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Anonymous Poster

Why is Output D/P Transmitter not Equal with Rising Level?

07/31/2010 10:31 PM

I use d/p transmitter for measuring level at LPG tank,length between low tap point and high are 1900 mm,from high to transmitter 1000 mm and total length 2900 mm,density are 0.54.But output transmitter not equal with raising of level,for example,level up 10 % but output only appear 5%,I was sett URV,LRV. etc.would you please give advise what point must be set again,thank's

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Power-User

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#1

Re: Why output d/p transmitter not equal with raising level

07/31/2010 11:57 PM

Analog DP transmitters do not indicate actual level output (unless it's Foundation Fieldbus with math block calculations), rather the transmitter provides (typically) a head pressure value, generally mm or inches water column.

That head pressure output has to be divided by the specific gravity, 0.54 in your case, to get the actual level.

actual level = head pressure (w.c.) / Specific Gravity

For a DP scaled 0-2900 mm w.c., an acutal level of 290 mm (10% of 2900 mm) would be 156.6 mm w.c.,. or about 5.4% full scale.

156.6mm (w.c.)/ 0.54 (S.G) = 290 mm (actual)

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Power-User

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#2
In reply to #1

Re: Why output d/p transmitter not equal with raising level

08/01/2010 2:55 PM

My bad. The GA is undeserved. It's what happens when an answer is composed late at night after a couple wedding receptions that same day.

Upon re-reading the OP the next morning,

a) From the description, it is likely that this is a remote seal DP transmitter, since there are varying distances (elevations) for the high and low connections. I didn't catch that previously.

If such is the case, the density of the fill fluid in the remote seal capillaries needs to be known to calculate the LRV and URV for the transmitter.

Is this a remote seal unit?

b) The transmitter's span might be incorrect, which is what the question was about, and SG does play a major role.

If this were a typical flange mounted DP level transmitter down at the bottom of the tank with a dry leg on the DP's low side, the transmitter's 100% span/range for an actual level of 2900 mm water column would be calculated as

actual level * S.G. = head pressure in water column

100% actual level * S.G. = 100% head pressure (in water column or in the eng units the transmitter is set for)

2900 mm (actual) * 0.54 = 1566 mm water column

The 4-20ma output would then span 0-100%; 0-2900 mm LGP, or 0-1566 mm water column.

Any receiver device (PLC, DCS, digital meter, recorder, whatever) can be then be scaled for whatever units are appropriate.

A span of 2900 mm is presumably water column engineering units. That is way too wide a span, by almost a factor of two. The SG needs to be taken into account.

example:
span = 2900 mm w.c.
level change = 290 mm (actual)
head pressure = 290mm (LNG) * 0.54 = 156.6 mm w.c. = 5% of 2900 mm span

span = 1566 mm w.c.
level changes 290 mm (actual)
head pressure = 290mm (LNG) * 0.54 = 156.6 mm w.c. = 10% of 1566 mm span

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Anonymous Poster
#3
In reply to #2

Re: Why output d/p transmitter not equal with raising level

08/02/2010 3:38 AM

Dear sir

Thanks very much for you answer,type of transmitter are d/p with sensing tap point High fill with process and low tap point fill with vapor of gas lpg.this is the digital tramsmitter.distance High and low tap point 1900 mmwc,from high tap point to transmitter 1000 mmwc.transmitter to be instal at under tank.you have formula of URV and LRV.

regards

efi

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Guru

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#4

Re: Why is Output D/P Transmitter not Equal with Rising Level?

08/02/2010 10:03 AM

In an earlier occasion I have read the factors affecting it.If you go through the website you will find it.The parameters of the cable connecting the transmitter to the digital readout/indicator should be considered. In an OMEGA(US) level sensor/indicator we didn't get the correct reading.We then adjusted the "k" value until we got the correct reading and handed over to the client.

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Anonymous Poster
#5

Re: Why is Output D/P Transmitter not Equal with Rising Level?

08/18/2010 11:29 AM

Please check the zero and span settings are correct. Zero when tank is empty then change span to max when tank is full .

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Guru
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#6

Re: Why is Output D/P Transmitter not Equal with Rising Level?

02/20/2024 7:01 AM

Well, if the tank is not vented, then a change in level is accompanied by a change in pressure in the headspace above the liquid; that might do it if the low pressure port on the <...d/p transmitter...> is vented, perhaps.

The thing is that no-one at CR4 can see the installation other than the Original Poster, and no-one else is so deeply invested in it.

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