# Video tutorial: Differential pressure flowmeters

## Using differential pressure to calculate flow is a very basic and common method. Here's how it works.

02/16/2010

 Measuring flow with differential pressure

One of the most basic ways to measure fluid flow is by using a differential pressure measurement of fluid pressure up- and down-stream of an obstruction that creates a pressure drop. The pressure drop will be proportional to the amount of fluid flowing through the pipe, so the greater the flow rate, the greater will be the pressure difference. (The concept works with liquid or gas, but having a compressible fluid makes it somewhat more complicated. For purposes of this discussion, we'll stick with liquids.)

A simple way to introduce such an obstruction is a plain orifice disk, where a device is inserted in the pipe with an opening that is somewhat smaller than the normal cross sectional area of the pipe. Liquid flowing through the orifice loses some pressure. That loss can be measured using a differential pressure device comparing readings above and below the obstruction.

This approach is very common commercially, and companies that have refined the technology have found sophisticated ways to optimize orifice size and shape, select the most desirable pressure measurement points, and otherwise create the highest accuracy and turn-down ratio with the least actual pressure loss.

There are many advantages to this approach:

• Simple sensor that can be compact with no moving parts;
• Bi-directional flow is possible;
• Scalable over a very wide range of sizes and flow rates; and
• Relatively inexpensive.

Of course there are drawbacks and limitations too:

• Limited accuracy and turndown range-If high precision is your primary objective, there are probably better technologies;
• Causes a pressure drop-Compensation for this may require higher pumping energy;
• Reduces the pipe's free passage-As sensor technologies go, this is one of the more invasive, but the extent varies among manufacturers;
• Accuracy and turn-down are related to pressure drop-In other words, the more accuracy and range you need, the greater pressure drop you will have to tolerate; and
• Orifice is prone to wear-While there are no moving parts, the orifice itself is often a wear-prone point due to the high liquid velocity. If the orifice gradually enlarges with wear, the flow measurement will read low. On the other hand, if it becomes partly obstructed with debris, the measurement will read higher. The extent of this also depends on the configuration.

The same application and installation guidelines for good pressure sensor practice apply here. Anything that interferes with the pressure readings, such as clogged impulse lines, will interfere with a true flow calculation. Additionally, changes in fluid characteristics, including viscosity, density, multiphase flow, and even temperature, can affect readings. Of course these are generalities, so discuss your specific application needs with any prospective suppliers.

The video shows a demonstration of a home-made sensor that illustrates the basic concepts.

-Peter Welander, process industries editor, PWelander@cfemedia.com
Control Engineering Process Instrumentation & Sensors Monthly eNewsletter

The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Robotic integration and cloud connections; SCADA and cybersecurity; Motor efficiency standards; Open- and closed-loop control; Augmented reality
Controller programming; Safety networks; Enclosure design; Power quality; Safety integrity levels; Increasing process efficiency
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.