Understanding The Top 8 Flow Sensing Technologies & Which Types Should You Use? Vol. 2

January, 2020

Click here to view Vol. 1 (1-4)

In case you haven't noticed, there are several types of flow meters and flow sensors. There are also a number of different operating principles. Have you ever wondered if there were other, more efficient methods than what you are currently using? If you have, you're in luck. We've compiled a list of 8 different flow meter types and operating principles as well as pros and cons for each. Let's take a look at numbers 5-8...

8 Flow Meter Types & Principles Vol. 2 (5-8)

5 THERMAL FLOW METERS

When fluid comes into contact with a heated object, the fluid takes heat away from the object, increasing the temperature of the fluid. A thermal flow meter uses this principle to measure flow. Thermal flow meters can be divided into two groups:

(1) The temperature difference measurement method
A heater is installed in the fluid, and fluid temperature is measured at 2 points, upstream and downstream. Then flow is determined by the temperature difference between these 2 points. This is primarily used for applications with low flow rates.

(2) The power consumption measurement method
A heater is installed in the fluid and fluid temperature is measured at 2 points, upstream and downstream. The heater is controlled to constantly maintain a fixed temperature difference between these 2 points. Flow is determined based on the amount of power required to maintain this temperature.

PROS

• Able to detect gases
• There is basically no pressure loss
• Can measure mass flow

CONS

• Errors occur when the temperature of the fluid changes
• It is difficult to set-up specifications that match the type and composition of the fluid
• Errors occur when there are deposits/buildup on the temperature sensors or heating element

6 PRESSURE DIAPHRAGM FLOW METERS

This is classified as a differential pressure flow meter. Differential pressure flow meters currently have the top market share for flow meter production volume. Using Bernoulli's principle, an orifice plate is installed into the path of flowing fluid, deliberately causing pressure loss. The pressure differential before and after the orifice plate is measured. The relationship between the pressure loss generated by the orifice plate , the flow velocity, and the flow rate are as follows:

PROS

• Able to detect gases, liquids, and vapors
• Generally low priced
• No moving parts

CONS

• Because there is an orifice plate, great pressure loss occurs
• Not suited for fluids that include solids (clogging occurs)
• Because it is sensitive to turbulent flow, a long section of straight pipe is required

7 ULTRASONIC FLOW METERS

An additional type of flow meter technology uses an ultrasonic sensor. The most popular form of this utilizes the principle known as either transit time or time of flight. These can clamp-on to the outside of a pipe and eliminate the need for any pipe modification or risk of contamination.

[Transit time flow meters]
Sensors transmit and receive ultrasonic waves that are sent diagonally through the fluid in the pipe. The ultrasonic waves are accelerated as the flow rate increases. In turn, the ultrasonic waves are decelerated as the flow rate decreases. The difference in the transmission time of the ultrasonic waves is measured as a flow rate.

PROS

• There is no pressure loss
• A type that can perform detection from the outside of piping is available

CONS

• A long section of straight pipe is required
• Liquids that have a large solid content will cause malfunctions
• Measurement is not possible when there are many air bubbles

8 CORILOUS FLOW METERS

Coriolis flow meters are flow meters that use a physical phenomenon called the Coriolis effect inside the flow meter itself. As seen in the illustration below, when fluid flows through a vibrating U-shaped pipe, the Coriolis effect goes into action, reversing the flow between the entrance-side, A, and the exit-side, B, and twisting the pipe. With the Coriolis effect, the weight and speed of an object are proportional to each other, so the amount of twist is measured and mass flow is understood.