What does "constant pressure, variable orifice" indicate about mechanical flow meters?

• A. The pressure drop varies while the orifice size remains constant
• B. The pressure drop is constant as the orifice size changes
• C. The orifice size remains unchanged regardless of pressure
• D. The flow is equalized across all types of gases

Answer: (B)

The phrase "constant pressure, variable orifice" indicates a relationship in which the pressure remains steady while the size of the orifice can change. This means that as the orifice size varies, it affects the flow rate through the meter while keeping the pressure drop consistent.

When the orifice increases in size, the flow rate will also increase due to the lower resistance to flow. Conversely, if the orifice decreases in size, the flow rate decreases, but the pressure drop across the orifice remains constant. This principle is used in mechanical flow meters to provide accurate flow measurements under varying conditions.

This concept is fundamental when considering how flow meters are calibrated and function in practice, particularly in anesthesia machines where accurate gas flow is critical for patient safety and effective anesthetic administration.