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# Level Measurement by differential pressure smart transmitter

## Level Measurement by differential pressure smart transmitter

(OP)
Hello,

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

(OP)

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.

https://files.engineering.com/getfile.aspx?folder=...

### 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

(OP)

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

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