Can you dive deeper than 140fsw with nitrox?

• A. No, 140 fsw is the limit
• B. Yes, up to 160 fsw with CO permission and surface supplied diving
• C. Up to 200 fsw with Nitrox permit
• D. Only with training.

Answer: (B)

Oxygen partial pressure at depth controls how deep you can safely dive on nitrox. Increasing the nitrox percentage raises the amount of oxygen you’re breathing, and as you go deeper the same mix pushes that oxygen pressure higher, which raises the risk of oxygen toxicity. In recreational nitrox, you manage this by sticking to mixes and depth limits that keep the partial pressure of oxygen within safe ranges (often around 1.4 to 1.6 atm, depending on policy).

The reason this particular scenario allows going deeper than the usual recreational limit is that, with authorization from the commanding officer and using surface-supplied diving, you can tailor and control the gas on the surface for the planned depth. Surface-supplied setups enable precise gas blending and higher safety margins, so a limit of about 160 fsw can be accepted under those controlled conditions. It’s not a general rule for all nitrox diving; it requires the special authorization and the gas-delivery method to manage the oxygen exposure risk at that depth.

Training alone doesn’t automatically grant access to these deeper operations, and going up to 200 fsw would typically require different breathing-gas strategies beyond nitrox (such as trimix).