Flow Measurement (Mechanical Transducers):
The measurement of flow rate and quantity is the oldest of all measurements of process variables in the field of instrumentation. It is used to determine the amount of materials flowing in or out of a process. Without flow measurement, plant material balancing, quality control and the operation of any continuous process would be impossible. Flow velocities are also measured by inductive transducers.
The measurement of liquids containing suspended solids, such as sewage or feed to paper mills, present considerable problems. This is overcomed by the use of a flowmeter.
The transducer can be used to measure the flow of any flowing material that is electrically conductive. (The meter can be regarded as a section of pipe that is lined with an insulating material.)
Two saddle coils are arranged opposite each other and electrodes diametrically opposite are arranged flush with the inside of the lining. If the coils are energized, the moving liquid, (as a length of conductor) cuts the lines of force, resulting in the generation of an electromotive force that is picked up by the electrodes. By suitable circuitry and amplification, an electrical signal proportional to the flow can be obtained.
Many accurate and reliable methods are available for measuring flow, some of which are applicable only to liquids, some only to gases and some others to both. Fluids measured may be clear or opaque; clean or dirty, wet or dry, erosive or corrosive. Fluid streams may be multiphase, vapour, liquid or slurries. The flow may be turbulent or laminar, and viscosity and pressure may vary from vacuum to many atmospheres. Temperature may range from cryogenic to hundreds of °C.
Flow rate may vary from a few drops per hour to thousands of gallons per minute.
Mechanical Flow Meter:
In mechanical flow meters, there is a mechanism in the path of the flow which moves continuously at a speed which is proportional to the flow rate. These are generally used for metering liquids however with certain modifications, they can be designed to meter gases also. They can be divided into two main categories, with further subgroups.
- Displacement type
- Inferential type
The displacement type are volumetric in operation, the cyclic displacement of the detecting element, e.g. piston, being directly proportional to the volume of the fluid passing through the meter during each cycle.