Not in the strict sense. A dP transmitter just reads the pressure difference between the bottom tap and the top tap. That is then converted into a level using the liquid density. The vapor density might also be used but in most cases, it's so much less than the liquid density that its effect is minor. Lots of things can throw this off, the densities you are using versus actual, foam on top of the liquid, another liquid phase (say a HC layer on top of amine), etc.
I have a feeling there's more to your question than you've posed.
It can be used to measure interface level if both taps stay full submerged and the sense lines are purged with an appropriate fluid. If the total fluid level is held constant, then the upper tap could be in the vapor space.
Under fixed (and many times, not continuous) conditions, the answer is a qualified yes; DP can provide an inferred interface level reading.
In reality, a DP measurement is the ratio of the head pressure (heavier/lighter) that each of the two fluids contributes to the total head pressure. That ratio, as a percentage, indicates the interface elevation above the lower tap.
The required fixed conditions are
- known and stable densities of the heavier and lighter fluids.
- level must remain at or above the upper tap, or at a fixed level below the upper tap (sure . . .)
The taps are a fixed elevation distance apart which constitutes the 100% range for the interface level.
When the level between high and low taps is 100% lighter fluid, the head pressure span*lighter fluid Specific Gravity (SG) becomes interface LRV/minimum/zero %/4mA value.
When the level between high and low taps is 100% heavier fluid, the head pressure span*heavy fluid SG becomes the interface URV/maximum/100%/20mA value.
When the interface of the two liquids is half way between the taps, then 50% of the head pressure is contributed by the lighter fluid, 50% by the heavier fluid and the resulting output is 50%/12mA.
Issues are impulse leg fill fluid contamination or diaphragm seal temperature response.
@danw2: How will the transmitter know the percentages? I know that it only measure pressure difference, how come it can know the percentage?
Is the formula like this?
dP = x1*rho1*g*h1 + (1-x1)*rho2*g*h2
where x1 and x2 are mass fraction.
However, there is 2 unknowns... x1 and h1...
And what if the liquid level became higher than the 2nd transmitter?
@Compositepro: I think tapping it in the vapor space will not really read interface. Maybe you are indicating interface between liquid and vapor. I'm considering interface between oil and water.
To answer your other question, X1 and X2 should both be 100%. The DP measurement technique will not work if the fluids mix or do not have constant density.
> I know that it only measure pressure difference
> And what if the liquid level became higher than the 2nd transmitter?
Correct that a DP measures pressure difference between the low side and the high side. A DP subtracts the low side pressure from the high side pressure.
But there's only one transmitter with two pressure ports, a low side and a high side (not a 2nd transmitter). If the fluid rises above the top tap, it will exert an additional head pressure equally on both the lower tap and the upper tap. But a DP subtracts the low side pressure from the high side pressure, so an additional head pressure applied equally to both the high and low sides is zeroed out in the subtraction.
>How will the transmitter know the percentages?
Here's an example:
The taps are 1000mm apart in elevation.
The transmitter reads in mm wc (water column)
The 1000mm column is filled with 0.80 SG fluid = 800mm w.c.head pressure
Then the 1000mm column is filled with 1.10 SG fluid = 1100mm w.c.head pressure
If the transmitter is ranged for 800mm w.c. = LRV/zero and 1100mm w.c. is URV/100%, then the span is 0 to 300mm w.c.
If the column is filled with lighter fluid its head pressure of 800mm wc is zero output (because 800mm wc = zero output), meaning zero percent heavy fluid.
If the column is filled with 100% heavy fluid, its head pressure of 1100mm wc is 100% output, meaning 100% heavy fluid.
The example for half one fluid, half the other:
500 mm of the heavier fluid will contribute 550mm wc head pressure on the 1000mm column
500mm of the lighter fluid will contribute 400mm wc head pressure on the 1000mm column. The total of these two contributing head pressures is 950 mm w.c.
950mm w.c. minus 800mm w.c. = 150mm w.c. head pressure on a range of 0-300mm w.c. which is 50%, the mix of the two fluids.
I agree with CompositePro. Having the high tap in the vapor space above the liquid mix requires a constant upper level to read the liquid/liquid interface level.
You could use dP for interface, I suppose. If you knew that the interface was somewhere between the taps and you knew what the two fluid densities were, I think it would be a "lever rule" kind of calculation to locate the interface.
That said...
What is wrong with capacitance or guided wave radar?
You can measure interface with a DP transmitter IF you also know the overall level, either by means of a 2nd DP transmitter or another type- AND you accurately know the density of both phases (i.e. you know their temperatures and compositions). You can only make the measurements if the interface remains between the correct taps.
You do not need to purge the impulse legs unless there's a risk of plugging.
Guided wave radar or capacitance give better results for interface. Some GWR units will give both level and interface from one transmitter (Magnetrol in particular), so the installed cost is similar. If the DP units need remote diaphragm seals, the GWR will be both better and cheaper.
If the column is filled with 100% heavy fluid, its head pressure produces a 100% output, so the (non-existent) interface level will be at the highest elevation: 100%.
0% and 100% both represent the lack of an interface or the presence of 100% of either the heavier or lighter fluid.
