Calculating Average Depth

[D_B]

I actually took my computers downloaded data for one dive, added all the depth data together (30 sec sample rate) divided by the number of data points ... wow, did that take way too much time to do
... I now have a Citizen dive watch knockoff that can average my dive ... I wonder why can't they have the computer do that too

 

[H2Andy]

well ... thank God my computer does ... i dont think i'd have the patience to do it manually

 

[TSandM]

What are you trying to determine? If it's your baseline SAC rate, longer determinations at a relatively constant depth, at a similar level of exertion, are the best way to get a consistent determination.

When there are multiple variables -- varying depth, changing exertion, differing temperatures, different TYPES of dives (my SAC has varied by a factor of 2 from touring dives to line running) -- then there will be a lot of scatter, and a difficult determining one valid number.

If you are interested in finding one SAC rate for a baseline reference, the best way to do it is a dive where you spend a significant relaxed period at the same depth. For example, our Redondo site involved a tour of various structures all located within the 40 to 50 foot range. You can easily spend more than a half hour in that depth range, kicking idly forward. I did that dive a few days ago, and spent my "depth averaging" intervals calculating my SAC rate, which matched my reference.

You know you have serious SCUBA sickness when you are underwater, noting your SPG readings and depth, and calculating your SAC rate on the fly. Either you are sick, or the site is a much better night dive . . .

 

[Willie]

((depth1 x time at depth1) + (depth2 x time at depth2) + (depth3 x time at depth3) ...) ÷ total dive time = average depth

I wear a Sensus data recorder on every dive these days and it calcs avg depth for me. Before that I would use a very simplified calculation based on my dive plan. Say I was at 80' for 15 min, then 40' for 20 mins, then 20' for 10 mins, then gradual ascent from there, say 2 mins to make the final ascent. I would use the following.

(80 x 15) + (40 x 20) + (20 x 10) + (10 x 2) = 2220
2220 ÷ 47 = 47.23 avg depth​

I set up a simple spread sheet that would take as many as 10 pairs and would just enter the data and it would give the answer at the bottom of the column.

Sometimes I would be very specific, but most of my dives were no more than 3 pairs of data, so I could even work that out fairly simply long hand if I wanted.

Willie

 

[Tonio Anastasi]

If you have the money.
Trade your computer for an air integrated one.
This will let you know:
How much pressure you have left,
How long that air will last you at any given depth. So it will for example indicate 20M
No Stop time 20mins.
130 Bars, ,
Air duration 15mins.
This will change as you change depths and or if you are working hard under water like in a current.
For safety, the duration of the air reserve is not included
so when it shows
Air Duration 15mins. that will be over and above the reserve.
Advise you when you your breathing rate increases.
On the PC download.
Because of the graph you will be able to see your average depth, some will indicate that for you.
Indicated to you any moments when your breathing rate in creased.
I've been diving with an air integrated computer now for around 8 years now, and it has made some big changes to my profile management.

 

[coldsmoke]

Thanks for all of the replys. I'm afraid I will be in the stone age for a while with regards to upgrading my computer though.

 

[dsteding]

I'll add to what other said (including the offers to dive with you when you come out).

FWIW, Bob does this in his AOW class. We went to 33 fsw, noted pressure, swam hard for 5 minutes, stopped, noted pressure, relaxed a bit, then took 10 minutes to return. That is enough to gather a baseline RMV from which to do gas planning for the deep dive.

One way to do this on regular dive is to choose the deeper part of the dive, the flat part of a square profile, for instance. Doing this deeper (at 2-3 total ATAs) will lessen the variation of pressure due to depth changees, and will also give you a longer sampling interval.

However, most shore dives--particularly if they are out-and-back dives--can have average depth simply calculated. It is usually something right around half your max depth, as the profiles are often relatively V shaped. So, that isn't a bad way to do things.

One other thing to remember: note your pressure in the water before you drop rather than before you get in the water, tanks cool and pressure changes to the point that it can affect your calculations.

 

[nwbrewer]

I find EUP good for determining SAC rates. Doing a surface swim out and back will result in a fairly square profile, and you can cover a lot of gound out there in the outer edges of the park checking things out without changing depth very much. (Although that surface swim is not a lot of fun).

I think the last time I dove there my max depth was 40 fsw, and I'd say my depth varied maybe 2-3 feet from that for most of the dive. A total dive time of 40 minutes results in a nice long time to average the consumption over at a given depth without resulting in a boring dive.

Jake

 

[MXGratefulDiver]

You guys seem to be some of the most knowledgable on this site not to mention very willing to help.

I have a pretty simple question - what is the best method for determining your average depth. I have a computer but it doesn't display average depth. The way I was taught, which might not be the best, is to document your depth every 5 minutes or so and then average. Is there a better way?

I have searched around and haven't found much. Most seem to use computers.

As an aside - Bob (aka NWGratefulDiver) I wish I had taken my AOW with you! After reading what you seem to teach from either your posts or other divers giving you credit - my AOW sucked in comparison.

Maybe I can get out and do something with you this summer. How would I go about that?

Thanks.

The way I do it in class is to do a couple of timed swims at 33 feet. For each swim, take psi readings before and after the swim. Fgure out the total gas consumption and divide by the minutes, and dive that number by 2 (33 fsw = 2 ATA). That'll give you an average psi per minute consumption at 1 ATA = surface consumption rate in psi.

You can figure out your consumption rate in cubic feet by converting the pressure rate to volume. First you calculate the number of cubic feet per psi in your tank. For example, an AL80 would be 77.4 cubic feet divided by 3000 psi ... or about 0.026 CF/psi. Multiplying your consumption rate by this number will give you the same value in CF, rather than psi.

In class we do two swims ... one swimming at "normal" speed, and one kicking like hell to simulate swimming into a stiff current. We do the calculation for each, to find a "resting" consumption rate and a "working" consumption rate. Your actual rate for any given dive will be somewhere in between those two numbers, depending on diving conditions and a few factors that have to do with how you're feeling on dive day.

If you want a better understanding of how this all works, come on down to Tacoma Scuba next Friday (Feb. 23) for a free seminar on Gas Management.

I'll happily dive with you anytime. Just drop me a PM and let's set up a day (or evening) to go out ...

[edit]Just noticed you're from Montana. Let me know when you're heading out this way. I'm sure we can get in a dive or two. [/edit]

... Bob (Grateful Diver)