Flow meter is capable of measuring quantity of fluid flowing in a circular pipe.
Classification of Flow Meters
Flow measuring meters are classified as,
- Rate meters
- Quantity meters
- Special meters
Rate meters (Inferential)
The term “Inferential” is applied to those flow-meters, which do not measure the flow directly. Such type of flow meters measure other quantities, such as, pressure, temperature etc., which are related to flow. Therefore, they are called as “Inferential type flow meters”. Primary sensing element of inferential type flow meters gives an indication, which is converted into velocity of flow. This velocity of flow (in conjunction with cross-sectional area “A” of the flow meter) gives the discharge (volume flow rate) by using the relation,
Q = A × v.
Rate meters are further classified as,
- Constant Area Variable Head Meters such as Venturimeter, Flow nozzle, orifice meter, Pitot tube etc.
- Constant Head Variable Area Meters such as Rotameter, Cylinder and Piston arrangement.
- Variable Velocity Meters such as Hot wire anemometers, Current meters.
Quantity meters (Positive displacement meters)
In this method, primary sensing element of meter divides the flow of fluid into isolated quantities. Then, each quantity is separately measured either by weight or by volume. Secondary element of meter is called as counter, which acts as a totalizer. Instruments belonging to this category are called as positive displacement meters.
Examples:
- Coriolis Flow Meter,
- Oscillating Piston Flow Meter,
- Rotating Vane Flow Meter etc.
Special meters
- Ultrasonic Flow Meter,
- Electromagnetic Flow Meter,
- Turbine Meter.
FACTORS TO BE CONSIDERED WHILE SELECTING A FLOWMETER
- Measuring range.
- Chemical compatibility.
- Accuracy requirements.
- pressure requirements.
- Acceptable pressure drop.
- Calibration facilities.
- Desired measurement units (such as volume flow rate, velocity or mass flow rate).
- Fluid viscosity imitations.
- Quality of fluid.
- Properties of the flowing fluid.
- Cost consideration.
- Installation.
- Accessibility and ease of maintenance.
Names of Flow Measuring Devices Most Suitable for different types of Flow
| Types of flow/fluid | Device used |
| Flow of exceptionally clean fluid only | Orifice meter |
| Flow of fluid containing solid matter | Ultrasonic flow meter |
| Water flow in river | Ultrasonic flow meter |
| Ocean current | Ultrasonic flow meter |
| Fluid having rapidly fluctuating velocity | Ultrasonic flow meter |