Low flow is one.

Look here for the basics. [PDF]

Fundamentals of Orifice Meter Measurement

orifice meter is most widely used primary flow element.In case of low flow which is below 25% of full scale PV we will get error. If u plot the non linear curve in semilog paper u can easily understand.

Even you study the book, accurate knowledge comes only by experience.

Thanks

Sudip

I&C ENG

2-phase flow is another.

Drainage pipe runs, where there is an air space above the liquid, is a third.

1. When high turndown ratio is required. (Orifice plate has low turndown due to square root function)

2. When low pressure drop is required.

3. Solids suspended in the fluid.

4. For large pipe diameter (due to the cost)

5. Limitation of the striaghtrun (more likely for plant modification, not for the new plant)

6. Gas in liquid or liquid in gas.

7. Some clients prefer to use other types, i.e. Vortex Flowmeter. Client or project specification can tell.