Computer Setting - Fresh or Salt?

My dive computer requires me to select a fresh or salt water input prior to a dive. My question is if I descend into a cave system where the top 30 feet of the water column is salt water and the next 70 feet of descent is freshwater which input would you start with and why?
I know this question is splitting hairs and won't change the actual PO2 much but curiosity has just got the better of me. I personally would choose the saltwater setting to be on the conservative side. I came across this as I was doing some dive planning on my computer and noticed the NDL times were different as were the deco times.

Thanks in advance
WB

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Originally Posted by SRQ

My dive computer requires me to select a fresh or salt water input prior to a dive. My question is if I descend into a cave system where the top 30 feet of the water column is salt water and the next 70 feet of descent is freshwater which input would you start with and why?
I know this question is splitting hairs and won't change the actual PO2 much but curiosity has just got the better of me. I personally would choose the saltwater setting to be on the conservative side. I came across this as I was doing some dive planning on my computer and noticed the NDL times were different as were the deco times.

Thanks in advance
WB

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ppo2 and tissue loading are functions of pressure, fresh/salt really doesn't matter....unless I'm misunderstanding the question?

What James was driving at is the deco schedule will be right - the only thing is, if you set it for fresh and are in salt, or vice-versa, the depth reading will be off by one foot per 33 feet, but that really won't matter. Unless you're more OCD than me!

Thats what I meant by splitting hairs. One foot difference is getting pretty well , uh - yeah I guess OCD covers it - LOL
But in reality would the salt water being on top and the heavier of the two would that in effect raise the pressure of the freshwater at the lower depths? My guess yes.

The fresh/salt selection just determines the number displayed on the depth display. The actual pressure is used for the deco calculations. It doesn't make any difference to the ppO2 or the deco which is displayed.

What it does change is the depth logged. If you set it on fresh water you will log the dives as 2-3% deeper.

The reason the NDLs change is because the pressure you are exposed to at "80 ffw" is different than the pressure you are exposed to at "80 fsw" so even though the computer displays 80' in both cases the depths are actually different so the displayed NDLs are different.

If the first 30' is seawater than at any depth deeper than 30' the "actual depth" can't be correct regardless of your setting: choose the one that reads the deepest. :)

On the other hand if you are using tables as a backup it could be a good idea if your depth readout matches up with the tables you are using. If you are unsure then fresh will read deeper and be more conservative (not that you may know what depth you are at anyone if your computer failed and you didn't have some sort of backup).

I've heard mud is much more dense than water. If you were "mud diving" your effective depth might be several times your actual depth. Regardless of your depth in mud the dive computer will read out and compute deco for the actual pressure you are at.

Gary - you confirmed what I suspected. I just chose a poor way of asking the question.

Thanks
WB