Flow Sensor Liquid
A Closer Look At The Liquid Flow Meter
Measuring liquid flows has uncountable applications affecting all of our lives that are measured with a huge range of liquid flowmeter types, from the high-tech as used in the Processing Industry to more basic types used for land drainage and river flows. We will trip the light fantastic across the various types of liquid flow meters in common use today and give some technical insights to give you, the reader, a better idea of the best to use in a particular application.
Accuracy of flow is an important consideration when considering your application. The most demanding applications tend to be at the small end of the range where positive displacement metering flow meters are appropriate such as dosing a reactor vessel or to administer accurate dosage of a medication over time to a hospital patient. These diverse applications have one thing in common in that they work in association with processing equipment to deliver precise dosages.
Another factor to be considered for liquid flow measurement is whether or not the measurement of the flow will affect the flow itself and would that matter. Obstructions in the pipeline or restrictions formed by the flow meter, will affect the flow and this could be an issue for the process being monitored. It may be that the liquid flow contains solids that would quickly block the flow meter if used. It is therefore a maintenance issue.
There are many other factors that affect the specification of flow meter such as electrical conductivity, the presence of air or gas bubbles, laminar flow or turbulent flow, the pipeline restrictions for flow meter location (e.g. length of straight pipe upstream of the flowmeter location), the presence of stray magnetic fields, liquid temperatures and operating pressure and physical size and profile of the flow meter and, of course, the cost of the meter.
One of the most widely used flow meters in use today, mainly because of their accuracy and clear pipe bore profile is the Electromagnetic Flowmeter (or Magmeter or Magflow meter as they are known). The main restrictions on the use of this type of flow meter are the electrical conductivity of the fluid should be between 5 to 20 micro-siemens/cm minimum. This means that drinking water and all other water-based fluids, even slurries, as well as a whole range of process liquids with the notable exception of most petrochemicals that don't contain free ions.
The Vortex flowmeter and Coriolis flow meters come under the heading of meters with obstructions and these to some extent, will modify the flow that they seek to measure. The vortex flow meter uses a blunt object in the flow that causes eddies downstream in pipeline and these can be detected as vibrations occurring at a frequency proportional to the flow. The Coriolis flow meter works in a similar way except that the immersed pipe vibrates in proportion to the flow. These types of flow meter are most often found in the Processing Industries as they can work in extreme temperatures and pressures and are very accurate, so much so, that they are used to measure high-value liquids.
Ultrasonic flow meters are another form of no pipe restriction flow meter where they use the Doppler effect of ultrasonic pulses passing diagonally across the pipeline flow take differing times according to whether they are passing upstream or downstream.
There are a whole range of mechanical flow meters such as vane meters, turbine flow meters, ‘V', Trapezoidal and flat weirs (for open channel flow), orifice and venturi meters and many other types. All these and the preceding types use sensor analogue outputs to convert into digital outputs that can be used industry–wide by using well-recognised data buses.