Differential pressure (DP) transmitters are used to measure the interface of two fluids that have different specific gravities (S1 & S2). To make an interface measurement, the overall level must be at or above the low pressure tap at all times. It is important that the level be large enough to create a reasonable DP between the two specific gravity extremes. This measurement can be done with or without remote seals.
However, from a maintenance point of view, it may be easier to use a remote seal assembly; keeping the wet leg at a constant height can be difficult in some applications.
To determine the calibrated range for the transmitter, four assumptions need to be made:
 At the lower calibrated value, 4 mA point, the tank is filled with the lighter fluid.
 At the upper calibrated value, 20 mA point, the tank is filled with the heavier fluid.
 The taps leading to the transmitter are flooded at all times. The overall level should be equal to or higher than the upper (low pressure) tap.
 There is always a reference level seen by the low pressure side. This can be accomplished with either a remote seal system or a wet leg. The reference level must have a constant height and density. The calculation is modified slightly for wet leg systems when the density of the high and low pressure wet legs differ.
The measured level is composed of a combination of the two fluids:
L = L1S1 + L2S2
When the tank is filled with the lighter fluid, the transmitter will be at 4 mA (or 0% of span) and L2 = L:
HP = L2S2 + dSf
LP = dSf + hSf
At 4 mA, DP = HP – LP = L2S2 – hSf
When the tank is filled with the heavier fluid, the transmitter will be at 20 mA (or 100% of span) and L1 = L:
HP = L1S1 + dSf
LP = dSf + hSf
At 20 mA, DP = HP – LP = LS1 – hSf
Interface Level Measurement

Calculation for interface measurement 
The formulas used to calculate calibration ranges of a DP transmitter for interface measurement application
∆Pmax or 20ma = LS1 – HSf
Using the above tools, we are calculated the transmitter ranges. Now calibrate the transmitter and take into line. Now note down the present DP value of transmitter for calculating the interface level.
Formula for interface level measurement 
LRV = Lower range value of transmitter in DP scale
I = Interface level
Interface Measurement using DP Transmitters
Advantages
 Low cost
 Simple to install
 No additional components required
Limitations
 Span must be sufficiently large enough to measure; either the distance between taps or the specific grabity difference m7ust be large
 Upper tap must be covered at all times.
 Small spans are susceptible to temperature induced errors.
Example Problem:
A vessel requires an interface measurement where the level is 150 inches. The vessel has two fluids with specific gravities of 1.0 and 1.08.
At 4mA, DP = L2S2 – hSf
DP = (150 x 1.0) – (150 x 0.934)
So at 4 mA = 9.9 inH2O DP
At 20mA, DP = LS1 – hSf
DP = (150 x 1.08) – (150 x 0.934)
So at 20 mA = 21.9 inH2O DP
The calibrated span is 9.9 to 21.9 inH20. When the transmitter reads 9.9 inH20, the tank is filled with the lighter fluid. When the transmitter reads 21.9 inH20, the tank is filled with the heavier fluid.
To determine where the interface of a mixture of fluids is, use the DP reading as a percent of span. For example, if the transmitter output is 18.4 mA or a DP of 20.7 inH2O, the interface is calculated by using the formula:
13 comments
Really helped me…Sharing is good.
Hi,
Can you please share interface level measurement using radar.
Hi, Vardhan,
Interface level measurement using radar tool will be posted soon.
Thanks,
S Bharadwaj Reddy
Mind Blowing tools dude. Thanks for sharing these applications.
Could you please explain,how to calculate the range if the transmitter is installed above the HP side
thanks n advance
Sajeesh sathyapalan
why you are not using d value in calculation
Hello Ritesh, The Post is updated with detail information. Thanks for the query.
Regarding remote seal or wet leg, assumption no. 4 said the tube must always be filled with the liquid and the liquid should has constant density. I’m confuse, if the tube always be filled with known liquid, means that at the LP side, the pressure is constant even though no liquid in the tank, because of the liquid in the tube?
Hello, Yes, Even though there was no level in the tank, the Wet leg or LP side always pressure will be there and in negative number. In calibration or range calculation we compensate this extra pressure. Click Here to read the article.
There is an text error in your formula calculator, please change it , S2 is lighter fluid but typed heaver fluid with it in calculator.
Dear sir,
thanks for the sharing, I have one doubt regarding to interface level formula that Can u please explain in detail that how this formula came? then it will be very helpful for us.
For non interface the leg in the HP side allows the process liquid, but in interface type, both legs are filled with same leg liquid, am I right ? Plz make my doubt clear.
your calculations did not include the height d.
for eg, at 4 mA it’s supposed to be (L2S2 + dSf) – (d+h)Sf, right?