What is the difference between correlated and direct reading rotameters?

A direct reading flowmeter indicates the flow rate on its scale in specific engineering units (e.g. ml/min or scfh). Direct reading scales are designed for a specific gas or liquid at a given temperature and pressure. While it is more convenient than a correlated flowmeter, a direct reading flowmeter is less accurate and limited in its applications.

A correlated flowmeter is scaled along either a 65mm or a 150mm length, from which a reading is taken. The reading is then compared to a correlation table for a specific gas or liquid. This will give the actual flow in engineering units. One correlated flowmeter can be used with a variety of fluids or gases.