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  • Rick

Curing the Cross-over Conundrum

Updated: Jun 25, 2023


Sometimes mixing things is good, but sometimes it can cause lots of problems and not everyone likes chocolate in their peanut butter.


For cooling, a mist is often desired which is why your handpiece has air and water mixed to provide coolant. But if you're trying to dry a prep, you don't want water in your air line.


Cross-over is simply defined as the undesired mix of air and water within a dental delivery system. Water in the air line can not only prevent proper drying, but can also impede the performance of your handpieces, wear out turbines prematurely, and even lead to failure of valves to operate.


Water in the air line isn't always from cross-over, so if you have water in your air line, you should eliminate some more simple possibilities first. One of the most common places to encounter water in the air line is in the air water syringe. If you hit just the air button and get mist, you've got something going wrong. Disconnect the air line from the bottom of the syringe and blow the air into a tissue to check for moisture. If you have moisture within the line, it's time to start trouble-shooting the delivery system. If the air is dry there, you'll need to re-build (or replace) your air water syringe. We'll cover syringe repair in other posts.


Another common symptom of water in the air line is water inexplicably dripping out of the bottom of your delivery system. This is often caused by water in the drive air which then comes out the handpiece exhaust inside the delivery system.


If you find water in the air line in more than one room, you should check your compressor. Most compressors will have a moisture indicator to show if moisture is present. Check yours. Typically, they go from blue (ok) to pink (wet) but other colors are possible. The indicator will often have the color code imprinted on it, otherwise check your owner's manual.

You can also open up the air line at the compressor and allow air to blow onto a tissue to check for moisture.


If you have moisture at the compressor, start by draining the tank. Usually there will be a petcock or similar manual valve on the underside of the tank. Just open it up to let any moisture out. Many modern compressors have auto drain features and there are also such things you can retrofit. Regardless of how it is accomplished, it's not a bad idea to open up your compressor tank once a week to drain any moisture. Most of the time, it should be dry.

In addition, your compressor will have filters to clean and dry the air. If oil lubricated, there will also be a coalescent filter to remove oil. These all generally have indicators to show when they are saturated. Check them monthly. Most filters should be replaced annually.

If your compressor checks out ok, you should then move on to the delivery systems as you probably have a ruptured diaphragm. It is more common for a ruptured diaphragm to lead to air in the water line (rather than water in the air), however.


Air in the water line can lead to intermittent water flow, spitting and sputtering of the water, failure of the water to shut off, and (in extreme cases) even squealing pipes. If you have any of these symptoms, you almost certainly have one or more ruptured diaphragms. To determine where you have a ruptured diaphragm, perform the "syringe test" below, this will narrow down which room(s) may have a ruptured diaphragm:


First, turn off the water to each room using the manual shut-off found on your main water line. This is a knob just like you'd find under your bathroom sink, or at the toilet (another common name for this valve is "toilet valve"). This is only if connected to the municipal water supply. If your units are ONLY supplied with water from bottles, remove the bottle from the system and flip the toggle on the bottle to "off" before proceeding to the next step.


Next with the water turned off, take your air water syringe and empty the water from the line by depressing the water button. Once the water is exhausted, take the syringe and submerge the syringe tip in a cup of water. With the syringe tip submerged, hit & hold the WATER button again. Look for bubbles. If you have bubbles, this means you have air in the water line in that room and have a ruptured diaphragm in that room (note this is the case regardless of what symptom you may have been originally experiencing, water in the air or air in the water).


Repeat the above steps in EACH room with the water turned off. If connected to the municipal supply, it's extremely important to turn off the water manual shut-off in each room. Not only to remove the supply of water, but to isolate each room from the others. Make a note of which room(s) yield bubbles with the syringe test and thus, have one or more ruptured diaphragms.


Unfortunately, there is no easy way to determine which diaphragm has failed, but fortunately, there are normally only a few valves with diaphragms in any unit so it still shouldn't take too long to figure out. A few such valves follow:


One of the most likely valves in the water master valve in your junction box (where air and water come into the room). The diaphragm of a master valve will be beneath a square cap of some sort, with an air line attached to it (most often of red or yellow, but other colors are possible). The master valve will be attached directly to the manual shut-off, a standard plumbing fitting with a knob just like you find under the bathroom sink.



Note the gauge on the valve, not all masters will have a gauge directly attached to them, but most will have a gauge connected, sometimes at the end of a short piece of tubing. Also note the knurled cylinder just before where the valve attaches to the manual shut-off. This is the filter housing. Most filters are in some sort of cylindrical housing, check yours regularly (quarterly is usually good). Almost all masters have some sort of filter assembly attached, so look for a cylindrical filter assembly to help identify your master valve as well.




Above is a Beaverstate Delivery System. The three blue and white valves on the right are the water relay valves. These turn the water to each handpiece on and off. A rupture in the diaphragm of one of these valves could lead to cross-over. It's a bit difficult to see in the photo, but these are typical in many ways of most water relay valves: they have 3 tubes attached, two for water and one for air. When looking for your water relay, look for a valve with 3 tubes attached to it. Generally, 2 of one color and a 3rd of a different color. The Beaverstate valves have 2 blue and one clear line, for example.


The large rectangular chrome valve near the center of the photo is the handpiece block. A rupture of the diaphragm in this valve could also cause cross-over in the above system, but would never lead to cross-over in some other brands of delivery system, such as DCI or Marus as these brands do not have water going through the handpiece block. Handpiece block diaphragms are rarely the cause of cross-over regardless of brand (such a rupture simply isn't that common). Nonetheless, it is possible and certainly warrants investigation if other possibilities have been eliminated.


Many cuspidors have air activated water valves of a design similar to the master valve. If you have cuspidors in the office, check the valves in them as well as they could also lead to cross-over.


Tracing down the source of cross-over can be time consuming, but it isn't complex nor difficult and is something you and your staff can certainly accomplish. Once you've traced the source, repairing is comparatively simple.









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