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Water flow venturi meter problem! Heeeelp please

 i have worked out the experimental rh but i dont know how to find out the therotical rh, and im not sure what Cb (show on results table is).
I am not quite sure what your question refers to. What is "rh?" What is "Cb?" Do you mean Cd (which is the discharge coefficient)? Also, I am having a lot of troubles reading your results page that you scanned in.

So...let's talk in general.

The theoretical flow through the venturi is given by the equation you have already mentioned:

$$M = A_{2} \sqrt{\frac{2 \Delta P \rho}{1-k^2}}$$

That is the theoretical flow (in a perfect world). With the measurements you have you should be able to calculate both the theoretical and what actually happened. There will be a difference. That difference is the discharge coefficient of the venturi:

$$M_{actual} = M_{theoretical} C_{d}$$

I guess the first big question is do you understand what the data you took is for and where it fits into the venturi equation?