Negative and or diagnostic positive checks are failing.


If the negative test is failing there could be a hole in the counter-lung. Remove mouthpiece and breathing hose assembly complete and remove the counter-lung. Fit the exhale (green) hose end to the counter lung and breathe through the mouthpiece in closed circuit position until the counter-lung is inflated (positive). Submerge the inflated counter-lung in water and look for leaks. If leaks are noted replace counter-lung.

With the Over Pressure Valve (OPV) cover removed (Note. See FAQ on tight OPV cover) if the valve can be heard to leak on negative then the OPV is bypassing gas. Open the OPV fully (counter-clockwise) hold the canister base upright and fill it with warm water. Let the water flow through the OPV to clean it through, repeat several times and re-test. If the OPV continues to bypass gas on negative test return the OPV/Canister base assembly to Avon Underwater Systems for service.

If greater than normal effort is required to vent the CCR via the OPV/balance tube system during drills such as ‘Partial flood recovery’ test the balance tube one-way valve as follows, remove the balance tube from the OPV cover by unscrewing counter-clockwise. You should then be able to blow through the tube but not suck. If the tube allows gas flow in both directions flush the tube through with warm water several times and re-test. If the tube continues to allow gas flow in both directions contact Avon Underwater Systems to order a new balance tube one-way valve and shrink wrap for fitting.

The negative test could fail due to damaged, dirty or dislodged gas block o-rings. See FAQ on Gas block problems.

The negative test could fail due to damaged, dirty or dislodged isolator O-rings in either the Oxygen or Diluent isolator. See FAQ on Gas block problems.

The negative test could fail after a more complete unit disassembly to replace the CO2 seal or an Oxygen sensor. When the Flow cone is removed from the Sentinel head you will note a radial O-ring on the flow cone and another in the head where the flow cone screws into the head. If during negative test the leak is heard around the centre of the head where the exhale hose enters the head these O-rings should be checked.

Further Resources

Assembly & Testing Document