# Tutorial on flow metering (Part 1)

17 Jul 2012 | Mohit Arora, Prashant Bhargava**Share this page with your friends**

A new generation of electronic flow meters provides better control and accuracy of fluid measurement, however it still leaves several choices on how fluid is measured. Part I of this series covers basic flow meter fundamentals including types of flow meters and the main considerations and challenges in selecting a flow meter.

**Fluid flow measurement and reynolds number**

Flow is generally measured inferentially by measuring velocity through a known area. With this indirect method, the flow measured is the volume flow rate, Q_{V}, stated in its simplest terms:

Q_{V }= A * V (1)

Where

A = Cross-sectional area of the pipe

V = Fluid Velocity

A reliable flow indication is dependent upon the correct measurement of A and V. If, for example, air bubbles are present in the fluid, the area term "A" of the equation would be artificially high. Likewise, if the velocity is measured as a point velocity at the center of the pipe, and it is used as the velocity term "V" of the equation, a greater Q_{V} than actual would be calculated because "V" must reflect the average velocity of the flow as it passes a cross-section of the pipe.

The following are the major factors affecting the flow of fluid through a pipe:

• Velocity—speed at which a fluid moves through a pipe

• Density—weight per unit volume

• Viscosity—ease of flow of a fluid

• Pipe size—diameter of the pipe carrying the fluid*Velocity of the fluid and pipe size:* Fluid velocity depends on the head pressure, which is forcing the fluid through the pipe. The greater the head pressure, the faster the fluid flow rate (all other factors remaining constant), and consequently, the greater the volume of flow. Pipe size also affects the flow rate. For example, doubling the diameter of a pipe increases the potential flow rate by a factor of four.

*Viscosity of the fluid: *Viscosity negatively affects the flow rate of fluids. Viscosity decreases the flow rate of a fluid near the walls of a pipe. Viscosity increases or decreases with changing temperature, but not always as might be expected. In liquids, viscosity typically decreases with increasing temperature. However, in some fluids viscosity can begin to increase above certain temperatures. Generally, the higher a fluid's viscosity, the lower the fluid flow rate (with other factors remaining constant).

*Density of the fluid:* Density of a fluid affects flow rates such that a more dense fluid requires more head pressure to maintain a desired flow rate. Also, the fact that gases are compressible, whereas liquids essentially are not, often requires that different methods be used for measuring the flow rates of liquids, gases, or liquids with gases in them.

*Reynolds number*: The most important flow factors mentioned above can be correlated together into a dimensionless parameter called the Reynolds number, which indicates the relative significance of the viscous effect compared to the inertia effect. The Reynolds number is proportional to inertial force divided by viscous force. The Reynolds number is proportional to fluid flow means velocity and pipe diameter and inversely proportional to fluid viscosity.

Reynolds number (Re) = ρ * D * v/µ (2)

Where

D = Internal pipe diameter

v = Velocity

ρ = Density

µ = Dynamic Viscosity

At very low velocities of high viscosities, Re is low and the fluid flows in smooth layers with the highest velocity at the center of the pipe and lower velocities at the pipe wall where the viscous forces restrain it. This type of flow is called laminar flow and is represented by Reynolds numbers below 2,000.

Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

*Ooops, that doesn't look right. Please enter a valid email.*

*That email's already registered. You may wish to*

__update your subscriptions__or try using another email.Got to make sure you're not a robot. Please enter the code displayed on the right.

*Please enter the valid code.*

*Sorry, you have reached the maximum number of requests allowed. You may wish to try again after a few hours.*

**Time to activate your subscription - it's easy!**

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.

**Robotic glove helps restore hand movements**

The device is an improvement from conventional robotic hand rehabilitation devices as it has sensors to detect muscle signals and conforms to the natural movements of the human hand.

**Control this smart glass with the blink of an eye**

K-Glass 2 detects users' eye movements to point the cursor to recognise computer icons or objects in the Internet, and uses winks for commands. The researchers call this interface the "i-Mouse."