Peter Gluck has been babbling on about how it is impossible to make a mistake with a flow meter. I pointed him to a handy guide to flow meters from Omega, which he then ignored. Anyway, let me point it out here, because it is handy. If you are going to work with flowmeters you should read this carefully. I wish I had read it more carefully years ago.
http://www.omega.com/prodinfo/flowmeters.html Below are some quotes from it. If you deliberately install a flowmeter with one of these parameters wrong, such as bubbles for a meter not designed for them, or "whether the flow can reverse" (Defkalion's trick), you get the wrong answer. Fluid and flow characteristicsThe fluid and its given and its pressure, temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (Newtonian or not?) and vapor pressure at maximum operating temperature are listed, together with an indication of how these properties might vary or interact. In addition, all safety or toxicity information should be provided, together with detailed data on the fluid's composition, presence of bubbles, solids (abrasive or soft, size of particles, fibers), tendency to coat, and light transmission qualities (opaque, translucent or transparent?). Pressure & Temperature RangesExpected minimum and maximum pressure and temperature values should be given in addition to the normal operating values when selecting flow meters. Whether flow can reverse, whether it does not always fill the pipe, whether slug flow can develop (air-solids-liquid), whether aeration or pulsation is likely, whether sudden temperature changes can occur, or whether special precautions are needed during cleaning and maintenance, these facts, too, should be stated. The sections on "Piping and Installation Area" and "Flow Rates and Accuracy" shows many more ways to screw up. As I said, if you are a nefarious person and you want to get the wrong answer, this guide is a bonanza of information on how to cheat. Piping and Installation AreaConcerning the piping and the area where the flow meters are to be located, consider: For the piping, its direction (avoid downward flow in liquid applications), size, material, schedule, flange-pressure rating, accessibility, up or downstream turns, valves, regulators, and available straight-pipe run lengths. The specifying engineer must know if vibration or magnetic fields are present or possible in the area, if electric or pneumatic power is available, if the area is classified for explosion hazards, or if there are other special requirements such as compliance with sanitary or clean-in-place (CIP) regulations. Flow rates and AccuracyThe next step is to determine the required meter range by identifying minimum and maximum flows (mass or volumetric) that will be measured. After that, the required flow measurement accuracy is determined. Typically accuracy is specified in percentage of actual reading (AR), in percentage of calibrated span (CS), or in percentage of full scale (FS) units. . . .
