I would like to measure air flow rate. I have a orificemeter but do not know its coeficient of discharge. Will the Cd remain same for both the liquid and gases or it will be different.?
The discharge coefficient is a function of many factors, therefore, it will not be the same for liquids and gases. The discharge coefficient is a ratio between the effective flow area and geometric flow area of an opening. The effective area changes based on fluid properties and compressibility effects. http://kb.eng-software.com/eskb/ask-an-engineer/theory-equations-and-calculated-results-questions/relationship-between-flow-coefficient-and-discharge-coefficient
If you want to test, you can use this online FREE orifice plate calculator..