How important is a depth gauge as part of a reg set?

[boulderjohn]

From what I remember they were good for three different levels but you needed to know the levels before or it threw the whole thing off.

Yes--it was part of planning the dive--I will spend X minutes no deeper than 90 feet, followed by Y minutes no deeper than 70 feet, followed by Z minutes no deeper than 50 feet, and then you had to do that dive as planned. I was shocked to learn, not that long ago, that the idea for the wheel was in the planning from the start of PADI's research for the RDP. In fact, the desire to be able to do a multi-level dive was the true impetus for the research. Prior to that, there was no way to do it, meaning that multi-level dives were unnecessarily very short.

and then if you were one notch off it also threw the whole thing off.

I had to do the wheel for both DM certification and instructor certification, the only times I have ever seen it done. With the wheel, you had to calibrate it by making sure the dot on one part lined up with the circle on another. On my instructor exam, I did one of the questions as carefully as I could, but my answer did not match any of the multiple choice answers. I did it over and over and over and got the same answer--precisely halfway between two of the choices. I took a guess and, of course, got it wrong. The instructor showed me how to do it using his wheel, and he got the right answer. I handed him my wheel and had him do it--he got the same wrong answer I did.

 

[Hoag]

When I learned the tables, you used the deepest part of the dive (Max Depth) and assumed that the entire dive was done at the deepest part even if you were only there for a minute. Several devices (my dive watch for instance) offers not only a max depth but also an Average depth. I have often wondered what the purpose of an Avg Depth would be other than interest.

Could some people be using the average depth instead of max depth with the tables as a way of trying to gain some of the advantages that a computer gives while still using the tables? To me, that sounds like it has the potential for catastrophic consequences.

 

[boulderjohn]

When I learned the tables, you used the deepest part of the dive (Max Depth) and assumed that the entire dive was done at the deepest part even if you were only there for a minute. Several devices (my dive watch for instance) offers not only a max depth but also an Average depth. I have often wondered what the purpose of an Avg Depth would be other than interest.

Could some people be using the average depth instead of max depth with the tables as a way of trying to gain some of the advantages that a computer gives while still using the tables? To me, that sounds like it has the potential for catastrophic consequences.

I don't know how others use it, but average depth is very valuable if you are trying to determine your air consumption rate. If you know how much gas you consumed on a dive and know how long you dived, you can obviously figure out how much you used per minute. You can then use your average depth to calculate your surface consumption rate--the rate with which you would breathe through your gas if you were on the surface. Do that often enough to get a consistent number, and you can predict how fast you will go through gas at any depth.

 

[Zef]

The only way an analogue (or digital for that matter) depth gauge can be safely used as a backup should one's dive computer fail is to aid in timing one's ascent.

If one has adopted 9 meters/min as a safe ascent rate then one should be ascending approximately 1 meter roughly every 7 seconds (6.6 seconds).

In the event of a computer failure, one can potentially look at their backup depth gauge and count seconds in their head (one alligator, two alligator three alligator, etc.) and control their ascent rate that way...BUT...this presumes a few things:

1. That one is ending the dive as a function of their computer failure.
2. That they have the presence of mind to look at the backup depth gauge and count seconds in their head.
3. That they can do #2 without being task loaded and control their buoyancy while doing.

To be honest, for most divers it would be safer/easier/better in such a situation to just ascend as slow as possible using the the old standard of ascend slower than your smallest bubbles (or slower). More experienced divers will either have a back up computer, or will resort to a very conservative (slow) ascent rate in a computer failure situation.

What is scary are those divers who seem to have experience, who have participated in this discussion and have given explanations how they would employ their backup depth gauge to continue/salvage their dive, some citing reasons such as salvaging an expensive trip or whatever.

-Z

 

[Bigbella]

The only way an analogue (or digital for that matter) depth gauge can be safely used as a backup should one's dive computer fail is to aid in timing one's ascent.

What is scary are those divers who seem to have experience, who have participated in this discussion and have given explanations how they would employ their backup depth gauge to continue/salvage their dive, some citing reasons such as salvaging an expensive trip or whatever.

-Z

I may have mentioned "salvaging" a dive, whether for work or pleasure, in the event of a computer failure; but I did not mean to suggest that I was ever fully dependent upon a computer or even a simple depth gauge. I had always carried analogue equipment, along with a dive watch, putting them to full use, regardless of what else I carried; and had been diving fifteen years or more with them, before I ever saw my first computer.

I had always desired a contingency plan, aside from simply aborting a dive, if at all possible, for "when the lights go out;" and I saw serial computer failures, when they first hit the market; and still do, to a slightly lesser degree.

Years ago, while doing a job in Queensland, I took pity and had doled out my entire cache of redundant electronic equipment to others on the boat -- two of whom, who had experienced electronic failures, of one form or another; and who had probably not seen dive tables since their first openwater classes.

I dove tables for ten days or more, sans computer, with my watch and analogue gauges; and have lived to tell the tale . . .

 

[halocline]

With the wheel, you had to calibrate it by making sure the dot on one part lined up with the circle on another. On my instructor exam, I did one of the questions as carefully as I could, but my answer did not match any of the multiple choice answers. I did it over and over and over and got the same answer--precisely halfway between two of the choices. I took a guess and, of course, got it wrong. The instructor showed me how to do it using his wheel, and he got the right answer. I handed him my wheel and had him do it--he got the same wrong answer I did.

I remember having a similar experience, then learning that some of the wheels were slightly off kilter when they were assembled. Since it's a mechanical thing, if the printing or alignment is slightly off, when you rotate the thing and try to line up the little arrows you can be a pressure group off. Ah, the good old days....

 

[Kaveman]

Did not miss that at all

 

[boulderjohn]

I remember having a similar experience, then learning that some of the wheels were slightly off kilter when they were assembled. Since it's a mechanical thing, if the printing or alignment is slightly off, when you rotate the thing and try to line up the little arrows you can be a pressure group off. Ah, the good old days....

In reality, though, if you were actually diving, it would be no big deal--plenty clase enough. That kind of precision is only needed on multiple choice test answers.

 

[Eric Sedletzky]

In reality, though, if you were actually diving, it would be no big deal--plenty clase enough. That kind of precision is only needed on multiple choice test answers.

There are a lot of things like that in diving. I get a chuckle out of the trim and weight placement fanatics that agonize over every ounce of weight and where it’s placed. One good healthy day of diving where I live when conditions are a little feisty and all of it’s out the window, along with tables, the wheel, and computer arguments. Most people are just grateful they survived it.

 

[Hoag]

In reality, though, if you were actually diving, it would be no big deal--plenty clase enough. That kind of precision is only needed on multiple choice test answers.

We have a saying in the Air Force that addresses this: "Close enough for government work."