Robert Lange, a PADI Tec Trimix Instructor, explains how he does positive and negative pressure checks on normal open circuit scuba gear.
Before you go out with your favourite push-bike you would always check the tyre pressures to make sure your journey will start nice and smoothly. Having the correct tyre pressure and double checking for leaks helps you avoid the tyres developing a problem during your journey.
We perform a similar pressure check with our regulators and Buoyancy Control Device before diving. We set both systems under pressure and close the valve. The regulators and the BCD should retain the pressure until we release it. If the regulator gauge starts moving we should have a close look and find the leak. After curing the leak, repeat the check until the system holds the pressure for at least three minutes. The BCD should also hold the pressure for at least three minutes.
We should only dive when the gear is perfect and has no leaks.
There is no such thing like the saying ‘little bubbles – little troubles’.
Even the smallest leak can become a serious problem under water. This test is called a ‘positive pressure test’and should be performed for any single dive.
The second check we perform on the regulators is called the ‘negative pressure test’.
After the regulator has passed the positive pressure test we set the regulator under pressure and close the valve once more. We breathe through the main regulator until the gauge is showing zero bar of pressure and then attempt to suck gas through the system. You should not be able to suck gas through the system when the valve is closed. We repeat the same test for the alternative gas source and making sure both regulators are sealed.
If you can breathe through the system while the valve is closed, (although it is quite difficult to do so) the most likely reason is a problem with the exhalation valve and you need to find the problem and fix it. Breathing through a faulty regulator can cause salt water aspiration syndrome and water can develop on your lung. This may block the lungs from absorbing oxygen. We recommend performing both tests before any dive.
Please feel free to pass on this information to your instructors, students, and dive buddies.
We hope this information makes scuba diving a little safer for everyone.
Robert Lange PADI TecRec Trimix Instructor