"All manometers and pressure switches must be mounted on the standard flange of a threeway valve,with a needle valve. The using of a gate valve is forbidden." The reason for this is that needle valves are more reliable than gate valves because of its construction. The needle valve is fitted in between the three way valve and the manometer/pressure switch, so that either the pressureswitch or the manometer could be used whenever required for checking the pressure of flow through the threeway valve.