Gradient Factors on Shearwater Predator

[hfig]

I finally bought a shearwater predator (which I love) and I need some help. Since I've started doing cave dives that require deco stops I've always set whatever computer I've owned to a more conservative setting then how it comes right out of the box. It looks like the only way to do this on the predator is by adjusting the gradient factors. And I don't know anything about gradient factors.

Can someone please help explain gradient factors in simple terms? And if you have the ability to change the gradient factors settings on your computer, what do you use and why?

Just to be clear, I do plan on taking a deco class a little later but until then I'd really like to make sure my computer is set to be a little more conservative than just plain default, which on the shearwater predator is 30/85. For all I know this could be a good setting, but on my dive last night my DR duo (backup comp) said I had 10 - 12 minutes of deco and the predator said I only had 3 minutes.

Thanks in advance for your help.

[outofayr]

Harry:

In simple terms, GF are used to adjust the shape of the deco curve. What they actually are, is the percentage of the leading compartment in the decompression model at any point during the ascent.

The first GF is the LO GF. It is used to set the depth of the first deep stop. For most air-based or nitrox-based deco diving, a GF LO of 30 seems to be about right. When switching to TMx-based dives, a GF LO of 10-15 is typically more suitable. How to select that factor should be covered in your deco course, but basically it is picked so that your first deep stop occurs approximately 1 ATA above the depth where you switch from on-gassing to off-gassing during the ascent.

The second number is the HI GF. This GF is used to extend the shallow portion of decompression. The higher that number is (max of 100 IIRC on the Predator), the more saturated the leading compartment is at surfacing; i.e., if using 100 for GF HI, the model would have the leading compartment at 100% of the M-value (the maximum allowed supersaturation). Most divers use something more conservative than this, with 85 being very common for most deco dives. Cold water, strenuous dives, or other factors may lead you to select a more conservative GF HI for your diving.

Hope this helps explain w/o getting too technical (no pun intended!).

Brian

[netmage]

This is one of the better writeup's I've seen explaining GF's....

http://www.rebreatherworld.com/hamme...tml#post212002

[DA Aquamaster]

The Shearwater site has some good links on deco and gradient factors with some charts and graphs that help explain what's going on.

But basically yes, the first number is the % of the M value at which the first stop is triggered (smaller number = deeper first stop) and the second number is basically the % of the M value at the end of the last stop (smaller = more conservative, farther away from 100% of the M value).

As noted above 30/85 will work for advanced nitrox/deco procedures type dives on air and nitrox mixes and you may want to lower the first GF on trimix dives. 10 is often thrown around but I know several trimix divers who use 20 or 25, and it's something of a moot issue as it really does not make much difference until you are doing deep hypoxic mix dives with very high helium percentages and long runtimes.

With the new dive planning option on the Predator you can play with the GF's and see how it impacts the deco curve and run time.

----

Per your expoerience with the Duo, it is a lot more "conservative" than the Predator on one and two gas deco dives than the Predator, but I'd argue it is a little too conservative and I've never felt bad getting out of the water with the Predator set at 30-85 on those types of dives. I'd recommend leaving it set on those defaults until you better understand what is happening with GF's.

[DA Aquamaster]

The Predator also does some neat stuff. You can set it to show you the actual % of the critical compartment on ascent or to show you the actual ceiling. And regardless of the option set, you can page through the screens and see the current % of M value. That can be both something to do on deco as well as instructive to see how the value changes on a stop, then on ascent from that stop.

It also has an "@5min" setting that lets you see how much deco (total time to surface) you will have after 5 minutes at your current depth. That will let you see how efficiently you are offgassing even if you are not at the suggested stop depth. For example, after a dive to Hendly's castle, the computer may tell you that you can ascend to 20' when in fact you are still 50-60' deep with another 15 minutes to swim before you can ascend any higher. If your TTS is 20 minutes and the "@5min" number is 16 minutes then you know you will still be scrubbing off three minutes of deco over the next 5 minutes so you are still offgassing with about 80% efficiency. If on the other hand the TTS is 20 minutes and the "@5min" number is still 19 or 20 then you know you are not off gassing much at all and will need to do the full stop at 20 ft when you finally get there.

[RN]

What computer were you using before this?

Also, you really should do that deco course sooner rather than later. Flying your computer is no way to do dives. What happens if your computer craps out on you during your dive? Make sure you also interview potential instructors. GF wasn't covered in my initial deco courses.

[Bob Cree]

I guess I am a little surprised by the apparent difference between the DR duo and the Predator. I would question that since the 30/85 settings are already conservative - meaning the 35/85 forces you to stay deeper and offgas less quickly to begin with, then slowly ascend to build offgassing up over time.

I have always used gradient factor tables in my deco diving using Erik Bakers decoplanner software. For a long time I used 20/80 decoplanner tables as my primary and a Nitek Duo as the backup. Since trimix diving, I use the Nitek HE and spent quite a bit of time comparing tables cut with differing gradient factors to the HE. The HE works out to be a roughly 80/80 computer relative to deccoplanner and my old duo (conservatism level 0 or 1, I forget now) and the HE agreed normally quite well (so a little conservatism) there.

I found my Nitek Duo with conservatism set to the highest level = 2 gave me about a 35/80 decoplanner result...which is why I am surprised by the DR duo...anybody have any idea what it's effective gradient factors are - also, what conservatism factor do you have it set for?

I am considering a predator myself - so I am very interested in a comparative analysis.

[hfig]

Thanks guys for all the info, I'll get to reading.

What computer were you using before this?

Also, you really should do that deco course sooner rather than later. Flying your computer is no way to do dives. What happens if your computer craps out on you during your dive? Make sure you also interview potential instructors. GF wasn't covered in my initial deco courses.

I was using an Oceanic VT3 but that was way to liberal and the conservative setting on the Oceanic forces the computer to think it's at 3000' higher then whatever your current elevation is, I didn't really like that so that is why I bought the Duo. I sold the Oceanic on SB and bought the predator. I do however always dive with two computers and I carry dive tables in my pocket as a third backup.

Thanks,

[hfig]

My Duo is set to a 1, Last nights dive was 68 minutes long max depth was 96' average depth was 60'.

Thanks,

I guess I am a little surprised by the apparent difference between the DR duo and the Predator. I would question that since the 30/85 settings are already conservative - meaning the 35/85 forces you to stay deeper and offgas less quickly to begin with, then slowly ascend to build offgassing up over time.

I have always used gradient factor tables in my deco diving using Erik Bakers decoplanner software. For a long time I used 20/80 decoplanner tables as my primary and a Nitek Duo as the backup. Since trimix diving, I use the Nitek HE and spent quite a bit of time comparing tables cut with differing gradient factors to the HE. The HE works out to be a roughly 80/80 computer relative to deccoplanner and my old duo (conservatism level 0 or 1, I forget now) and the HE agreed normally quite well (so a little conservatism) there.

I found my Nitek Duo with conservatism set to the highest level = 2 gave me about a 35/80 decoplanner result...which is why I am surprised by the DR duo...anybody have any idea what it's effective gradient factors are - also, what conservatism factor do you have it set for?

I am considering a predator myself - so I am very interested in a comparative analysis.

[hfig]

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