[Aorus]

My fluid mechanics textbook is rather shoddy, and I can't find any decent information online regarding my question.

I'm wondering what factors affect the accuracy of readings taken from these two types of meters? More specifically, why are Venturi meters more accurate than orifice meters? This is assuming that the Reynolds number is sufficiently large (i.e. Re = 0.60 for the orifice meter).

I know that head losses are obviously greater in the orifice meter, but I would think that the flow coefficient, C_d, accounts for any error this would introduce.

The equation I'm going by is V₂=C_d[2(P₁-P₂)/p(1-B⁴)]
where V₂ is the velocity at the neck/orifice, p=density, B=ratio of neck/orifice diameter to pipe diameter.

Help is much appreciated!

[eugenedakin]

Hi Aorus,

I think that you very close to the answer.

Ok.. step back... and stop thinking of formula's for a second.

First part: Now, when the reynolds number increases, this means we are in turbulent flow.  Fluid is no longer flowing in a nice symmetrical pattern.  In fact, some of the fluid is turning back on itself and 'fighting' against the flow.  No matter what type of constriction you have, turbulent flow will always be less efficient as laminar flow (from a flow-efficiency point-of-view).

Last part: Most venturi's that I have seen have a nice slow constriction into the centre of the venturi, and a nice slow slow away from the constriction.  Most orifice's that I have seen have sharp constrictions and sharp slopes away from the centre. I am simplifying this next part: Venturi's tend to promote laminar flow due to the slower transitions from one part of the venturi than the other -when compared to an orifice, there tends to be sharper transitions which tend to promote turbulent flow.

When a fluid is in turbulent flow, it is extremely difficult to model the effects because we have to make many assumptions (how many flow eddies are turning backwards against flow in turbulent flow?).  With turbulent flow, there are sharper pressure increases and decreases because of turbulent flow. Venturi's have more laminar flow which have less pressure fluctuations.

If you had to read a pressure gauge in turbulent flow, the dial would be vibrating back-and-forth, and you would have to guess at the pressure reading.  If you read a pressure gauge in laminar flow, it would stay in one position, and be very easy to read.

There is the answer - accuracy

I hope this helps.

Sincerely,

Eugene