Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Level Measurement by differential pressure smart transmitter

Level Measurement by differential pressure smart transmitter

Level Measurement by differential pressure smart transmitter


I started a new experience in a refinery and We are planning to change a pneumatic transmitter by smart transmitter which measures the level inside a Vessel.

After reading manuals and watching tutorials, I did the calculation and I found delta P at minimum level (0%) = -236 mbar and delta P at maximum level (100%) = -91 mbar. So I suggest to my collegues (the instrument technicians) to configure LRV as -236 and URV as -91 because the smart transmitter can accept the negative pressure but they didn't agree with me and they said that they have an old method: in fact, to eliminate the zero: they will consider LRV = 236 and URV = -91 - (-236) = 145 (span). I want to verify with you if someone has worked with this method before?

Thank you.

RE: Level Measurement by differential pressure smart transmitter

The LRV/URV numbers you calculate are very typical for dual remote seals with capillaries, assuming the transmitter's high side seal is connected at the bottom, and the low side seal is connected up top. In theory, reversing the connections so the high side seal is connected at the top and the low side seal is connected at the bottom, reversing the LRV and URV as suggested would produce the internal (high side minus low side) calculation you provided. I've always seen dual remote seals connected with the high side on the bottom, but I don't know why it wouldn't work as long as the transmitter is designed for it (Early Honeywell smart DP's would read when connected backwards but had significant error unless the transmitter was ordered with an option called 'compound characterization'. But this could be a wet leg (reference leg) installation and it's been so long since I've done a wet leg level calc that I'm drawing a blank at the moment.

Two considerations

- the output is 4-20mA, which has to be interpreted in some level or volume units by the receiver (DCS, controller, indicator). So it's really irrelevant whether the range is -236mbar to -91 mbar or 236mbar to 91 mbar if what the user/operator sees is mm, cm, m, l or bbls. If this new transmitter has a local indicator, then there's a reason for have level units that make senses.

- if the plant/mill uses a particular convention for connecting level-by-DP transmitters, as to whether the high side or the low side connects to the bottom of the tank, then it would seem prudent to continue with the convention in order to avoid longterm ownership issues with 'oddball' installations.

RE: Level Measurement by differential pressure smart transmitter

Thank you Drawn2 for your answer.

In fact, we still use wet leg and as you said the transmitter's high side is connected at the bottom and the low side seal is connected up top.

please find attached a file that describe my calculation, my methode to configure lrv and urv and the method of my collegues.


RE: Level Measurement by differential pressure smart transmitter

I work in meters of water column or mmH2O because I find water column units intuitive when working with level.

Units aside, my calculation agrees with yours,
LRV = -2405 mmwc or -236 mBar
URV = -926.6 mmwc or -91 mBar

To create positive values for LRV and URV, the transmitter has to be connected 'backwards': low side at the bottom, high side at the top.

I find that doing so does not make it any more intuitive with LRV/empty/4.0mA/zero level = 236mbar and URV/full/20.0mA/100%level = 91mbar, where an increasing level has a diminishing pressure. But in the end, who reads pressure? Isn't the user value onverted to level or volume units in the HMI?

RE: Level Measurement by differential pressure smart transmitter


The signal 4-20 mA will be transmitted to the HMI and dispayed as % of level. So % of level eqquivalent to % of delta P.

RE: Level Measurement by differential pressure smart transmitter

thanks for the valuable information provided by @danw2

Transmitter Shop

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close