Flow Meters

A flow meter is an instrument used to measure linear, nonlinear, mass or volumetric flow rate of a liquid or a gas. Some flow meters measure flow as the amount of fluid passing through the flowmeter during a time period (such as 100 liters per minute). Other flowmeters measure the totalized amount of fluid that has passed through the flow meter. Selecting a Flow Meter The basis of good flowmeter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation.

Flowmeters consist of a primary device, transducer and transmitter. The transducer senses the fluid that passes through the primary device. The transmitter produces a usable flow signal from the raw transducer signal. These components are often combined, so the actual flow meter may be one or more physical devices. 

Type of flow meter is used for closed pipe system In general, can be classified as differential pressure, positive displacement, velocity, and mass meters. Differential pressure devices (also known as head meters) include orifice, ventury tubes, flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target meters, and variable-area meter

There are several types of positive displacement flow meters are used include piston, oval-gear, nutating-disk, and rotary-vane type. While the Velocity type flow meter consists of a turbine, vortex shedding, electromagnetic, and ultrasonic. Flow meter is used to measure the Coriolis mass flow meters and thermal types. Measurement of liquid flow in open channels generally involves weirs and aqueducts.

Things to note the installation of flow meters for liquid application ensure that the flow meter is installed in such a way that the fluid in the flow meter must be full, because the steam / gas flowmeter can alter the geometry and affect accuracy. Flow meters are used to measure gas / vapor, must make sure that the flowmeter is installed such that the flowmeter remains full of gas / vapor, because liquid in the flowmeter can alter the geometry and affect accuracy. Therefore the flow meter should be installed in horizontal lines and vertical pipe flow with flow from bottom to top

There are some key questions which need to answered before selecting a flow meters :
•    What kind of the fluid being measured by the flowmeter or flow meters (fluid, gas, water, air, stem, etc)?

•    Is the fluid clean?

•    What is the minimum and maximum flow rate for the flow meter?
•    What is the minimum and maximum process pressure?
•    What is the minimum and maximum process temperature?
•    Do you require rate measurement and/or totalization from the flow meter?
•    If the liquid is not water, what viscosity is the liquid?
•    Do you require a local display on the flow meter or do you need an electronic signal output?
•    Is the fluid chemically compatible with the flowmeter wetted parts?
•    If this is a process application, what is the size of the pipe?