Nitrox 1st dive, air the second, can my Sherwood Insight computer do it?

I have a Sherwood Insight dive computer (which is nitrox compatible) that I recently purchased and I'm in the process of getting nitrox certified - after which I'm going on a two tank boat dive with my local dive shop on a club dive. The first dive is deep, second shallow and since nitrox tanks are extra, I will dive nitrox the first dive then air the second....

My question is, can the Sherwood Insight dive computer store the info for the first dive as nitrox then go to air the second dive and still take into account your nitrogen levels and time limits based on your profile from the first dive? Or do I need to dive nitrox on an air profile the whole day? The owner's manual does not seem to mention this situation, at least not that I've seen so far and I've read it pretty much cover to cover.

I'm going to be using my nitrox cert on the Star Dancer in Papua New Guinea this April so I'll be diving nitrox every dive on the same blend then so I won't have to reset it each dive. I don't know how often I'll use nitrox at home since not that many people dive nitrox during club dives, but I figure it'll be a good cert to have for the knowledge and for use during travel...

Any info would be greatly appreciated...

I do not have a Sherwood Insight & do not know a good deal about them, but most Nitrox compatible computers the user programs the gases into them before the dive. All Nitrox computers I've had, you must have the computer in Nitrox mode & program the gas % into the computer. Let's say you have 32% for your first dive, then you would program in 32% on the second dive you use air, then before the dive, you program in 21% (the percentage of air). One thing I've noticed about the computers I've had, is you must program the gas mix BEFORE you get into the water. The best thing you can do is read the computer's instruction manual. If you do not have it, you should be able to go tho Sherwood's Web site & download it.

On your trip, you still will need to analyze your cylinder between each dive. Even with banked gases or membrane systems, there can be minor fluctuations in the % of gases. As you will learn in your class, for your safety & accuracy you should analyze the cylinder & program your computer to the analyzed mix, unless you intend to breathe the mixture as air instead of Nitrox. In that case, you must still adhere to the depth limits & your bottom time will shorter.

Contact Sherwood directly

if it can't, get a new computer. this is basic stuff, changing the percent mix between dives.

but i'm sure it can. and yes, it would keep your first dive group info when starting your next dive, even if you switch mixes. that's what a computer excels at.

Specs say it is programmable from 21 to 50%. You may not be able to reset to air (my Oceanic won't rest for 24 hours) but you can set it for 21% and you will be good to go.

The computers that I have used (Suunto Cobra and Vtec and Oceanic VT3) allow you to set to nitrox and this setting will remain until changed by the diver. I leave my computer in the nitrox setting all the time with the O2 on 21% (when diving air) and I change the % when using nitrox.

...the Sherwood is a typical single-gas-at-a-time recreational dive computer. To dive it correctly you need to enter in the correct % of oxygen you will be using for THAT next dive into the computer...BEFORE getting into the water, as you CANNOT change/correct the oxygen % IN the water once the dive has begun...unless you come back to the surface in the middle of the dive and trick the computer into thinking your dive has ended, change it, and then resume the dive. IF you operate it correctly, and enter in the correct oxygen % prior to each dive, the computer will correctly track what gas you are diving....and correctly calculate nitrogen and oxygen buildups, deco requirements, bottom time limits, etc. The computer doesn't care if you switch back and forth between 'air' and nitrox....do it as often as you want, as long as you are telling the computer the truth.....which brings me to the concept of 'diving nitrox on an air profile'.......MAJOR BAD IDEA !!!! If you do that, the computer will underestimate both your MOD limits....and your 'oxygen toxicity' exposure...which can definitely be fatal! (ask your instructor about it)....also be careful, many recreational dive computers auto-default to 'air' (EAN21) after a period of non-use.....for example, day one you dive nitrox, then go to bed for the night, wake up to go diving on day 2 and there is a chance your computer will reset itself automatically to 'air' overnite....so you ALWAYS double check your settings on EVERY dive!

How deep is the first dive? What mix are you considering?

Were it me, I would look at using air first and Nitrox second. That way if the 'deep' dive gets too deep, I'm on air. It really depends on the definition of 'deep' and the mix.

Richard

scubafanatic:

which brings me to the concept of 'diving nitrox on an air profile'.......MAJOR BAD IDEA !!!! If you do that, the computer will underestimate both your MOD limits....and your 'oxygen toxicity' exposure...which can definitely be fatal! (ask your instructor about it)

Click to expand...

As long as the diver does not exceede the MOD of the mixture by careful monitoring of depth & keeps track of the O2 exposure clock, diving Nitrox as air is in fact safer, from a DCS standpoint, as the actual amount of nitrogen absorbed in the diver's body will be less than the computer has recorded. I dive Nitrox as air quite a bit, yet monitor my depth & O2 exposure & have done fine to date. I do it because for me, I feel much better & less fatigued after the dives. The biggest draw backs are reduced bottom time & longer surface intervals.

Hi ewaiea,

Extremely common practice. Set your computer for the mix of the 1st dive and dive. During the surface interval, often after a short lock out of 10 minutes, reset your computer to air or 21% and proceed with the 2nd dive.

I do this very commonly in Key Largo with a deep 1st dive on the Spiegel Grove or the Duane and a shallow dive on a reef or the Benwood for the 2nd dive. This is also true for Oahu with a 1st dive on the Sea Tiger or the YO and a shallow reef dive for the 2nd. When I first started diving nitrox, I occasionally forgot to reset for air after the 1st dive. This was not so important for NDL as the 2nd dive generally had unlimited NDL on air, but it does screw up your nitrogen exposure a bit for multiple dives/multiple days.

Good luck and good diving, Craig