Understanding The Top 8 Flow Sensing Technologies & Which Types Should You Use? Vol. 1

January, 2020

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 the first 4...

8 Flow Meter Types & Principles (1-4)

1 ELECTROMAGNETIC FLOW METERS

Electromagnetic flow meters detect flow by using Faraday's Law of induction. Inside an electromagnetic flow meter, there is an electromagnetic coil that generates a magnetic field, and electrodes that capture electromotive force (voltage). Due to this, although it may appear as if there is nothing inside the flow pipe of an electromagnetic flow meter, flow can be measured.

PROS

• Unaffected by the temperature, pressure, density, or viscosity of the liquid
• Able to detect liquids that include contaminants (solids, air bubbles)
• There is no pressure loss
• No moving parts (improves reliability)

CONS

• Cannot detect gases and liquids without electrical conductivity
• A short section of straight pipe is required

2 KARMAN VORTEX FLOW METER

Classified as a vortex flow meter, this device utilizes a law that was theoretically proven by Theodore von Karman in 1912. When there is a column-shaped obstruction (vortex shedder) in flowing fluid, it will generate alternating vortices downstream. The flow velocity of the fluid and the vortex generation frequency are proportional to each other. Therefore, detecting the number or pulse of vortices makes it possible to measure flow. The main method of detection involves sensing the vortex vibration with a piezo element. However, a more robust method utilizes ultrasonic waves to detect the vortex vibration.

STRUCTURAL DIAGRAM OF A KARMAN VORMAN VORTEX FLOW METER

PROS

• No mechanical moving parts
• Can detect liquids, gases, and vapors
• Because there are no electrodes, it has specifications that offer excellent chemical resistance
• Has large range ability and good accuracy

CONS

• Because it restricts the flow path, pressure loss will occur
• Scale deposits and liquids that include solids become the cause of "clogging"
• Not suited for high-viscosity fluids
• Sensitive to pipe vibrations
• A section of straight pipe is required

3 PADDLE WHEEL FLOW METER

This is classified as a turbine flow meter. Paddle wheel flow meters are generally divided into two mechanical classes as described below:
(1) Tangential-flow paddle wheel flow meters, with a water wheel structure
(2) Axis-flow paddle wheel flow meters, with a windmill structure
The flow and the revolutions of the paddle wheel are proportional to each other. Thus, by spinning the paddle wheel with the force from the flowing fluid, it becomes possible to measure the rate of this flow from the number of revolutions. By embedding a magnet in the rotation axis and on the edge of the paddle, pulses can be extracted as signals, converting the number of revolutions into the flow rate.

STRUCTURAL DIAGRAM OF A PADDLE WHEEL FLOW METER

PROS

• Excellent reproducibility and responsiveness
• A simple structure with a low price
• Compact and can perform large-capacity measurement

CONS

• Extremely sensitive to foreign objects (Causes clogging)
• Because the paddle wheel spins at high-speeds, periodic maintenance is needed to deal with axial wear or the paddle wheel must be replaced

4 FLOATING ELEMENT FLOW METER

This is classified as an area flow meter. Its mainstream method involves suspending a float in a tapered pipe (a pipe that progressively widens heading upwards). When the fluid is forced in between the tapered pipe and the float, a differential pressure is generated. When this happens, the float stops at an area where the upward force caused by differential pressure and the downward force caused by the weight of the float have been equalized. The resulting display is the instantaneous flow. Commonly, the tapered pipe is manufactured from transparent materials calibrated for flow and measurements are read directly from the pipe. There are also types available that have a magnet built into the float and perform detection using a magnetic sensor.

FLOATING ELEMENT FLOW METER

PROS

• Able to detect gases, liquids, and vapors
• Generally low priced
• Comparatively low pressure loss

CONS

• Does not offer very high accuracy
• Not suited for fluids that include solids (clogging will occur)
• With the directly-read type, the float cannot be seen if the tapered pipe becomes dirty (frequent maintenance required)