Video tutorial: Measuring liquid depth using pressure

One of the simplest and most reliable ways of measuring liquid depth is via pressure. This article, and its accompanying video demonstration, offers some practical suggestions. See outtake and other photos.

12/07/2009


Photo inset, upper right, shows an outtake from the pressure gage tutorial demonstration, flipping its lid.

Photo inset, upper right, shows an outtake from the pressure gage tutorial demonstration, flipping the demo's lid, says Peter Welander.

If you want to measure the level of liquid in a tank, one of the simplest approaches is to use a pressure gage. The fact that liquid has weight means that you can measure that weight in the form of pressure.

If you drill through the side of a tank and insert a gage, the reading can be translated into the depth of liquid above the gage.

If your gage is set up to read in inches of water column, the conversion is really easy. If it reads in PSI, one pound equals 2.307 feet of water, or 27.68 inches. ( Watch the video demonstration .)

 

Fire water tank

A 500,000 gallon fire water tank


Yokogawa pressure sensor

The sensor measures static pressure from the liquid weight

For all practical purposes, any unit of pressure can be converted to the required unit of depth. Most assume a liquid density equal to water, however there are conversion factors for liquids with other densities. If your tank is full of ethanol, one PSI equals about 3.00 feet of liquid since ethanol is less dense than water. If you want to be really precise, a temperature correction is required as well since density is affected by temperature.

Keep in mind that the gage is going to tell you what the depth is above itself. If it is mounted halfway up the side of a tank, that is its starting point. It can't read anything lower. In the same way, if you put the gage on an impulse line (with no air in it), the position of the measuring device is still the relevant point. The position of the penetration into the tank doesn't matter.

The accompanying photos illustrate a 500,000 gallon fire water tank at Arkema Chemical . The depth of water in the tank is monitored by a pressure sensor mounted near the bottom. It is configured to monitor the level and provide an alarm if it drops below a specific point. (Source: ISA100 Wireless Compliance Institute )

This discussion so far is based on the assumption that the tank is open to atmosphere and that you are taking a normal gage-pressure reading. If the tank is sealed, there is the possibility that the process might cause the pressure to go above or below atmospheric which will throw off your reading. In these situations, you have two choices: Either adjust your level reading to compensate for the internal pressure, or use a differential pressure device.

A manual correction is simple enough, but it means you need a second gage mounted on top to read the interior tank pressure. If it's reading 2 psi, you deduct that from the depth reading on the bottom.

It's simpler to use a differential pressure gage. The high end should be connected to the bottom of the tank, or at least below the liquid level, just as you would if the tank were open. The low end of the pressure gage should be connected to the top of the tank, where the line will be above the liquid level. This line should be filled with air or whatever gas is in the headspace of the tank. With both sides of the device connected thusly, the gage will self correct. You don't need to pay attention to changing pressure levels within the process.

This type of application is very common given its simplicity, accuracy, and versatility:
• You can use any type of pressure measuring device, electronic or mechanical;
• It can be as precise as the process requires;
• With the right type of device, you can interface with a control system, set alarm levels, etc., as needed; and
• Once the device is permanently positioned, even if it isn't at the bottom, you can compensate for most placement issues.


There are some practical limitations, including:
• If there is sediment or debris in the liquid, it can clog impulse lines;
• You have to add one or two more penetrations into the process; and
• If a tank can hold multiple products, you have to use appropriate density corrections.

 

Pressure measuring devices are available from a wide variety of suppliers, including:

Ashcroft
AST Sensors
Foxboro (Invensys)
GE Sensing
Honeywell Process Solutions
Omegadyne
Rosemount (Emerson)
Setra


-Peter Welander, process industries editor, PWelander@cfemedia.com
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 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...
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
Safety standards and electrical test instruments; Product of the Year winners; Easy and safe electrical design
Safer human-robot collaboration; 2017 Maintenance Survey; Digital Training; Converting your lighting system
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Big Data and bigger solutions; Tablet technologies; SCADA developments
Automation modernization; Predictive analytics enable open connectivity; System integration success; Automation turns home brewer into brew house
Commissioning electrical systems; Designing emergency and standby generator systems; Paralleling switchgear generator systems
Natural gas for tomorrow's fleets; Colleges and universities moving to CHP; Power and steam and frozen foods

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.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
Compressed air plays a vital role in most manufacturing plants, and availability of compressed air is crucial to a wide variety of operations.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
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.
click me