Niagara on Rebreather

Niagara on Rebreather

by Dave Apperley

It was the documentary video Niagara’s Gold, which was recently shown on television, and persistent harassment from my friend Tim Cashman, that brought about my desire to dive this famous shipwreck. I had first come to know Tim on a cave diving trip in New Zealand two years ago, where I realised his passion for wrecks. After diving one of the best caves in New Zealand and Australia, Tim’s comment was ‘What’s so great about cave diving? You see the same at the bottom as what’s at the top. Bloody rocks!’ With comments like these, I knew I could never sway his preference away from wrecks.

As one of the first two free-swimming divers ever to descend on the Niagara, I used the Buddy Inspiration closed circuit rebreather. This, in my opinion, is the best readily available closed circuit unit on the market today. While basic rebreather design is fairly simple, many manufacturers have added numerous extra features to their models. AP Valves, the makers of the Inspiration, have decided to keep it simple and have only added a few features to add to diver awareness and safety. The unit consists of front-mounted inhalation and exhalation bags, a carbon dioxide scrubber canister, three oxygen sensors and an LCD digital screen for oxygen (O2) readouts.

Mounted on the front of the breathing bags is an O2 addition button and a diluent button. The Inspiration also allows for an O2 setpoint – a ed partial pressure of oxygen that can be maintained throughout the dive by an oxygen solenoid controlled by a computer in the handset. This works by monitoring all three oxygen sensors and taking the readings from the two that are closest together. When the oxygen level in the rebreather’s breathing loop (bags, cannisters, hoses, mouthpieces etc) drops below the setpoint, oxygen is added through the solenoid. As the ratio of oxygen in a mixed gas rebreather changes with your depth, there is a chance of exceeding the oxygen tolerance limits. The Inspiration has a warning buzzer coupled with a visual display to warn when high or low limits are exceeded. A secondary slave computer also monitors the O2 content. Should there be a fault in the master control unit, the diver can compare its readout to the slave.

You are probably wondering which control you believe. During rebreather training and continuing education, we are taught that there are always constants that can be relied on. For example, on a dive to 100 metres I check my master control display and find all the sensors reading 0.9atm. I look at my slave control unit and find all the sensor readings to be 1.1atm. Which one do I believe? Before the dive I have tested my diluent gas and found it to be a 10% O2 content mix. Through the use of Dalton’s Law (before the dive), I know that if I flush the unit with diluent gas my correct O2 sensor reading should be approximately 1.1atm. In this scenario I would manually switch to the slave control unit. Of course, it may be that neither control unit is reading correctly. In this case, a bailout in semi-closed mode would be the next course of action. This means flushing the unit with diluent and breathing for a predetermined number of breaths before reflushing. A failure on a rebreather therefore does not necessarily mean a major catastrophe. There are usually three or four emergency options before having to bail out to an open circuit regulator supply. One of the golden rules of rebreather diving is to always carry enough open circuit gas to safely ascend and decompress.

If you think rebreather diving means you don’t have to carry much equipment, you couldn’t be further from the truth. My Inspiration had been modified to accept two seven litre cylinders as opposed to the standard three litres. I also had three 12-litre cylinders with open circuit regs for decompression purposes in case the rebreather had a major malfunction. Having strapped all this on, I entered the water and remained on the surface to pre-breathe the unit before beginning the dive. This is to start the reaction of the CO2 scrubber, which takes about ten minutes to ‘warm up’.

Tim, diving on open circuit, had basically the same configuration with two 12 litre bottles for bottom gas and three for decompression gas. Our bottom gas was Heliair, 9% oxygen and 57% helium, which gave Tim a partial pressure of oxygen (PO2) of approximately 1.2atm. I decided to run my PO2 setpoint at 1.3atm. I would be breathing a helium mixture all the way down, and all the way back up again to six metres. I can hear all the Trimix divers out there saying ‘No, your decompression will be too long!’ But remember, the rebreather would hold the PO2 at 1.3atm for the whole dive, so I didn’t have to swap to high PO2 gases on the way up.

Finally we were descending effortlessly down a near vertical shotline. The wreck started to come into view at about 80 metres, my first impression being its huge size, and the second being rows and rows of portholes as far as the eye could see. The whole stern was covered in trawlers’ fishing nets. Lying on its port side, the Niagara had suffered very little. The first two levels of superstructure are broken up, and easy penetration was possible. Several large sections of walls had fallen away allowing ample ambient light. Our main concern as we swam slowly along the corridors was the large amount of net above us, which meant exit from this area would always have to be out to the side.

As we approached an open area at deck level, a huge hole loomed. I swam toward it, instinctively reaching for my reel. As I approached I realised the enormity of the ship’s hull. My UK600 torch did not find any floor or walls when I shone it through the hole. I checked my time: enough left for a quick look! It was just then that it hit me – I was down at 110 metres on a closed circuit rebreather, which I had never used at this depth before. It did not take me long to reclip the reel to my rig and turn away from temptation.

Tim, meanwhile, had found a beautifully intact deck light with a decorative brass cage. I could hear Tim shouting through his reg, ‘Where’s my tool bag?’ Even if he’d had tools, I don’t know whether he would have bothered with such trivial treasure when there were telegraphs, binnacles, and bells to be found all over the wreck. All too soon we had to commence our ascent. Our ascent to the first decompression stop at 78 metres was short, and we had only a one minute stop before we were on our way again. All our deep stops were of the same interval, and it wasn’t until around the 30 metre mark that we really settled in for our 100-minute decompression obligation.

The whole deco was fairly uneventful, except for the embarrassing moment I had around the 21 metre stop, described in Tim’s account. My major mistake was telling Tim, who nearly choked while laughing at my stupidity. After completing deco I unclipped all equipment and it was hauled into the chase boat by our ever-efficient and reliable surface support, Captain Nemo. Chris Ash, one of Reel Passion’s crew, could only mutter, ‘You guys are insane, bloody insane’ as I climbed back into the inflatable. Yes, well, he spends his working day dragging live four metre mako sharks into his boat. It was all relative, I explained to him. To us, the dive was an adventure, but to our New Zealand team mates it was also an historic moment.

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