Proper air pressure and the many functions of your delivery system rely on accurate pressure to operate— we've talked about this in previous Practice Tips. Today's issue will discuss the differences between "normally open" vs "normally closed" holder valves.
Most delivery systems are “automatic” systems. Being automatic means that whichever handpiece you pick up is the one that runs when you step on the foot control. The activation of the handpiece is controlled by a valve in the handpiece holder. The valve in the holder allows air to flow through and into the handpiece block to control activation. These valves will have two hoses attached to them – one for air flowing in, the other for air to flow out (and to the handpiece block). There are two different configurations of valve used with these systems, referred to as “normally open” and “normally closed.”
In a normally closed system, the valve closes when the handpiece is removed from the holder—stopping the flow of air to the block, which is pushing down on a diaphragm to prevent the handpiece from running. This type of system was employed by A-dec for many years and is often employed in other brands of current systems (A-dec has moved to a relief valve system that only has one hose attached to it). Some common brands that use this type of valve are A-dec (prior to c. 1992), Beaverstate, Belmont, DCI, Engle, Forest, Marus, and Pelton & Crane. The normally closed valve is more commonly used than the normally open valve. See below for a diagram of this system.
In a normally open system, the valve opens when the handpiece is removed from the holder—allowing air to flow through the valve. In these systems, the air flows into the block and pushes up on the diaphragm to allow the handpiece to run. The only brand we’re familiar with that uses normally open valves is Dental-Ez, but there are certainly others. See below for a diagram of this system.
NOTE: In both types of systems, the handpiece runs when it is removed from the holder. A unit that doesn’t allow the handpiece to run when removed from the holder wouldn’t make much sense, would it?
The key to determining between a normally open or closed holder valve: determine when air flows through the valve. Is it when you hang the handpiece up or is it when you lift the handpiece up?