# Venturi tube help needed in determining pressure difference

1. May 20, 2011

### timothy1875

1. The problem statement, all variables and given/known data

Water flows through venturi tube at 0.3 litres/s. Diameter of venturi tube is 32mm to 20mm to 32mm and the water density is 1000kg/m^3.

2. Relevant equations

a) Assuming head loss is neglible, what is pressure difference between 1 and 2.

b) If pressure head difference between these is 45mm what is head loss.

3. The attempt at a solution

So far I have V2 = P1A1V1/P2A2 = 0.9548 and V1 (Q/A) = 0.3731

delta H 1/2 = H1 - H2 = 1/2g (V2^2-V1^2))
delta H 1/2 = 39.3mm

Where do I go from here, if this is even correct ?

Thanks a lot for your help.

2. May 20, 2011

### tiny-tim

welcome to pf!

hi timothy1875! welcome to pf!

(try using the X2 and X2 icons just above the Reply box )

use the fact that the volume per time is constant, and Bernoulli's equation

3. May 20, 2011

### timothy1875

Hi Tim,

Thanks for getting back to me.I am trying to answer the question but think I may be going about it the wrong way.

Using the Bernoulli equation do I need to transpose to find P ?

4. May 20, 2011

### tiny-tim

show us what you have so far

5. May 20, 2011

### timothy1875

V2 = P1A1V1/P2A2 = 0.9548 and V1 (Q/A) = 0.3731

delta H 1/2 = H1 - H2 = 1/2g (V22-V12)
delta H 1/2 = 39.3mm

This where I have got to so far, :(

6. May 20, 2011

### tiny-tim

what does this mean?

where does it come from?

7. May 20, 2011

### timothy1875

Basically we have been set this question and there are numerous equations on the back that we have to pick from. I take it from your response I have selected the wrong equation initially. Should it be 1/2g (V22 - V1 2) ??