# Pressure gage ranging and accuracy

08/29/2010

Dear Control Engineering: I was reading the recent article on differential pressure sensors, and I had a question coming out of that discussion. The author made the following statement: “The pressure in the line may be upwards of 1,000 psi, but the pressure drop is usually very small. Using two separate gage pressure devices and subtracting the readings would result in the combined error of two devices having wide measuring spans. The result would likely be a very poor measurement seldom suitable for control.” Could you explain that in greater detail?

That statement was made in the context of using a DP (differential pressure) sensor to measure flow using an orifice plate flowmeter. His point is that a single DP device is better than trying to do the same job with two pressure gages and calculating the difference.

Here’s why: Using his example, let’s say your process line is running with a pressure of 1,000 psi, and you have an orifice plate. Under normal operating conditions, the pressure drop across the plate is 5 psi. That means if everything is perfect, the upstream gage reads 1,000 psi and the downstream gage reads 995 psi. For appropriate ranging, both gages are identical with a scale of 0-1,500 psi. The problem emerges when you consider the accuracy of the two devices. If they have an accuracy rating of ±1% full scale, that means any reading can be high or low by 1% of the full scale, or 15 psi. So the first gage could indicate 1,000 psi as anything from 985 to 1,015 and still be in tolerance. Similarly, the second gage could indicate 995 psi as anything from 980 to 1,010 and still be in tolerance. Do you see the problem? You’ll never be able to know when your pressure drop is, in fact, 5 psi.

The differential pressure device, if it has a range of 0 to 15 psi, will give you a much higher accuracy since 1% of 15 is 0.15 psi.

Now, the tricky part is making sure you’re gage isn’t exposed to 1,000 psi on one side and 0 on the other. This could damage it, so you will have to ensure that the device won’t see pressure level imbalances outside of its ability to withstand them.

Peter Welander, pwelander@cfemedia.com

Also read an earlier article on pressure measurements. article

The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
Detecting security breaches: Forensic invenstigations depend on knowing your networks inside and out; Wireless workers; Opening robotic control; Product exclusive: Robust encoders
Smooth motion control strategies: Integrated systems, efficiency, motion profiles; Understanding optimizers; Capture far-flung data; Lean automation architectures; Artificial intelligence: Smarter sensor systems
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.

## Integrator Guide

 Search the online Automation Integrator Guide

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

## Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.