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Participant

Join Date: Feb 2013
Posts: 3

DP Level Transmitter Terminology Question

02/05/2013 12:31 PM

Hello,

I'm not an engineer but I hope it's still ok to post my question here.

I have to translate the following text, and am having a lot of trouble understanding the sentence in bold:

"Calculations for DP level transmitters shall assume that sensing lines to the low- and high-pressure transmitter legs are filled with glycerin with a specific gravity of 1.26.

Exceptions to glycerin fill are water or steam-water service when both legs are filled with water, and rare services when the process liquid has a specific gravity higher than 1.26, which requires a special fill fluid.

Length of sensing lines does not affect the calculation because fill fluid in both legs is the same and, therefore, is cancelled out below the elevation of the bottom tap."

My questions are:

  1. The calculation of what?
  2. "Fill fluid in both legs is the same", meaning that the pressure or specific gravity is the same? Or simply that the same fluid is contained in both legs (e.g. glycerin)?
  3. What is "cancelled out"? The fluid pressure?
  4. What does "below the elevation of the bottom tap" mean?

I've been reading all kinds of level transmitter documentation for the past few hours (including threads on this board), but I am still completely clueless as to what this means. So any help would be gladly welcome!

Thanks in advance!

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#1

Re: DP Level Transmitter terminology question

02/05/2013 1:33 PM

What language?

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#2

Re: DP Level Transmitter Terminology Question

02/05/2013 2:29 PM

Why not just say,"Length of sensing lines is not critical".

The rest of the sentence is not needed, nor relevant to the operation of the device.

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#3

Re: DP Level Transmitter Terminology Question

02/05/2013 5:51 PM

Calculation means calculation of LRV (lower range value, what the 4.0mA signal represents) and URV, (upper range value, what the 20.0ma value represents)

If the fill fluid is the same fluid, then its specific gravity will be the same, at least until condensate pollutes it.

"Cancelled out" refers to the pressure exerted by gravity of the fill fluid on the sensing diaphragms of the DP transmitter.

Tank level has a lower tap, the elevation at which a hole is poked through the tank and the high side of the DP is connected, in this case with remote seal capilliary tubing.

Level is measured as elevation difference, so the elevation of the lower tap plays a role in where level is actually measured from. The lower tap might not be at the bottom of the tank, it might well be above or below the bottom of the tank. The amount 'aboveness' or 'belowness' is a difference in elevation.

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#4
In reply to #3

Re: DP Level Transmitter Terminology Question

02/05/2013 5:56 PM

And rightly so.

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#5

Re: DP Level Transmitter Terminology Question

02/05/2013 8:28 PM

I read the original too quickly and need to clarify:

>"Length of sensing lines does not affect the calculation because fill fluid in both legs is the same and, therefore, is cancelled out below the elevation of the bottom tap."

The descent of sensing (impulse) lines below the bottom tap is cancelled out because both legs are filled with the same fluid.

The fill fluid pressure component from the bottom tap elevation to an even lower elevation transmitter port will be the same pressure for both the high side and the low side. A DP transmitter operates by subtracting the low side pressure from the high side pressure. Since the 'below bottom tap' pressure component is the same for both sides, subtracting that component 'cancels out' any contribution to a level measurement.

The elevations of the upper and lower taps, that constitutes 'zero' or 'span' for liquid level, used to be referred to as 'datum', as shown in the graphic.

Here's a graphic of a DP transmitter mounted below the bottom tap. The green levels cancel one another and the magenta levels cancel one another.

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Participant

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#6

Re: DP Level Transmitter Terminology Question

02/06/2013 3:23 AM

Iris, thank you ever so much for the comprehensive explanation! The graphic helped a lot too, because I initially thought that "below the elevation of the bottom tap" meant the level of fluid inside the tank that was beneath the bottom tap, which obviously made no sense at all. Thanks for clarifying everything for me. My translation actually means something now! I've also saved this thread for future reference.

Lyn, as a translator, I don't have authority to remove part of the sentence (even if it might be appropriate in some cases), I really have to translate every word that is written to the best of my ability. By the way, I'm translating this into French. :)

Thanks again, I really appreciate it!

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#7

Re: DP Level Transmitter Terminology Question

02/06/2013 9:18 AM

  1. The calculation of what?

= Span and Range calculations, with elevation and suppresion due to the fill fluid =

  1. "Fill fluid in both legs is the same", meaning that the pressure or specific gravity is the same? Or simply that the same fluid is contained in both legs (e.g. glycerin)?

= Same fluid always =

  1. What is "cancelled out"? The fluid pressure?

= The hydrostatic head caused by the fill fluid pressure =

  1. What does "below the elevation of the bottom tap" mean?

= The elevation changes due to the location of the seal legs and the bottom tap =

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#8

Re: DP Level Transmitter Terminology Question

02/06/2013 12:15 PM

Hi Erudd

While I'm late to the game, and the answers you have already, nothing can be added, I just had to say while you state you're not an engineer, your question was clear, precise, to the point and you presented it well.

It's a shame that some of those that do use this forum, who claim to be "engineers" don't put their question together in a manner such as yours. It was a pleasure to read!

Out of shear nosiness... into which language are you translating?

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#9
In reply to #8

Re: DP Level Transmitter Terminology Question

02/06/2013 1:12 PM

It's a shame that some members would have missed this in #6: "By the way, I'm translating this into French."

I agree that the OP did a good job of presenting the problem.

I understand thet my editing suggestion was not sound advice.

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#10
In reply to #9

Re: DP Level Transmitter Terminology Question

02/06/2013 4:35 PM

Not some members.. only me it would seem.... opps!

I can understand the OP's problem with not wanting to remove any part of the text, a friend who was fluent in French would translate her company's tech manuals on occasion and ask for my help, and while translating French to English in the literal form may in some case not make much sense, once the meaning was understood, we would write an English amendment for that sentence.

Seemed to work. She got the gold medal at work, I got taken to dinner!! Bon!

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#11

Re: DP Level Transmitter Terminology Question

02/07/2013 9:45 AM

Thank you for the compliment, Brich and Lyn :)

Whenever my own research hasn't yielded anything and that I have to resort to asking questions on technical forums (which fortunately doesn't happen too often), I'm always concerned about wasting people's time by asking things that probably sound stupid or completely obvious to professionals of a particular industry. This is why I try to make my questions as precise as I can. And as they say, context is everything, so I make sure to provide as much as possible as well. In this case, I must admit I wish I had an engineer sitting next to me full time, lol!

Thanks to vargaalex for your input as well :)

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#12
In reply to #11

Re: DP Level Transmitter Terminology Question

02/07/2013 10:54 AM

Compared to many posts we get, your was a treat.

We also have a Japanese translator, Wingman1985 (I think) and a Canadian Import/Customs agent that come here sometimes. Apothocus is his name, I think.

Their questions are always interesting, too.

Welcome to the site!

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#13
In reply to #11

Re: DP Level Transmitter Terminology Question

02/07/2013 11:06 AM

you're welcome anytime!

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brich (3); Erudd (2); Iris (2); lyn (5); vargaalex (1)

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