Flow Sensor

Flow sensor measures the flow rate or quantity of fluid whether it will be a gas, fluid or solid. Flow sensor is a sensing element in flow meter or a flow logging device to record the flow of fluids. The flow sensor can measure velocity, flow rate or totalised flow. The flow sensor technology can be based on such things as light, heat, electromagnetic properties, ultrasonic and many other technologies in a wide spectrum.

With most fluid flow sensors, the flow rate is determined directly or inferentially by measuring the liquid's velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the liquid through a pipe. Because the pipe's cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate.

As these systems work without any mechanically moved parts they can be mounted independent of mounting position and flow direction.
A very basic relationship for determining the fluid's flow rate in such cases is:
Q=V/A;
Where Q=liquid flow through pipe,
V=average velocity of flow,
A=cross sectional area of the area.

Other factor that affects the flow rates are:
• Viscosity
• Density
• Temperature
• Frictional force
• Pipe configuration

Flow sensor can be classified into three categories:
• Mass flow sensor: It measure flow rate in terms of the mass of the fluid substance and have units such as lbs/min.
• Velocity flow sensor: It measure flow rate in terms of how much of the material is flowing and use units like mol/min.
• Volumetric flow sensors: It measure flow rate in terms of how fast the material is moving. These use units like ft/sec.

Flow sensors can also be selected based on their technology.
• Magnetic flow sensor
Ultrasonic flow sensors use sound frequencies above audible pitch to determine flow rates. They can be either Doppler effect sensors or Time-of-Flight sensors. Doppler flow sensors measure the frequency shifts caused by fluid flow. The frequency shift is proportional to the liquid's velocity. Time of flight sensors use the speed of the signal traveling between two transducers that increases or decreases with the direction of transmission and the velocity of the fluid being measured.
• Turbine flow sensor
Turbine flow sensors measure the rate of flow in a pipe or process line via a rotor that spins as the media passes through its blades. The rotational speed is a direct function of flow rate and can be sensed by magnetic pick-up, photoelectric cell, or gears.
• Ultrasonic flow sensor
Magnetic flow sensors apply Faraday’s law to measure liquid flow. The sensor contains two electrodes that produce a magnetic field when energized. When a conductive liquid passes through the electrodes in the flow meter, a voltage is induced. The voltage is proportional to the electric field strength, diameter of the pipe, and flow velocity.

Flow velocities of fluid i.e liquid or gases can be measured using thermal methods. Generally one of two principles are applied.
• Heat transfer from heated structure into fluid.
• Heat transport from one area of heated structure to another area

Principle 1 referred to as a hot wire anemometry, a heated wire immersed in the flowing fluid. The heat power p transfer from wire to fluid by p=(c₁+c₂√v) ΔT where ci and v denotes system dependent constants and velocity.It is the temperature increase of hot wire above reference temperature defined by fluid.

Principle 2 referred to as a hot film anemometry. Sensor structure heated centrally and temperatures are measured at downstream and upstream direction.
An important advantage of hot film over a hot wire anemometry the strong vv nonlinearity does not appear in response.

Liquid flow sensor uses tubes made of silica, stainless steel or PEEK, and either constant power or constant temperature methods in conjunction with a heater and temperature sensor. The tested instruments were capable of measuring flow ranges between 25-500 nl/min (smallest flow range) and 100-2000 µl/min (largest flow range) water, with operating pressures up to 100 bar (up to 400 bar for flow meters with flow ranges below 100 µl/min).

It's low cost and its protective properties against humidity, pollution, high temperature, EMC interference and vibrations make the sensor suitable for application in many different environments.

The air flow sensor contains a silicon sensor chip, a special actuation circuit, an amplification circuit and a signal conditioning circuit. The sensor chip is actuated to heat up the air locally. The movement of the heated air is detected by the sensor in both directions. This principle makes the sensor very sensitive. Therefore the amplification could be held at a low value. The signal conditioning circuit keeps the sensitivity to a constant level at different temperatures.

The air flow sensor is designed to detect air flow in the free field. There is no physical connection to the flow conductor and the sensor just captures bypassing gas flow. The sensor is supposed to be placed in a flow channel of approximately 15 x 15 mm. Placement in smaller or larger channels is possible, but the sensitivity will change. In all situations the air flow should be laminar.

Typical Specifications
• Air speed: 0-25 m/s
• Resolution: 1-5% f.s.
• Response: 1-10 ms
• Power: <0.05 W
• Chip size: >(2x1x0.5) mm3

Typical Applications
• Bi-directional air flow sensing
• Breathing measurement
• Control of building flows
• Natural gas flow monitoring
• Process control

Advantages of velocity flow sensor:
• It does not introduce thermal turbulence.
• It can detect very low velocity changes (few microns/second).
• It can be introduced into very small cavities.

When selecting a flow meter consideration must be given to:
• Flow measurement type - momentum (velocity), volumetric or mass flow measurement
• Media - type of media (liquid, gas or solid) and any special condition such as particulates in the media and viscosity of the media
• Media conditions - pressure and temperature of media and whether media conditions are likely to remain constant or vary
• Flow range - required flow range of media (min and max readings required)
• Accuracy - required accuracy of the readings
• Environmental considerations - special installation considerations such as hygienic installation, installing in to an ATEX zone or requiring tamper proof readings.

• The signal is very sensitive to particles gathering to the tip. Even small dust grains attached to the tip will create a significant signal increase due to the associated stagnant water. For this reason it is recommended that a fine brush is implemented into the measuring setup, so the sensor tip can be cleaned at regular intervals
• The flow sensors are always somewhat directionally sensitive. This is because it is impossible to build the sensor completely symmetrically. Thus, you either need to know the direction of flow in your system and calibrate in the same direction, or you operate with substantial uncertainties, especially at high velocities.
• You need a dedicated calibration setup preferable consisting of a system with stagnant water, which can be moved at well-defined speeds.
• Like all other sensors, the flow sensor is temperature sensitive.

• Medical devices
• Diagnostics
• Process technology