Cave Diving, Cave Exploration and Cave Mapping in Yucatan, Mexico
Scuba Diving with Big Manta Ray in the Coral Sea Austrlia

Sometimes You Get the Bull, Sometimes the Bull Gets You.

Last week I wrote about an awesome cave dive to The Wakulla Room at The Pit.  Well, three days later, Saturday, I returned to make another try.

Over the last two weeks, I also wrote about some oxygen sensor trouble, trouble might be overstating the issue.  My 02 sensors had come to the end of their lives and needed replacement.  Unfortunately, after many hours of preparation for my dive on Saturday including staging all my gear in the water and starting my dive, I experienced another sensor failure, or sensor abnormality.

To recount the chain of events, on Saturday, 1/17/09, I went to Vaca Ha to do some cave diving on the unit.  The night before as I prepared my Megalodon, I discovered the number 2 sensor was dead, no voltage.  I didn’t think anything of it, the sensor was old and I hadn’t fired up the unit in three months.  I replaced the sensor with a new one dated July 2008, calibrated the unit and dived it.  During the dive at Vaca Ha, the number three sensor became current limited.  It wasn’t a big problem; I took the appropriate steps and exited the cave.  In preparation for my Pit dive on Wednesday, 1/21/09, I replaced the number three sensor with a new one dated March 2007.  That sensor was pretty old, but I wanted to give it a go.  Maybe it only lasts 6 months, maybe it lasts a year.  It was vacuum packed from the factory.  I calibrated the unit and went for a dive at The Pit.

During my 2 plus hours of deco I noticed the number 2 cell started to read a little lower than the other two sensors.  I checked to see if was current limited and I flushed, both checked out.  I didn’t think much of it. I assumed that the cell had come out of calibration as it baked in during the dive.  I figured, I could recalibrate the unit and all would be well.  The number one and number three sensors agreed.  Since I calibrate before every dive, it would be taken care of in my next pre-dive.

And that brings us to Saturday, 1/24/09.  I had another dive scheduled at The Pit.  Again, the setup and gearing up process went very smoothly.  I was super relaxed and ready for an awesome dive.  I finished my in water meditation and dropped down the deco line to check my staged tanks.  At the surface I had a PO2 of .4.  At the 20ft station I stopped and gave my gear the “In Water Two’s Check.”  My PO2 looked fine.  But for some reason, I decided to watch my primary handset as I descended.  This is not something I normally do, usually I check the handset and my HUD periodically to confirm the PO2, but I don’t watch it.

As I approached the 70ft station to check on my 50%, my number two sensor spiked to 1.97.  The other two sensors were in range at 1.0.  I had been adding diluent (7/71) on the way down.  I stopped and hovered staring at the handset trying to figure out what I was looking at.  I watched the PO2 fall on the number two cell from 1.97 to .8, while the other two sensors held steady at 1.0.

At this point I made a mistake; I didn’t flush the unit and put a known gas in the loop.  I just stared at the numbers trying to figure out if I should go for a dive.  Luckily on at least two separate occasions in the past week, I flushed the loop instinctually.  This time due to some mental twist, it never even crossed my mind. I think because it was so near the beginning of the dive and I thought I knew what should be in the loop.  The truth is I had no idea what was in the loop!  I wrongly assumed that I started the dive at .4, maybe I started the dive closer to 1.0.  I really didn’t know at that point, and what is worse is I didn’t know that I didn’t know.  I thought I knew.

After less then 1 minute, I decided to call the dive.  I couldn’t see going for a 4 hour 350ft cave dive with a unit that may or may not be broken.  I decided that when I surfaced, I would replace the number two cell again and try and go for another dive.

When I surfaced four minutes later, I hoisted the unit out of the water and changed the number two sensor with a brand new one.  I fired the unit up to calibrate it and the brand new sensor read 9.4mv.  I stared at it for a minute and realized, I couldn’t remember the proper range for an oxygen sensor.  My sensors had always started above 10mv, and that was my mental low limit.  I asked another CCR diver, Victor an Evolution diver, and he told me 8-13mv was the acceptable range, though I wasn’t sure if that same range applied to the R-22D sensors.  Later, I found out that the sensor was in range.

I decided against diving that day.  It was unfortunate that I couldn’t remember the proper range because I would have been able to continue my diving.  It amazes me how the smallest issue can sometimes put a dead stop to something that took hours and hours to prepare for.  It was a case of not being sure of a fundamental piece of information and paying the price.  In any event, I had hit my limit for the day.  Jorge and I packed up the gear, waved to Santiago and Victor and headed back to Playa for an afternoon on the beach.

The next day I called Patrick Widmann and Andrew Driver to discuss the sensor issue.  The very first thing they both did was give me a good flubbing for not flushing the unit.  The lesson is, know what you are breathing and don’t assume.  It was a good reminder that at any point in the dive, I may need to flush the unit to determine what gas is in it.  It may save my life.  As a result of my discussions, the sensor that spiked has been pulled from service. I am going to take it with me to Florida and put it on a cell checker.   The 9.4mv sensor is going to stay for now; however, I am going to watch it closely.  The bull got me.


1 MK { 01.28.09 at 12:15 pm }

Yes, yes, that bull is tricky.

2 Paul aka Hammerhead { 01.29.09 at 8:08 am }

Sounds like the bull gave you a little bump. Be careful you don’t get the horns next time.

3 John K { 01.30.09 at 9:26 pm }


On other thing to consider. You should carry a card (I have one if you want) that shows the cell mV in both air and O2 (at various percentages) so that you know both the lower limit and also the linear calculated max mV for the cell when you are calibrating.


4 Hans { 01.30.09 at 11:25 pm }

Hate to be a noob John, but I have no idea what you mean? When I calibrate in o2 it is usually around 51-54mv, when I cal in air is is usually around 10mv. What else do I need to know? Please enlighten me.