Warning about Garmin Descent!

[stiebs]

The first dive has you surfacing at a gradient factor of 100%. That’s pretty aggressive, and the 12 hours is just to illuminate that a 24hr lockout might be a bit excessive

You have taken almost the most conservative "aggressive" example possible in order to (try) and prove a point. If someone has done multiple dives with "normal" dive holiday surface intervals, your example all of a sudden looks much more conservative by comparison.

 

[crofrog]

You have taken almost the most conservative "aggressive" example possible in order to (try) and prove a point. If someone has done multiple dives with "normal" dive holiday surface intervals, your example all of a sudden looks much more conservative by comparison.

How about you post some proof from subsurface or similar. You're not going to ongas your slow tissues sufficiently while staying below NDL to make a 12 hour surface interval not be sufficient with even the smallest a safety stop.

On a typical dive vacation you’ll be hard pressed to not get at least 12 hours overnight between dives.

 

[stiebs]

How about you post some proof from subsurface or similar. You're not going to ongas your slow tissues sufficiently while staying below NDL to make a 12 hour surface interval not be sufficient with even the smallest a safety stop.

On a typical dive vacation you’ll be hard pressed to not get at least 12 hours overnight between dives.

You are still taking a conservative approach. Is someone has done a dive with their computer in gauge, who is to say that they stayed within NDL limits? Who is to say that they didn't violate a deco stop?

In gauge mode, the computer is being told not to track dive and deco parameters, so it doesn't. It cannot make any assumptions that the dives done were anything close to a reasonable profile.

 

[Sascha314]

I would be careful just to use a short bit deep dive to the bottom and direct ascend close to the NDL as an extreme example.

From a Buhlman point of view:
On multiple dives its the medium to long half life tissues that will saturate more and more each dive, as they do not have time to fully desaturate. These also tend to not be saturated a lot, if you are at depth only a short period of time. If you then have some time and intermediate depth they are not of a concern at this single dive, but eventually add up during multiple dives.

 

[rongoodman]

From my experience it's the medium half life tissues which bear watching, at least on the typical liveaboard doing 4-5 dives daily, and they're clear in the morning when I get up. The slower compartments never really come into play. The Shearwater tissue loading graph is interesting to watch over the course of a week.

 

[crofrog]

You are still taking a conservative approach. Is someone has done a dive with their computer in gauge, who is to say that they stayed within NDL limits? Who is to say that they didn't violate a deco stop?

In gauge mode, the computer is being told not to track dive and deco parameters, so it doesn't. It cannot make any assumptions that the dives done were anything close to a reasonable profile.

Here is a day of diving:
80ft 30'
-- 1hr SI --
50ft 50'
-- 2hr SI --
80ft 30'
-- 1hr SI --
50ft 50'
-- 12hr SI --
80ft for 30'

Without any safety stops, dropping 40ft/min on the deeper dive and 50ft/min on the 50' ft divers, and ascending without any safety stops at 40ft/min on the 80ft/min dive and 50ft/min on the shallow dive... Clearly a not very intelligent way of diving.

As you can see during the 12hr the vast majority the slower compartments return to ambient pressure. With compartments 14 and 15 still being slightly above ambient. It's hard to put enough gas into those compartments with anything close to resembling your typical vacation diving for them to become the leading compartment.

As I eluded to above, this is a very dumb way of diving and violates most if not all of what we consider safe diving practices and every dive has you getting out of the water at close to 100% of your GF.

 

[Christopher Kosseff]

Hmm, let's take a look at the manual. Page 3:

View attachment 860728

It is a safety feature. In gauge mode the Mk2i, and every other computer out there (as far as I am aware) does not track your nitrogen loading. So if it allowed you to go back into a dive mode straight after a dive in gauge mode, it has no idea what you nitrogen loading is, meaning the algorithm does not have a known starting point and cannot be trusted.

No disrespect, but unfortunately you are the exact person that Garmin is protecting itself from. If you did an aggressive dive in gauge mode that pushed your nitrogen loading to the max, and then decided to switch it back to dive mode and did another dive with a 20 minute surface interface, the computer would probably give you a pretty long NDL because it thinks you started off with zero residual nitrogen. So you follow your computer and get bent, and then try and pin your medical expenses on Garmin (or your insurance does).

