Video tutorial: Differential pressure flowmeters

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


DP video

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.)

Watch the video .

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.


See the growing catalog of instrumentation videos at CEtv.

-Peter Welander, process industries editor,
Control Engineering Process Instrumentation & Sensors Monthly eNewsletter
Register here to select your choice of free eNewsletters .

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Raising the standard: What's new with NFPA 70E; A global view of manufacturing; Maintenance data; Fit bearings properly
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Upgrading secondary control systems; Keeping enclosures conditioned; Diagnostics increase equipment uptime; Mechatronics simplifies machine design
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.