Caustic Cocktail

Howard Hall

Cranston and Hall film mantis shrimps. Credit © Peter Kragh

For over fifteen years the use of closed circuit rebreathers has been fundamental to our diving operations when making underwater IMAX films. Considering the extreme nature of so many dives my crew and I have made together during the past two decades, it was all the more surprising when one of our best divers found himself in a life-threatening emergency twenty feet below the boat in calm water.

We’d spent the last four days filming mantis shrimps for our latest IMAX 3D feature, Deep Sea 3D. The reluctant film star lived in a burrow on a silt-covered bottom seventy feet below the calm waters near Catalina Island, California. After four days our dives had become routine. The fifth morning seemed no different. I dropped off the Conception’s swim step and drifted down to the muddy ocean floor twenty feet below. Then, as I did on numerous previous dives, I swam down the slope to the marker buoy at seventy feet. The buoy identified the mantis shrimp burrow we had been filming and also marked the spot where we’d left a pile of gear including a half dozen heavy bags of lead shot, measuring sticks, movie lights, and a small hoe for removing mud out from under the 1,300 pound IMAX 3D camera so we could get the lens down to eye-level with the shrimp. As I swam down the slope I saw the lights come on in a flash or of incandescence announcing the beginning of another two-hour dive.

When I reached the buoy I unfastened my fin straps and slipped them off. Then I unclipped two lead shot bags from the buoy-line and attached them to my harness. As I did this, I looked up-slope toward the mantis shrimp den. A large male shrimp, about ten inches long, looked out from the entrance to his den and almost certainly thought, “Oh no! Not again!”

The rest of the film crew soon followed me to the “set.” Bob Cranston began removing his fins, Mark Thurlow began detaching several 20-pound shot bags from the buoy-line; weight that we would use to weigh-down the camera. Peter Kragh helped Richard Herrmann and Dave Forsyth maneuver the camera down onto the muddy slope for as gentle a landing as possible. After Mark finished digging a two-foot deep hole to settle the camera in, visibility had dropped from forty feet to less than five feet. I turned the camera power on and watched as the video viewfinder monitors began to glow. After the monitors warmed up, I looked to see if I could make out the mantis shrimp den through the murk. No luck.

“Cranston, let’s fan some of this murk down hill,” I said pressing the microphone button on my Kirby Morgan full-face mask.

“Okay, okay,” Bob gurgled back. He sounded like he was drowning in his mask.

“You okay?” I asked.

“Damn mask is leaking,” Bob replied. “Want a focus check?”

“Let’s get some of this murk out of here first” I answered.

Bob and I then began fanning water down hill. This fanning motion set up a tiny current that sucked murky water away from the front of the camera to be replaced by clear water from up-slope. After about ten minutes, the water was still murky, but clear enough to focus. “Okay Bob. Let’s get a focus.” Bob gently moved out in front of the camera and held his light meter over the mantis shrimp den giving me a good focusing target. I checked that the position of Bob’s light meter was directly over the den then focused on the image of his light meter that I saw in the video monitor. When the image was sharp I checked the focus readout in the upper right of the monitor screen. It read 2’ 5”.

“It’s too close,” I said after keying my microphone. “We’ve gotta pull it back about six inches.”

In normal film production the cameraman is concerned with focus, composition, and exposure. In 3D there is the added dimension of proximity – how far away from the audience the image will appear when projected in 3D. Too far away and the 3D effect is unsatisfying. Too close and the image becomes almost painfully uncomfortable for people to focus on. In IMAX 3D we adjust the proximity using a combination of lens selection, the actual subject to camera distance, and an additional lens setting specific to 3D called convergence. This latter setting changes the distance between the two IMAX 3D lenses thus changing the angle of the light path through the lenses to the film plane. Properly combining these factors is critical. Using our 80mm lens with the camera focused at 2.5 feet and the convergence set at five feet, the theoretical apparent distance to a viewer in the middle of a typical IMAX 3D theater should be 1.6 feet – too close for comfort. Moving the camera back six inches moves this apparent distance out to 2.5 feet, which should be comfortable for most viewers.

After Bob, Mark and I pulled the camera back a few inches (kicking up another enormous cloud of mud) I refocused the lens. The readout said 3’ 1” - almost perfect. “Okay, that’ll do,” I said. “Convergence set at five feet. Give me another focus Bob.” I tried to announce lens settings as I made them. Sometimes Bob will catch me in a stupid mistake and that can save us a fortune in wasted time and film not to mention the embarrassment of watching the screwed-up footage in the company of a crowd of IMAX and Warner Brothers executives.

