What is the most accurate gas meter?

What is Flow Meter Accuracy? - KOBOLD USA

Choose the correct flow meter that will meet all the exact needs of your application profile.

There are many elements of an application that can affect whether or not a flow meter delivers the factory-stated accuracy. For example, if you choose a flow meter that requires full pipes and no bubbles to operate correctly but you run the pipe half full with bubbles and foam, it will not deliver the intended accuracy. It may not even work at all. Running flows much lower than the stated minimum flow range for the meter can also impair accuracy or cause the meter to fail entirely. To ensure full pipes for correct operation, install the flow meter vertically, with the flow running upwards.

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Install the flow meter correctly.

Certain flow meters require a uniform, non-turbulent flow profile in the pipe. Failing to accommodate these needs can significantly reduce accuracy. For example, some flow meters require a straight, uninterrupted pipeline with no impediments, bends, or valves for a specified distance before and after the flow meter. Not adhering to these requirements will reduce accuracy as the meter cannot function properly under those flow conditions.

Make sure nothing is broken.

For flow meters that measure by mechanical means, if accuracy begins to suffer, verify that the functional elements of the flow meter have not been compromised. Some meters are simple enough to be easily repaired by the end user, while others must be sent back to the factory for repair.

Calibrate your flow meters in line with manufacturer recommendations.

Certain flow meter technologies require more frequent calibration than others, while some may not require any calibration at all during their service life. Make sure you are aware of the calibration needs of your meter and adhere to the maintenance schedule. Some meters can be calibrated in the field, while others require removal from the system and must be sent to a company for calibration before being returned.

4 Gas Flow Meter Types for Measurement

Natural Gas Flow Meter Types

There are four natural gas meter types often used for flow measurement: mass flow meters, velocity flow meters, differential pressure meters, and PD meters.

What is a flow meter?

A flow meter is a precision instrument that measures a pipe’s gas flow rate (or liquid flow rate). While there are four main meter styles for flow measurement, here are three characteristics to determine flow:

1. Positive displacement meters collect a fixed fluid volume, then release and refill the gas or liquid. They tally how many times the capacity fills to determine the flow.

2. When measuring the rate of fluid over a known area, one can determine the flow.

3. Other methods depend on the flowing stream’s forces as it overpowers a known constriction to indirectly calculate flow.

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Actual Flow vs Standard Flow

Since gas is compressible, accurate gas flow measurement is difficult. As the temperature increases, gas molecules move further apart. Conversely, as pressure increases, gas molecules move closer together.

Most gas flow meters (differential pressure, turbine, positive displacement, vortex shedding) measure the gas flow at the actual operating conditions. This flow rate is ACFM (actual cubic feet per minute). However, it is more important to adjust or correct the flow rate for a particular pressure and temperature. This adjusted flow rate is often called STP (standard pressure and temperature) and is usually in units of SCFM (standard cubic feet per minute).

For this reason, most gas flow meters require pressure and temperature correction to convert the flow rate from operating conditions (ACFM) to standard conditions (SCFM).

Flowmeter Styles

1. Direct Mass Flow

Mass flow meters determine mass flow passing through the meter. Two types deserve mention here:

Coriolis flow meters provide a direct mass flow measurement based on the fluid's deflection force moving through a vibrating tube. These meters are highly accurate with high turndown capabilities and are independent of fluid properties. They are also costly to purchase and install and unsuitable for larger pipe sizes.

Thermal mass flow meters measure mass flow based on heat transfer from a heated element. The gas molecules create the heat transfer; the greater the number of gas molecules in contact with the heated surface, the greater the heat transfer. This method depends only on the number of gas molecules and is independent of the gas pressure and temperature; therefore, additional pressure and temperature equipment are unnecessary. They provide excellent accuracy and repeatability and are easy to install.

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2. Velocity

In a velocity meter type, the rate of the medium passing through the meter determines the measurement.

Turbine flow meters measure volumetric flow based on fluid flowing past a free-spinning rotor, with each revolution corresponding to a specific volume of gas or liquid. The meters have high turndown and accuracy. However, due to the meter’s moving parts, its use is limited to clean, dry gases in gas applications, and pressure and temperature compensation are required.

Ultrasonic flow meters measure the difference in pulse transit times that travel from a downstream transducer to the upstream sensor, compared to the upstream transducer back to the downstream transducer. This meter style is highly accurate but very expensive, and pressure and temperature measurements are required.

The vortex gas flow meter has a shedder bar (an obstruction) in the flow path, causing the fluid to flow around the shedder bar and creating vortices on its backside. The frequency of vortex generation is a function of gas velocity. Fluid velocity is determined based on the principle known as the Kármán effect. The frequency of vortex shedding is independent of the fluid composition. The meter requires temperature and pressure compensation and a minimum flow rate to produce vortices.

3. Differential Pressure Meters

Differential pressure flow meters calculate flow by measuring pressure drop over an obstruction inserted in the flow path. Common types of flow elements are orifice plates, flow nozzles, venturi tubes, and averaging pitot tubes.

The orifice plate is a differential pressure meter frequently used for natural gas measurement. It measures volumetric flow, not mass flow. This meter’s limitations include reduced low flow sensitivity, limited turndown, and a pressure drop that impacts operating costs. Additionally, it requires temperature and pressure correction to achieve mass flow since it's a volumetric flow meter.

An averaging pitot tube is a differential-pressure flow measurement device commonly used for combustion air measurement. The device has limitations with measuring gas flow, especially low-flow sensitivity and turndown. The measure depends upon achieving velocity pressure, and if the flow is too low, the user may not obtain adequate signals.

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4. Positive Displacement Gas Flow Meter

Positive displacement meters require fluid to displace components mechanically and measure volumetric flow at the operating temperature and pressure. While they have sufficient accuracy, pressure and temperature compensation are needed to achieve mass flow, and since they have moving parts, gas cleanliness is a consideration. A PD meter may be called a PD flow meter or a volumetric flow meter. An example of a PD meter is the diaphragm meter.

Why is measuring mass flow important?

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