What Is a Flow Sensor?
A flow sensor is a measuring instrument that measures the flow rate of a fluid, from gas to liquid.
It is sometimes treated as a flowmeter. Flow rate is the volume or weight of fluid that passes through one cross-section in a unit of time.
There are various types of flow sensors, including analog flow sensors that monitor the fluid flowing in a fluid circuit for abnormalities, and digital flow sensors that are connected to a feedback control mechanism to adjust the gas supply to a certain volume.
Uses of Flow Sensors
Flow sensors are used in the following fields:
1. Hydraulics, Pharmaceutical/Chemical, and Food Industries
Flow sensors are mainly used to control the input of raw materials in manufacturing processes. The volumetric flow sensor is used in these fields.
2. Measurement of Industrial Gases, Fuel Gases, and Exhaust Gases
Flow sensors are used to control the amount of industrial gases, fuel gases, and exhaust gases used in factories. Thermal mass flow meters are the most common type.
3. Drinking Water, Sauces, and Flammable/Explosive Fluids
Food-related and highly hazardous fluids require non-contact measurement that does not touch the fluid For this purpose, electromagnetic flow sensors are used.
4. Industrial Facilities and Water Treatment Facilities
Flow sensors are used for flow control in industrial plants and water treatment facilities. Ultrasonic flow sensors are used especially when adding flow sensors to existing pathways.
5. LNG Plants, Automotive, Chemical, and Pharmaceutical Industries
Flow sensors are used in a variety of manufacturing settings. Coriolis flow sensors are most commonly used.
6. Chemical and Pharmaceutical Industries
Kalman vortex flow sensors are widely used in plants that handle natural gas and water vapor.
Principle of Flow Sensors
Flow sensors employ a variety of measurement formats depending on the nature of the fluid to be measured and the purpose of the measurement. This section describes the principle of differential pressure flow sensors, which are applicable to common fluids such as air and water.
The principle used in differential pressure flow meters is Bernoulli’s theorem. During the flow of a fluid of a certain size, if the flow path is intentionally narrowed, a pressure difference is created due to a pressure drop in the fluid. This pressure difference is applied to Bernoulli’s law to calculate the flow velocity.
There are various types of flow sensors, such as orifice-type and nozzle-type, depending on the mechanism used to narrow the flow path. However, they all share the same principle of measuring the pressure difference between the fluid before and after the narrowing mechanism.
How to Select a Flow Sensor
The appropriate flow sensor depends on the type of fluid to be measured. If you select the wrong flow sensor for a gas or a liquid, you will not only record the wrong flow rate, but also risk a serious accident, particularly if the chosen flow sensor is ill-suited for measuring the flow rate of a hazardous or flammable substance. Therefore, it is important to start by carefully examining the nature of the substance to be measured.
In addition, flow sensors generally cannot measure a wide range at once. If the expected flow rate is determined in advance, it is best to select a flow sensor with high accuracy for that range.
Other Information on Flow Sensors
1. About Clamp-on Type Flow Sensor
Clamp-on means that the instrument is mounted outside the piping through which the fluid to be measured flows. Clamp-on flow sensors include ultrasonic flow meters and ultrasonic flow sensors.
If the object to be measured is a fluid or other material that can be measured by ultrasonic waves, it can be used as a measurement device. Ultrasonic flowmeters include the propagation time difference method and the frequency measurement method using the Doppler method. The advantage of ultrasonic flowmeters is that they can be installed without removing or cutting piping, thus reducing the cost of installation.
Another advantage is that maintenance after installation is greatly improved and pressure loss in the piping is eliminated. Conversely, demerits include the need for a long straight pipe section since the meter is installed outside of the piping, the fact that the diameter of the piping is also a factor, and the fact that measurement cannot be performed if the fluid to be measured contains many solids or bubbles.
2. Air Flowmeter
Air flowmeters include clamp-on flowmeters and screw-connection flowmeters. The screw-connection type eliminates the need for an intuitive part, and the measurement device is compact and space-saving.
Conventional air flowmeters have used thermal mass flowmeters if the gas to be measured is a compressible fluid. There are several measurement methods for thermal mass flowmeters. In the capillary type, a flow element with ohmmeter is installed inside the tube and a heater is wound around the outside of the tube, and temperature sensors on both sides of the heater measure the flow balance.
However, thermal mass flowmeters are vulnerable to contamination and are not good in low-pressure environments, making them unsuitable for facilities with large amounts of oil mist. Currently, the flow sensor type is the most common type of thermal mass flowmeter.
This type is composed of a semiconductor flow sensor and can be used in low-pressure environments with a wide measurement range and without concern for contamination.