Appeared in DIVER January 2008

John John Bantin has been a full-time professional diving writer and underwater photographer since 1990. He makes around 300 dives each year testing diving equipment.


LIKE MANY PEOPLE, I HARBOUR A SECRET FEAR of computers. I am the man who bought my wife a copy of the all-singing, all-dancing Leopard Operating System, loaded it on to her Mac and instantly deleted four years of work from her hard drive.
So when Sergio Angelini from Uwatec in Switzerland sent me an advance copy of the latest firmware update to load on to the Galileo Sol dive computer I was using, I took it down to the guys at Scubapro in Basingstoke and let them do it for me. Its now available on the Internet for any Galileo Sol owner to use.
This new software introduces the concept of Profile Dependent Intermediate Stops to the dive plan. Although at first it sounds very much like a simple Deep Stop routine, it is not.
Deep Stops (or Pyle Stops, referring to the ichthyologist who formalised them in print) have usually been defined as a stop of around two minutes, the time it takes for your blood to circulate round your body, at a depth equal to halfway between your maximum depth and your first mandatory stop. You may choose to do more than one Deep Stop in an ascent.
PDISs are different. With the Galileo, the computer interprets the dive profile, suggesting intermediate pauses in the ascent that are a function of the nitrogen uptake calculated at that precise moment.
So the suggested intermediate stop data will change continuously, in line with the changing situation in your body.
PDISs also account for residual nitrogen-loading from previous dives. Conventional Deep Stop thinking ignores this.
Decompression theory depends on belief rather than anything else, and I know how dangerous it is to challenge anyones beliefs.
However, if you havent already torn up this magazine in disgust, Ill try to explain concisely how PDISs works.
The mathematical deco model (ZH-L8 ADT MB PMG) employed by the Galileo tracks a divers nitrogen uploading and off-gassing by dividing it into eight theoretical tissue compartments. These approximate to the various tissue types found within your body.
The depth of the PDIS is determined as the point at which the leading tissue model switches from on- to off-gassing.
The diver is advised to make a two-minute stop just shallower than this depth (contrary to the rule that you should make a deco-stop slightly deeper than the mandated depth).
Of course, very fast tissues are not considered during the very short dives for which they are leading tissues, and for which a PDIS is not appropriate.
The PDIS is not mandatory, and it is not a substitute for any mandatory deco-stop, or the 3-5-minute safety stop at 5m. In the case of nitrox-mix-switching during a dive, if the stop calculated is deeper than the gas-switch depth, it is displayed as such. But if it is shallower, it is calculated as a function of the next gas.
If you miss a gas-switch, the computer reverts to the original stop info. You have the option to turn off the PDIS function before the dive.
How the PDIS figure is displayed depends on whether you are no-stop or deco-stop diving.
Thanks to the dot-matrix display, its pretty easy to understand. The depth of the PDIS is displayed, and when you reach that depth or slightly shallower, the timer starts the two-minute countdown showing minutes and seconds in large figures.
When complete, it reads PDIS OK or, if you miss it, PDIS NO.
If you are deco-stop diving, the PDIS information is moved away from the deco information proper, to avoid confusion. There is no displayed timer function at this time, so the user merely waits at the stop depth until PDIS OK comes up, before continuing the ascent.
When using reduced micro-bubble levels its the same, but the stops tend to be earlier and deeper than with base-setting L0.
In some cases, PDIS readings will not be displayed, for example, with a long shallow dive with 21% O2 and a reduced micro-bubble level set at maximum.
If you miss a PDIS, it does not simply say: Missed stop. Use tables. It recalculates. PDISs take the voodoo out of Deep Stops!
You can update the computer to incorporate PDIS by going to You need to update SmartTRAK first. Its all free!

Divernet Divernet