Practice Tips #50: Dental Unit Cross-Over (Part 1)

Practice Tips #50 asks what is dental unit cross-over? What causes it? How do you fix it? And no, we don’t mean a 4 wheel drive vehicle that combines the features of a sedan, wagon, and SUV.

As we discussed way back in Practice Tips #3 Dental Pneumatics, most dental units run on air and require air for just about everything, including turning on the water (which is “air activated”). This means there are valves inside the unit which have both air and water flowing to (and possibly through) them. A failure in one of these valves can lead to cross-over, getting the air in the water line or water in the air line.

Verify Cross-Over

If you have water in the air line, start by checking at the compressor.

• Do you have good dry air coming out of the compressor? If the air is moist right at the compressor, the problem is at the compressor itself and you don’t have cross-over. Check the filters, dryer, and drain the tank. If the problem persists, additional trouble-shooting of your compressor may be required.
• If the air is dry at the compressor, then you probably have cross-over. Now you need to figure out where it’s happening.

Are you experiencing cross-over in just one room or several?

• If it’s in just one room, that will narrow things down considerably.
• If it’s in more than one, you could have multiple failures or you could have water or air working its way through the lines from one room to the others.
• Either way, we’ll next want to check within one room that’s affected.

Do you have trouble with the handpiece drive air?

• If you've got water in the drive air, you'll often notice that water drips out the bottom of the unit when you use your handpiece. The drive air typically exhausts into the unit. 

Are you getting cross-over in the air/water syringe?

• The syringe air and water doesn’t typically go through many valves. If you’ve got cross-over here, it’s likely from a failure within the syringe itself or in the master valves. See Practice Tips #23 ”Getting to Know Your Utility Center.”
• If cross-over occures in the syringe itself, this would be isolated to the syringe. Check for cross-over elsewhere.
• Do you have an assistant's side syringe as well? Is it affected?
• If it’s just one syringe, the most likely culprit is the small internal o-ring (#01-06). If this o-ring is missing or damaged, you can get cross-over in your syringe.

• If the water just fails to shut off, you’ve probably got a worn button or valve core. Check our previous issues on syringe repair and syringe troubleshooting for more information on resolving these issues.

Using an Autoclavable Syringe?

If using a certain brand of an autoclavable syringe, such as A-dec, check the manifold inside the handle. Its head attaches to this manifold and secures in place when the handle threads on. Its manifold has two male connectors that are off-center and correspond to two holes on the bottom of the syringe.

• One port is for air, the other for water.
• These ports are off-center, so they will only connect to the syringe if properly aligned. In theory, this will prevent cross-over, however in reality it can cause it.
• If the ports are not properly aligned, the handle can still screw onto the head of the syringe. However the air and water will just flow into the cavity between the manifold and the syringe head. This creates a constant mix of air and water in the syringe handle, which yields cross over in the syringe. This particular source leads to air or water flowing back down the lines and yielding cross over in other items in the delivery unit as well.

If you are getting cross over and have autoclavable syringes, always make certain that the syringe head securely attaches to the manifold before proceeding with other checks. If for no other reason, this is a very quick and simple thing to check. 

PHOTO: Shows an A-dec syringe handle with the manifold.

Other Potential Areas of Dental Unit Cross-Over:

• Every air activated valve in the delivery unit is a potential source of cross-over, and all of them should be checked.
• As mentioned previously, if you’re getting cross-over in the syringe, you could also have a failure in your master valve. The master valve is one of the most common sources of dental unit cross-over we encounter. There is a diaphragm on the master valves that corresponds to the air activation. Usually the diaphragm is under a square cover on the valve. This will also be where an air line attaches to the valve.

Common Master Valve Designs (shown with the location of the diaphragm):

NOTE: The square covers held on with screws at the corners in the second and third photos. This type of design is very common on master valves. Look for a square cover on your master to find the diaphragm.

The third photo is of an A-dec water master. Notice the yellow tubing with a red dash? This is the A-dec color-code for an air activation line (air out from the master switch on the front of the unit). The diaphragm will usually be near where this line attaches (another thing to look for to correctly identify where your diaphragm is).

American Dental Accessories, Inc. carries master valve repair kits which allows you to replace the diaphragm. In addition, the repair kits include o-rings, valve stems, and springs. We recommend replacing all of these components if you’re accessing the valve interior. You can also see a manual shut-off in the third photo. The master valve normally attaches directly to the manual shut-off. You will want to use the manual shut-offs to turn the air and water off at the delivery unit before servicing the master valve(s).

It is also common to have a relief hole in the side of the master valve just above the diaphragm. If water is coming out of this hole, this is another indication of a ruptured diaphragm. A worn or damaged diaphragm in the water master can also lead to getting air in your water line.

In the next Practice Tips, we cover more valves that could be the source of dental unit cross-over. As well as details on how to narrow down which operatory might be the source of cross-over and the effects of air in the water line. All of this and more, next month!