When is effective depth different from actual depth?

[FPDocMatt]

I notice that sometimes people refer to the depth as more than just the vertical depth from the surface of the water.

For example, after descending 60 feet and penetrating a cave for 20 feet, one might speak of 80 feet of something -- depth?

When exactly does one make such adjustments in the "depth"? And why?

Thanks!

 

[SnorkelLA]

If its cold (50s or lower Farenheit) they say to add ten feet to the depth. When I cave dive or wreck penetrate, I still go with the ACTUAL depth.

 

[dmoore19]

Matt,

I do not know about cave diving but I can tell you that there may be 2 depths. One (in my business) is called TVD or true vertical depth. The other is MD or measured depth.

For pressure you would use TVD. The MD or measured depth would also include horizontal displacement. I hope that helps. Without a picture I am having trouble trying to explain it to you.

True vertical depth would be the depth in a straight line up to the surface. Measured depth would include any horizontal departure from a straight line up.

So if you dropped from the surface to 60' and then went 20 feet horizontal into a cave the Measured depth would be 80' but the TVD is still 60'.

 

[PfcAJ]

There's no difference in 'depth' for cave diving. 80ft deep at 3000' in is still 80ft deep.

 

[TSandM]

Matt, I think the only place you run into that is in the definition of the cavern zone. There, the combination of depth and penetration is used to set the limit. PADI says you can't be more than a combined 130 feet from air and still be in cavern. Other agencies use different definitions.

It's kind of a useful way to think about things in an overhead -- once you exit the rock, you still have the distance to the surface to make, before you are "out".

 

[Kilili]

Ditto

Matt, I think the only place you run into that is in the definition of the cavern zone. There, the combination of depth and penetration is used to set the limit. PADI says you can't be more than a combined 130 feet from air and still be in cavern. Other agencies use different definitions.

 

[gdrseeker]

In altitude diving you have Theoretical Depth VS Actual Depth. For example if you are diving at Lake Tahoe we figure the lake at 7000 feet altitude. If I am going to do a dive to 30 feet the Theoretical depth I use for planning my dive is 39 feet based on the altitude chart. Tahoe is cold so we add another 10 feet so the total is 39+10 = 49 .....Using the RDP I plan my dive at 50 feet ( Theoretical depth ). I just did my Altitude Dive last weekend fro my AOW cert.

 

[DevonDiver]

I prefer to retain depth as (true) depth.

With regards overheads, I prefer the extra measurement/s of 'distance-to-surface' (meters) or even 'time-to-surface' (minutes).

When adding conservatism, or dealing with altitude or, high-exertion or cold-water diving, it's possible to have a theoretical/planning depth - as taught on OW courses, simply adding extra depth when calculating nitrogen loading via RDP tables. When using a dive computer, you'd achieve the same function by increasing the user changeable conservatism level of the algorithm.

 

[dfx]

The difference salt water vs fresh water also comes to mind. Everything related to compression and decompression is based on pressure, not actual depth. Salt water is denser than fresh water, so pressure at any given actual depth will be higher than in fresh water at the same depth. All depth gauges will therefore consider you to be deeper than you really are (or will consider you to be less deep than you really are in fresh water - depends on their calibration). The difference isn't a whole lot though.

 

[faze]

You may also come across this with respect to planning depth in terms of non deco limits with Nitrox. i.e. a situation when you are using "2" depths.

It's usually referred to as "Equivalent Air Depth" (EAD) and relates to the difference between the NDL on air and the equivalent NDL on Nitrox, e.g. at 31 metres your Equivalent Air Depth on EAN34 would be 24.4m (So you can dive to 31 metres but plan your bottom time as if you were at 24.4m)