Tutorial: Differential pressure sensors

Differential designs have many useful applications.


While working on the pressure sensor article in Control Engineering’s February issue , I was struck with fact that differential pressure (DP) sensors represent a larger portion of the market than expected, which speaks to their versatility.

As part of writing the article, I interviewed Scott Nelson, VP of pressure products for Emerson’s Rosemount division, certainly one of the largest manufacturers. He said that Rosemount’s pressure sensor sales break down along these lines:

35-40% are DP sensors for flow measurement;
20-25% are DP sensors for level measurement; and,
30-40% are standard sensors for gage or absolute measurement applications.

To review the basics, differential pressure sensors have two inputs and are designed to measure the difference between two pressure sources. (Technically, normal gage pressure sensors are differential designs in that they measure the difference between the process and surrounding atmospheric pressure.)

The two most common applications for DP devices are flow and level measurements, as suggested in the statistics above.

Level —One of the simplest ways to measure level in a vented tank is by putting a pressure gage at the bottom and converting the pressure measurement to a liquid height. However, if the tank is sealed, it can develop negative or positive internal pressure. This can throw off the level reading, unless the gage at the bottom is a differential design where the second inlet can measure the tank’s internal pressure above the liquid. This configuration compensates for the conditions inside the tank and corrects the level reading.

Flow —Differential pressure flow reading techniques are common and represent a very basic approach. As liquid flows through a pipe and crosses some type of flow restriction (or pitot tube, Annubar, or even pipe elbow), there will be a difference in pressure above and below the obstruction point. A DP sensor can measure the difference, which can be used to calculate the flow rate.

In both these applications, the pressure difference is usually relatively low. For a level application, water would have to be 27.7 inches deep to register 1 psi. Similarly, for a flow measurement, unless the obstruction is quite severe, up and downstream pressure readings should be within a few psi.

As a result, differential pressure devices have more specifications and limits than standard configuration designs. (Always consult with your suppliers for specifics.) For example:

A device will have two pressure ratings. The first is for the range of differences and the second is for the overall pressure limit. A DP device that shows an indication of 10 psi does not tell you if the difference between the high and low side is 20 and 10 psi, or 1010 and 1000 psi, but those two applications will likely require different units. (Some devices can also display a gage pressure reading of one or both sides, although this capability varies by design.)

More installation cautions may be required. Depending on the application, a device may have to measure a relatively low pressure differential but in a high pressure situation. Like the second example above, a sensor has to be sensitive enough to measure a 10 psi differential precisely in the context of total pressure of 1000 psi or more. That device may not survive having 1000 psi on one side while the other is open to atmosphere. Many manufactures offer manifolds with internal valves to simplify installation while protecting the device from such spikes. By following simple instructions, users can avoid damage to sensitive mechanisms.

One side may always be high. For some designs to operate properly, one side must be higher than the other. Others allow either side to be high. You will need to determine if your process is capable of swinging in both directions and select appropriately.

Devices can be either mechanical (e.g. Ashcroft) or electronic designs. All pressure gage device manufacturers do not necessarily offer DP units.

Differential pressure sensors are made by a variety of companies, including those on the list below. You can also search online at Control Engineering Supplier Search .

AST Sensors

Foxboro (Invensys)
GE Sensing

Honeywell Process Solutions
Rosemount (Emerson)

—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Process Instrumentation & Sensors Monthly

Register here and scroll down to select your choice of free eNewsletters.

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 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...
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

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.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me