The second function of the ADV is provide a simple means to verify the accuracy of the oxygen sensors during a dive. As mentioned in the Set up chapter, oxygen sensors are unreliable and your best defense is to check them frequently. Additionally, during the course of a dive, every time the ADV triggers the diluent gas from the ADV ends up right in front of the O2 sensors. If you have a look at the display when this happens you will see how the PO2 is affected. A pro active way to really find out if the sensors make sense is to have the diluent FgO2 x ambient pressure pre calculated in 5 m increments on your wrist slate or wet notes. Then go to the nearest depth and exhale half a breath from your nose and inhale slowly. If you inhale very slow for 10 seconds you will get a very accurate reading. If your inhalation is a bit fast you will get a slightly higher reading than actual PO2 due to increased pressure caused by the flow. If you don’t keep the inhalation long enough or interrupted the surrounding loop gas will rush in and affect the reading, again usually higher than actual. With some practice you will learn to keep a steady and slow inhalation and get accurate results. If you are at 40 m with air in your dil your display should read 0.21 x 5 BAR = 1.05 or close to it. Providing this works out, what exactly do you know? You know that your sensors ready accurately at 1.05 PO2 and you get an opportunity to compare the sensors response times against each other. If one cell shows abnormally long time to respond repeat, but this time you inhale forcefully to blow away moisture on the sensor membranes that could cause the slow response time. Next repeat instant cell validation. If the sensor is still slow, make a mental note to replace it, finish the dive on the CCR, but use frequent cell validations and follow the good cell! If in doubt – Bail out. Needless to say if both sensors fail – BAIL OUT!!! As instant cell validation is no guarantee a sensor works fine over its entire range, but only at the Po2 you expose it to it is important to always perform the 3rd point linearity check in the beginning of the dive. This means expose the sensors to a known gas at a known depth. After practicing flushing the loop with O2 at the surface for your 2nd point linearity check you will know it requires 4-5 flushes to get reasonable purity of O2 in the CCR. If you do this step properly and then close the DSV and descend to 6 m while breathing from your off board bail-out regulator or BOV you will bring that known gas sample to a known depth and the result should be around 1.6 BAR Po2 which is quite a bit higher than your recommended set point of 1.2 BAR. This should ensure that each dive is started with a confirmation that the sensors are linear from 0.21 to 1.6 BAR PO2 plus confirm that they are not current limited. During the dive, each time you manually add O2 with the bypass valve the Po2 momentarily over shoots the set point during the initial breaths before the added O2 mixes with the loop gas. Pay attention as this also confirms that the sensors are able to produce a higher PO2 than set point. Exactly how linear they are above the set point cannot be confirmed during the dive, which reinforces the importance to always perform the 3rd point linearity check in the beginning of each dive. Your instructor will make sure you do your linearity checks properly and make frequent use of the instant cell validation during your training dives. The Pelagian DCCCR together with these skills is a concept and a step towards making CCR diving safer.