Gradient Factors

[Lindan]

What should I set my high and lo Gf for a proposed dive to 200 feet using 18/33? ( approx) I have been reading and rereading and I am having trouble with the concept. My factory sets are 90/90 GF on the vision and that is where I have left it. Bob O?? John J?? Anybody? Thanks Linda

[ScubaAl]

Linda, I have my vision set at 30/85 has been working fine to date for dives to 260ft. For your up coming dive I would try getting your helium up. Something more like 45-50%

Al

[rcojr]

I'll be diving 60% helium
Bob

[john j]

See Page 75 of the Vision Electronics Manual.  If you don't have a copy, you can download a free PDF file copy from



A setting of 90/90 is very aggressive (too little decompression) and I would advise against that setting.

A starting point given in the manual is GF LOW = 15 and GF HIGH = 85.  I would call that an "average conservatism" setting and that's about what most people use from what I have seen.  However, I don't think that setting is overly conservative and I personally set the GF LOW to 10 (which is even more conservative than 15) because I want more deep stops.  Some people believe deeper initial decompression stops are helpful. Lower values of GF numbers give longer decompression stops.   GF LOW sets the deep part of the decompression and GF HIGH sets the shallower stops.  Al,s setting of 30 for GF low is less conservative than what I would call "average", but what works, works as they say.  Al is in pretty good shape.

Decompression theory is not well understood and there are many variables, so you should error on the side of caution of course.  Also, proper hydration and avoiding fatigue are super important in avoiding decompression sickness.

[Lindan]

thanks all you guys. John I have the same thought. I believe in a conservative profile and deep stops. I would rather prevent the bubble to quote Mel Clark. I will look at that area in my vision manual. Thanks all! Linda

[Deep Thought]

Think of Gradient Factors as a means to adjust the M values, or maximum tissue tension that is allowed for ascent without bubbling.  When the inert gas in your tissues reaches the Maximum, a stop would be required.  The straight M value or 100% of what is tolerated for ascent is the basis for the deco model.

Gradient Factors are then split into GF-Low and GF-high, the low affects the “Deep Stops” and the high affects your shallow stops.  If you set the first number at 50 then you are telling the computer that you only want to ascend to within 50% of the tolerable tissue tension, and it would add a stop deeper that required for the straight M value.  Keep in mind that the concept of Deep Stops is a concept only, that sound prudent but has most of its foundation in empirical data and not scientific.  The NAUI Deep Stop conference I attended about 6 years ago, has now been challenged by some researchers that think technical divers are spending too much time or emphasis on stopping too deep – too long.  The second number is typically set higher as general consensus is that shallow end of deco is time tested and sufficient.  Your setting for the second number is 90 and not unreasonable.

The Shearwater computers that I use have 30 / 85 as factory settings, and I have found this to be reasonable for a short fat mean basta rd ;-)

One thing to keep in mind is that decompression is not a perfect science, and people vary in susceptibility.  Nothing wrong with a bit of conservatism in deco planning.  Also remember the adage:  “is the person you ask advice on dive tables going to push your wheelchair for the rest of your life”?
 
Keep doing research, ask questions and do not accept the status quo, things change.  Just imagine, we used to tell divers to follow the smallest bubble on ascent.

Do you have Mel Clarks book on CCR Trimix Simplified?  She is a good diver and writes well.

Ron

[Anderson]

Is there a differance between open circuit and closed circuit on the Sheerwater settings?

[john j]

I personally wouldn't worry about being too conservative on the GF LOW settings in the Vision Electronics.  For example, a GF LOW setting as low as 10 still gives less deep stop time than a standard VR3 computer would give you on the same dive.

[Chris H]

I don't believe they can be set independently, if that is what you mean?