ft3; 100,000 ft3; 1,000,000 ft3

• On the larger bellows meters used for commercial application, there are five dials. The fifth dial being 10,000,000 ft3.

Gas Meter Indicator Dials

• The indicating dial is a 100 ft3 dial and is only found on some of the larger commercial bellows type of gas meters.

• The indicating dials function is to give accuracy to the first consumption dial.

• It is not to be read for billing purposes.

Gas Meter Test Dials

• Test dials on bellows type of gas meters range from ½ ft3, 1 ft3, and 2 ft3 found on domestic gas meters.

• On the larger commercial meters there are 5ft3 and 10ft3 test dials.

• The test dial has two functions, first as a device to determine the appliance input. The second function is to test the gas line for leaks.

Reading the Gas Meter

• The gas meter keeps track of how much gas flows through it by counting the filling and emptying of the four compartments.

• Since each compartment fills with the same amount of gas each time, the measuring is very accurate.

• As one compartment is being emptied another is being filled, delivering a smooth flow of gas.


• Always read the meter from right to left, smaller dial to larger dial.

• The dials are divided into tenths, and dials that are next to each other rotate in opposite directions.

• To read the four consumption dials, take the numbers that the dial pointers have just passed and add two zeros.


• The accuracy of the dial that you are reading is determined by the dial reading of the next lower dial.

• Example the 10,000 ft3 dial pointer has just passed a reading of 1 indicating that approximately 1,000 ft3 of gas has been recorded.

• The 1,000 ft3 dial has also just passed a reading of 1 indicating 100 ft3 of gas has also been recorded.


• Combining the two consumption dial readings gives a true reading of 1,100 ft3 of metered gas.

• To determine how much gas the appliances have consumed, take the readings several days apart.

• Subtract the first reading from the second to find out how much gas was used during time between readings.

Direct Reading the Gas Meter

• At present the gas industry meters are designed to totalize the cubic feet of gas measured, and provide a continuous indication of the gas volume registered.

• Today the trend is towards a direct reading digital index, known as an odometer index.

• The circular consumption dials are replaced by a direct digital read out displayed on the index.

Function of the Test Dial

• The two main functions for the test dial are to determine the appliance input, and testing the downstream gas line for leaks.

• Determining the appliance input or what is called clocking the burner input.

• Testing the meter and downstream gas line for leaks is another function of the test dial.

Clocking the Burner Input

• The following is a method by which the test dial is used to determine the gas appliance input.

𝟑𝟑𝟑𝟑𝐓

X D = Q

• 3600 = the number of seconds in one hour • T = the number of seconds for one complete

revolution of the test dial. • D = the size of the test dial, i.e. 2cf or .05 m3 • Q = the gas flow rate in ft3/hr or m3/hr

Clocking the Burner Input

• Determining the appliance input by clocking the meter is done using the test dial of the meter.

• When clocking appliances make sure there is no other gas flowing other than the appliance being checked.

• Metric meters register gas consumption in cubic meters and metric appliances are rated in Kilowatts.

Meter and Gas Line for Leaks

• With the appliance or appliances turned off, turn on the gas meter and mark the position of the test dial hand.

• If the test dial hand has not moved from its position in 10 minutes, then the gas line can be assumed to be gas tight and leak free.

• If the test dial hand has moved from its mark, then a gas leak is indicated since gas was recorded

Temperature Compensated

• The preferred meter location is outside. All outside meters are temperature compensated as well as inside meters.

• Temperature variations between summer and winter would vary the gas flow through the meter.

• To overcome this inaccuracy in meter operation, temperature compensation is designed into the gas meter.

Temperature Compensated

• The temperature compensating mechanism consists of two bi-metallic elements.

• The bi-metallic elements are linked into the meter’s tangent arms.

• The tangent arms control the movement of the meter valves.

Classify Meters by Capacity

• Standard or imperial gas meters are classified as to capacity in ft3/hr or cfh, at a pressure difference across the meter of 0.5 “wc

• Metric gas meters are classified as to capacity in m3/hr, at a pressure difference across the meter of 0.12 kpa

Rotary Displacement Gas Meters

• Rotary displacement gas meters are designed to be rugged yet compact and light in weight.

• The smaller rotary meters are supported only by the connected piping, and may be mounted in vertical or horizontal pipe runs.

• Larger rotary meters are foot mounted having side connections only.

Operation of the Rotary Meter

• The rotary meter consists basically of two contra-rotating impellers of two-lobe or figure 8 contour.

• Impeller contours are of such form that a continuous seal without contact can be obtained between the impellers at all positions during rotation.

• A seal also exists between the tips of the impeller lobes and the two semi-circular parts of the case.

Operation of the Rotary Meter

• The gas at the inlet side of the meter is always effectively isolated from the gas at the outlet side by the impellers.

• The impellers can be caused to rotate by a very small pressure drop across the meter.

• As each impeller reaches a vertical position, it traps a known specific volume of gas between itself and the semi-circular portion of the meter.

Reading the Rotary Meter

• The counter section of a rotary meter contains a 7 digit counter; 5 of the digits are visible with the smallest visible digit as the 1000 ft3 index.

• The counter reading must be multiplied by 100 to obtain the total displacement volume measured by the meter.

• The normal pressure drop across rotary meters at rated maximum capacities is 1”wc (.25 kpa)

Test Dial Arrangement for Rotary Meter

• On some rotary meters, the 10 ft3 test dial is a series of empty frames located on a test wheel which is the first 7 wheels that are located on the counter.

• The test wheel is marked in ten divisions without numbers, each division or frame representing one ft3.

• One complete frame, as it moves from the bottom of the counter window to the top of the window.

Test Dial Arrangement for Rotary Meter

• For other rotary meters, the test dial is located on the counter’s end section of the meter.

• The test dial is a regular 10 ft3 test dial as you would find on a bellows type of gas meter.

• Also a meter rpm dial is located on the end section next to the 10 ft3 test dial.

Correction Factor for Meter Capacity at Elevated Pressures

• Gas meters are simple, but accurate devices which measure the number of cubic feet of gas passed.

• The absolute pressure in gas meters is equal to the barometric pressure of the atmosphere, plus the pressure in the service line itself.

• The pressure in the service line, houseline pressure, is usually 4 ounces, ¼ pound or 7”wc.


• This absolute pressure within the gas meter is called the base pressure and the meter’s capacity is calibrated at this pressure.

• As we already know the volume of a gas is inversely proportional to the absolute pressure, assuming a constant temperature.

• Thus any quantity of gas will change its volume as the pressure upon it is changed.


• Hence it is necessary to correct flow rates through a gas meter when the pressure of the gas varies from the base pressure.

• The following is a formula of meter capacity of flow rate correction when the gas pressure has changed from the base pressure.

Qa = 𝑸𝑸 (𝑨+𝑷)

(𝑷𝑷)


Qa = 𝑸𝑸 (𝑨+𝑷)

(𝑷𝑷)

• Qa = Actual flow rate in ft3/hr or m3/hr at base

pressure conditions • Qr = Flow rate in ft3/hr or m3/hr as recorded at

the gas meter • P = Actual gas pressure in psig or kpa gauge • Pb = Base pressure in psia 14.73 psia • A = Atmospheric pressure at the point of measure

Gas Meters - PBworks

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