Discover the magic of the holder valve. Proper air pressure and the many functions of your delivery system rely on accurate pressure to operate. We talked about this in previous Practice Tips. This issue discusses the differences between "normally open" vs "normally closed" holder valves.
PHOTO: A typical holder valve (#10-10).
Automatic Delivery Systems
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. Control the activation of the handpiece 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 valves in these systems: normally open and normally closed.
PHOTO: Handpiece holder (#10-03) with access to the holder valve underneath (#10-10).
Normally Closed Systems
In a normally closed system, the valve closes when you remove the handpiece from the holder. This stops the flow of air to the block, which pushes down on a diaphragm to prevent the handpiece from running.
This type of system was employed by A-dec for many years and implemented in other brands of current systems. A-dec 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.
PHOTO: Normally closed systems provide air flow to prevent a handpiece from running when in the holder.
Normally Open Systems
In a normally open system, the valve opens when you remove the handpiece from the holder. This allows 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.
PHOTO: Normally Open Systems
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 magic of the holder valve helps to determine between a normally open or closed holder valve. It is important to locate where air flows through the valve. Is it when you hang the handpiece up or is it when you lift the handpiece up?
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