**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 inH_{2}O to 0 inH_{2}O”

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

Profile : Linkedin

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