How to do Interface Level Calculation using DP Transmitter ?

Interface level calculation is not complicated.

If we just have our basics concepts clear it is very straight forward.

First simple pressure measurement formula

The pic above shows a Container of Height 100 Inches

And this tank contains Liquid of Specific gravity 0.9

So what is the pressure at the bottom?

Height x Specific gravity = InH2O

100 (inches) X 0.9 = 90 inH2O

This Formula is adapted from Rosemount technical Datasheet (paper no :- 00816-0100-3207)

Note :- Specific gravity :- It compares with Density of substance with that of WATER (H2O)

Hence when we multiply ,

Height (Inches) x S.G ( Comparing with Density of substance with that of WATER (H2O))

= inH2O (The unit we get as product)

This our Simple Formula that we will be using.

Interface Level Calculation

One prerequisite recommended for this system is to fill impulse lines with Heavier fill fluid.

At 4mA Case ( Zero interface level ) :-

In this case the entire portion we are measuring will be filled with “Lighter fluid” .

So as per our formula

Pressure at HP tapping = Height X Specific gravity

Pressure at HP tapping = 100 inches X 0.5 = 50 inH2O

(One prerequisite recommended for this system is to fill LP impulse lines with Heavier fill fluid to get stable reading at 20mA )

Hence

Pressure at LP tapping = Height X Specific gravity

Pressure at LP tapping = 100 inches X 1 = 100 inH2O

Differential pressure of Transmitter = H.P – L.P

Differential pressure of Transmitter = 50 inH2O – 100 inH2O = -50 inH2O

It means at -50 inH2O transmitter will give 4mA

Hurry we are already half way !!!!!

At 20mA Case ( When tank is full with Heavy fluid)

As per our formula

Pressure at HP tapping = Height X Specific gravity

Pressure at HP tapping = 100 inches X 1 =100 inH2O

Pressure at LP tapping = Height X Specific gravity

Pressure at LP tapping = 100 inches X 1 =100 inH2O (as LP tapping is filled with Heavier fluid)

Differential pressure of Transmitter = H.P – L.P

Differential pressure of Transmitter = 100 inH2O – 100 inH2O = 0 inH2O

It means at 0 inH2O transmitter will give 20mA

So here’s our calibration range “-50 inH₂O to 0 inH₂O”

For Extra information, what if we need to know what would transmitter give at 50% of Interface

Case 3 ( 50% is filled with Lighter fluid and 50% is filled with heavier fluid)

50 % of transmitter output will be 12mA transmitter at-25inH2O (Half of total calibration range -50inH2O to 0inH2O )

Interface = { (D.P by transmitter – Lower calibration range)/Span } X Total C-C distance

Here C-C distance nothing but center to center of measuring distance, as shown in above picture.

I = { ( -25 inH2O – (-50 inH2O) ) X 50inH2O } X 100 inH2O

Interface = 50 inH2O

Cross checking

Our total distance is 100 inches of H2O

When interface is at 50% means Interface between Lighter and heavier fluid will be at 50 inH2O

Thanks for reading !

Author : Asad Shaikh

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