“Okay, Bob.” I said pushing my microphone button. “Let’s start moving this murk downhill.” We began fanning the water back away from the camera again. It took about fifteen minutes, but eventually the water was clear enough for a shot. I pulled a small mussel from a bag attached to the buoy-line and cracked the shell with my knife. Then I leaned forward and placed the damaged mussel next to a clump of mussels we had already placed about three feet from the mantis shrimp den. We were ready to roll.

“Hey, Mark,” I heard Bob call through a mouthful of water. “My mask is leaking, take a look.” Bob was gurgling badly and had finally decided there was a free moment to address his leaking mask. I watched Mark drop down and disappear behind the camera next to Bob.

“You’re microphone cable is stuck under your mask seal. Take a breath and close off your rebreather mouthpiece and I’ll fix it,” I heard Mark say. A moment later I hear Mark ask, “That better?”

“Yeah, that’s better,” Bob responded.

A few minutes later, I noticed the mantis shrimp rise up from his burrow and turn toward the mussels. He seemed to perk up and it looked to me that he had captured the scent of the nearby mussels. “Rolling,” I announced.

The mantis shrimp left his den and scurried over to the mussels. He pulled a mussel off the clump then ran back to his den to eat it. The scene was expensively recorded by the IMAX 3D camera.

We had been underwater almost two hours when the camera rolled out. “Surface, copy,” I said. “Recover the camera, recover the lights. We’re done with the shrimp and we’re coming back for lunch.”

“Surface copies, recover all gear,” I heard Michele Hall respond from the deck of the Conception.

Bob and I marched the camera up the muddy slope as Peter and Mark collected the weights and other tools we had left on the bottom during the last five days. As we approached the Conception, Dave and Richard descended to relieve us of the camera. I checked my computer and saw that I had ten minutes of decompression to do. I assumed Bob, Peter, and Mark had similar amounts of time to wait before surfacing. I swam up-slope past Conception and waited in a murky kelp forest in fifteen feet of water.

Bob waited on the sand twenty feet beneath the Conception’s stern. When his decompression was finished, he unclipped the weights attached to his harness then clipped them to a line hanging off the starboard side of the boat. Then he bent down to put his fins on. This is where things began to go wrong.

As Bob bent down a highly caustic solution of salt water and calcium hydroxide filled his rebreather hose. The water that had slowly leaked into Bob’s mask had partially flooded his carbon dioxide scrubber in his rebreather. The scrubber canister, filled with a combination of calcium, sodium, and potassium hydroxides had become saturated and the remaining water sloshing in his rebreather counter-lung had become a highly caustic solution. Now this deadly mix had filled Bob’s inhalation hose. When he straightened up from donning his fins and took a breath, a shot of this terrible mixture was blasted down his throat. The pain was instantaneous and acute. In terrible trouble, Bob struggled to find his buoyancy compensator inflation button and blasted himself to the surface next to Conception’s swim step.

When I reached the surface a few minutes later. Bob was still on the swim step coughing and vomiting violently. Even an hour later, he was not in great shape. His throat and mouth were burned by the nasty alkaline solution. Michele produced an enormous bowl of ice cream and that finally brought a weak smile to Bob’s face.

We had all heard stories of the “caustic cocktail” that sometimes kills rebreather divers. But after more than 1,900 hours of rebreather diving, I had largely discounted these stories as exaggeration. Of course, I’ve reconsidered that now. Seawater leaking into a rebreather is a far more dangerous situation than I had thought. Bob should have aborted his dive as soon as he was aware his rig was leaking; as soon as he tasted saltwater in his mouthpiece. I’ve only had a badly leaking rebreather once. It happened years ago and I finished my dive without incident. But when I took my rig apart, my water absorbers beneath by scrubber canister had melted from the caustic water mixing inside my counter-lung. Had I swallowed some of that water I would have been in the same shape as Bob. I doubt any of us would have aborted our mantis shrimp dive had we noticed minor leaks in our rigs. But, certainly as far as I’m concerned and almost certainly as far as Bob is concerned, that attitude has now changed. A leaking rebreather is grounds for an immediate abort. We were lucky the incident had occurred in twenty feet of water. I hate to think what would have happened if this had taken place a hundred feet below the surface prior to lengthy stage decompression.

We made one more dive that afternoon, our last of this leg of the trip. Dave Forsyth had been out scouting in deep water below the mantis shrimp den and had located an electric ray at 97 feet. We were eager to attempt filming an electric ray predation sequence and this would be our last shot at it. Bob sat out the dive givinng up his second cameraman position on the camera to Peter and we all swam down slope following Dave as he descended to the location he had marked earlier in the day. When we got out there, however, the ray was gone. For the first time since our DEEP SEA 3D project had begun, we returned to the boat with an unexposed load of film.