Pressure difference in two pipelines

[Link]

Homework Statement

A water manometer is connected to the wall of two pipelines in order to measure pressure difference between the pipes. Manometer shows a 0.5m height difference, pipe B is 3m higher than pipe A. Find pressure difference between pipes.
[PLAIN]http://img191.imageshack.us/img191/9528/fluid.jpg


Density water: 1000kg/m3
Density Oil: 850kg/m3

Homework Equations

Pressure difference = density of fluid x height difference x 9.8

The Attempt at a Solution

From the illustration it is evident that pipe B will have lower pressure than pipe A since the oil pressure in pipe A can depress the water in manometer to a certain height even as Pipe B is mounted higher than pipe A.

If the two pipes were located at the same height, the manometer alone would show the true pressure difference:
Pressure difference manometer = 1000 x 0.5 x 9.8 = 4900 Pa

Since pipe B is mounted 3m higher than pipe A we need to add this pressure difference:
Pressure difference altitude = 850 x 3 x 9.8 = 24990 Pa

Total pressure difference is the sum of these two: 24990+4900 = 29890 Pa.


The correct answer is 25751 Pa.

My reasoning must be wrong somewhere?

 

[ben133]

V.quickly I get 25725 Pa as the height difference is actually 2.5m due to pipe B having more water in it than pipe A (0.5m more). If you made the difference 2.50312 you'd get the answer given, maybe the error is due to assuming a flat fluid boundary instead of a meniscus, but that's just a wild guess sorry.