Much easier to just stop stupid people doing stupid things.

OK, maybe that was a little harsh. It's easier to stop people from overriding safety features that they don't fully understand, or don't fully understand the consequences of mitigating.

I really don't intend to get into a protracted arguement over this isssue. I do understand I made a mistake in the setting. I will certainly look to see how I can avoid doing this in the future. I am a pilot and fully conversant with redundant safety features. In fact, an aviation GPS I have used will WARN you if the database is out of date and you must acknowledge that you are aware of this. That makes total sense to me. Better that it would prohibit you from using the GPS? BTW, 25 years of flying with a perfect safety record. My point was simply that such a warning and acknowledgment would have been much safer for me. Having the lockout function does not prevent "dumb people (thank you for that, I am quite certain you're free of errors!) from doing dumb things". It does protect the company from lawsuits - as would the acknowledgement suggestion. I'm quite capable of assessing risk! My only dive the day before, and for the previous 5 months, was to 20'. I will not post any further responses.

 

[Christopher Kosseff]

This one is on you, not on Garmin. I do understand their position here. I'm not a fan of lockouts on dive computers in general, but I can see the point in some situations.

In this case, by diving it in Gauge mode, the computer knew there was a dive that happened. However, since in Gauge mode, it wasn't tracking tissue loading, it had no clue what your tissue loads were, so subsequent dives would have been displaying incorrect NDL/Deco information.

I've had my MK2s for a couple years now. I never once accidentally put it into Gauge mode. I just now decided to see if it would be possible by mistake. I'm guessing you left Gauge mode in your favorites as that is the default setup. I moved the activities I either don't use, or use very infrequently out of favorites so I don't have to scroll through a bunch, or risk selecting by accident. It would be very difficult for me to accidentally select Gauge Mode.

True. Garmin does have lockouts, but does give the user the option to disable the 24 hour lockout. I'm not sure it would apply in this instance, but, in general I find that Garmin's approach is reasonable with regards to lockouts. There if you want it, disabled if you don't.

However, this situation is a little different as the computer simply doesn't know the tissue loading from the previous dive, so I'm not sure if it's possible to avoid a lockout in this case.

Edit to add: Of course it's possible to avoid a lockout in this case. Set the computer to the correct mode and this wouldn't be a problem.

Thank you, I just removed it from favorites. That should prevent me from accidentally setting this mode in the future. A helpful suggestion!

 

[Christopher Kosseff]

Others have given you good feedback about how the device is working correctly. As a practical matter if you don't use Gauge then just remove that activity profile from the list. That way you can't start it accidentally.

Thank you, that's exactly what I just did!

 

[stiebs]

I really don't intend to get into a protracted arguement over this isssue. I do understand I made a mistake in the setting. I will certainly look to see how I can avoid doing this in the future. I am a pilot and fully conversant with redundant safety features. In fact, an aviation GPS I have used will WARN you if the database is out of date and you must acknowledge that you are aware of this. That makes total sense to me. Better that it would prohibit you from using the GPS? BTW, 25 years of flying with a perfect safety record. My point was simply that such a warning and acknowledgment would have been much safer for me. Having the lockout function does not prevent "dumb people (thank you for that, I am quite certain you're free of errors!) from doing dumb things". It does protect the company from lawsuits - as would the acknowledgement suggestion. I'm quite capable of assessing risk! My only dive the day before, and for the previous 5 months, was to 20'. I will not post any further responses.

Its really comes down to why the feature is there. With a GPS, there would be very few reasons, if any, to operate with a database that is out of date. It's not like its a feature that you can choose to use either current or an out-of-date database.

Gauge mode is a little bit different. There is legitimate reason to use it, and it would be downright annoying for someone who uses it regularly and consciously to have to acknowledge a warning every time they use it. Or then do you a menu option to enable/disable the warnings for gauge mode? Where do you stop?

The Garmin Mk2 manual is actually quite good, and in this particular case the information needed was pretty clearly stated. Maybe using the words that I did was not quite right.. the point I was trying to make was that the answer is there in the manual, but you still decided to contact Gamin support, and then post on a public forum to warn others?