Trimix computer algorithms Options

[Hoglooper]

I'm in the market for a trimix computer but I'm unsure as to the algorithms employed.

I'm looking at the Vr3, VrX, Helo 2, X1 (with Vplanner), Shearwater and possibly the OSTC.

The Vr3 uses something proprietary or VPM depending on S/W and the VrX does something which seems to be a mixture of the two.

The Helo 2 runs RGBM which is a bubble model like VPM but I don't know what the difference is.

V-Planner on the X1 I get as I use Vplanner on my desktop.

The Shearwater uses GF which is the same as the OSTC?

[MartinH]

Vplanner Live on the X1 has now been superseded by MultiDeco X1 which runs both VPM-B and GF. Same author as V-planner (Ross Hemmingway). I've not used it as I am happy with Vplanner but if I was buying it today I wouldn't bother with Vplanner.

[The Duck]

I have a VR£ and to be honest I'm not keen on the deco profile. The total amount of deco time is ok but the shape of the curve is not nice - it has too bigger jumps between the deeper stops and occasionally picks some odd depths (for example giving you a stop at 22m or 23m when you've got a gas switch at 21m).

I'm sure that when it came out it was the dogs danglies but things have moved on.

Currently I run the VR£ on +0 but shape the profile by adding in a slower ascent deeper and putting in some extra stops in the 21m - 9m region.

Allegedly the VR£ VPM isn't - and I've not heard a favourable report of it.

[Rob Dobson]

I'm in the market for a trimix computer but I'm unsure as to the algorithms employed.

I'm looking at the Vr3, VrX, Helo 2, X1 (with Vplanner), Shearwater and possibly the OSTC.

The Vr3 uses something proprietary or VPM depending on S/W and the VrX does something which seems to be a mixture of the two.

The Helo 2 runs RGBM which is a bubble model like VPM but I don't know what the difference is.

V-Planner on the X1 I get as I use Vplanner on my desktop.

The Shearwater uses GF which is the same as the OSTC?

The first point to note is that deco is all theory and to a certain extent it's all bollocks. Deco is best guess numbers and the precise algorithm which arrived at those numbers is to a certain extent not very interesting to most divers. That said it does annoy me when companies make false claims or let the marketing boys get carried away with stretching the truth beyond what is reasonable. Personally I like to know exactly what a computer is meant to be doing so when it starts making random stuff up I stand a reasonable chance of spotting that it has come off the rails.

The Shearwater and OSTC / OSTC MKII both use ZHL-16 with full user definable Gradient Factors. This is a well documented and understood algorithm that is freely available for anyone to look at and use. It's a reliable and trusted algorithm which is easy to customise to your preferred profile via the GFs (I like to run 10/90 personally). Buhlmann didn't finish his He work and the ZHL-16 based computers tend to be pretty conservative when handling He when compared to the true bubble models.

To date all of the Delta-P computers all use Buhlmann ZHL-16 (without user definable GF) as a core algorithm and then they overlay different stuff to give different shaped profiles. The VR3 and VR2 combine Pyle stops with ZHL-16 and then add 2% inert gas loading for every 10% conservatism defined by the user. The 'VPM' S/W upgrade was IMHO a bit cheeky as it was not VPM at all but rather a frig to the standard Buhlmann core which could produce a profile similar to VPM when run within certain common depth / time parameters. Unfortunately it seemed to come off the rails past certain point and produces some really odd profiles with horrendous shallow stop times; like in excess of an hour longer at 6m than a true VPM-B model.

The VRx took the same VR2/3 core Algorithm and introduced a 3rd lever so that the user could manipulate the profile to suit their preferences. A nice idea in theory but due the proprietary nature of the approach they never released what exactly it was doing and seeing as you can effectively achieve the same thing via GFs I never really saw the point in it or trusted it (coming on the back of the flakey VPM emulation attempt).

The Helo2 does not run RGBM despite claims in the crappy UK printed diving press to the contrary. Suunto have never released a computer that actually runs RGBM but they have licensed the name. To give Suunto their due they have never actually claimed to run RGBM but "Suunto RGBM" and "Suunto Technical RGBM". The Helo 2 seems to again run some form of Buhlmann at the core but with lots of added conservatism; like Delta-P they hold their cards close to their chest and have never revealed the detail of what it is actually doing. It seems to be a decent enough computer but the LCD screen and lack of CCR/constant PPo2 support means that it was outdated before it was even released. The Helo2 will keep you in the water longer than anything else on the market; I don't think that's a good thing when diving cold water.

The X1 and Dr-5 run 3rd party S/W so the algorithm is dependent upon what S/W is installed on the computer. The Dr-5 ships with ZHL-16 GF (same as OSTC MKII and Shearwater) and the X1 ships blank. The most popular X1 S/W is Ross Hemmingway's V-Planner Live (VPM/B) but he has recently released Multi Deco for it which is VPM/B and GFs (both are due to be released on the Dr-5). There is also GAP available for the X1 (ZHL-16 GF) although the last time I looked GAP was still in Beta. V-Planner is excellent but I have never used Multi Deco.

So conclusions?

1) Base your purchasing decision on more than just the deco algorithm.
2) Buy a computer that does what you need today and what you can see you needing in the future - i.e if you might go CCR don't buy a computer that doesn't support constant PPo2.
3) Price wise computers are coming down and there are some bargains to be had on the 2nd hand market. Look around and don't be fooled by flash advertising and fancy sounding technical phrases that don't mean very much in the real world.
4) Look for a good screen with well presented information.
5) It is important to have an intuitive User Interface - you can't read the manual at depth.
6) If you want cross platform consistency go for either ZHL-16 GF or VPM/B and stay away from proprietary algorithms as you will struggle to get them to match anything else.

Reason for edit: missed some words out!

[Aquaman]

Add to the list above: user replaceable battery and matches desktop planning S/W (ideally freely available).

[MartinH]

Bungee Mount if that's important to you. BTW does anyone know if the Deep Sea Supply Suunto mount works with the Helo2 or if there is one available?