How Deep Am I?

[PaulC]

I have an Uwatec depth timer and an Apeks analogue depth gauge. On reviewing (honest. I did read them the first time. A bit) the instructions for the Uwatec it states that it gives depth in fresh water; so that if I dive in the sea the gauge will read deeper than I actually am. Now that I think I understand, as salt water being presumably denser than fresh the water column will be heavier at a given depth than fresh water. What I was wondering was, is that an industry standard? So if I dived to 'x' metres in the sea off, say, Oz, using a locally hired gauge, it would still be giving me metres of fresh water? And consequently, if I become a tech diver and dive in the sea to '70' metres, I won't actually be 70 metres down? But if I did the same in Dorothea I would be? Does this matter? And should I be concerned that my Apeks gauge is reading significantly (>10%) shallower than my depth timer (yes I'm being conservative and working off the deeper reading; but I'd like to know...)? Thanks for your comments, Paul

[Jay]

I'll probably get shot down here as I'm trying to remember this from memory (will look it up when I get time). Salt water is 1.1 times denser than fresh water. I was under the impression that all SPGs were set up for salt water unless specifically designated otherwise. The reason being that it would err on the side of caution (ie would actually be shallower in fresh water). Therefore 70m salt water SHOULD equal 63.63m fresh water.

Anyone else agree with this mumbo jumbo?

I'll look it up later (honest!)

[Davey Willo]

Its very early in the morning so my excuse if I'm talking twoddle, is that I'm not fully awake yet.... ok disclaimer over.

Jay, isn't the way it's setup now the correct way, assuming that most dives will be carried out in saltwater the fact that its calibrated in fresh water will make it err on the side of caution when diving in the sea, i.e. tell you your deeper than you actually are?

[Jay]

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Originally Posted by [b

Quote[/b] ]And consequently, if I become a tech diver and dive in the sea to '70' metres, I won't actually be 70 metres down? But if I did the same in Dorothea I would be?

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I read this to mean it was currently the other way around - it would show deeper in fresh than salt.

You're right - its still WAY too early.

[Dominic]

The Aladdins are set up for fresh water. Some other gauges are set up for sea water.

It's vitally important that you know if the gauges &amp; tables you're using to calculate your deco requirements are calibrated for salt or fresh water. If your gauges work on one and the tables work on the other, you're either going to spend longer in the water than you need to, or not long enough.

If you're having your computer calculate your deco, you don't have to worry about it that much

IIRC, most tables work on fresh water, because salinity varies in sea water depending where you are, whereas fresh water is always fresh water.

[andy2tanx]

it IS too early but if you are set up for fresh water then in the sea your gauges/computers will be erring on the side of caution, yah?

[Dominic]

Your computer won't be erring on the side of caution any more in the sea than it does in fresh water.

Computers measure ambient pressure and calculate deco from it. It's the pressure that matters, not the depth. Your computer doesn't know or care what the water around you is made of, or how deep you are. It only worries about how much pressure you're under.

That's why the problems only come if you're using tables and gauges that are calibrated differently.

[PaulC]

Thanks for that folks...I think I understand...yes sorry to be obscure Jay :dunce: ; what I was trying to get my head round was the fact that I might be diving in the sea and my gauge reading 70m when I wasn't at 70m but was at 63 or whatever. I suppose that that could cause a difficulty if you planned a dive using Admiralty charts on the basis that a wreck, etc, was say 40m down. Imagine your surprise when you got to the bottom of the shot and your gauge read a cheery 44m. Thanks for the warinng about tables Dominic: I use good old DSAT tables which I suppose are calibrated for fresh water. Although I don't think they expressly say so...

[Davey Willo]

Hey Paul,

Whats all this talk of 70m... Are you a closet techie?

[andy2tanx]

if a techie comes out of the closet would said closet be a black one?