Salt - impact on weights?

[justwannadivenow3]

Hi - just got back from a dive trip in Hurghada. I was very surprised with how much weight I needed to use ( an extra 4 kg!). One of the dive guides mentioned the Red Sea had more salt and this was common...

anyone else have a similar experience?

 

[victorzamora]

The Red Sea has a specific gravity of 1.028 – 1.035. "Typical" ocean saltwater has a specific gravity of 1.025.

So, yeah, you'll need ~3.5% of your total weight in lead in the Red Sea than in freshwater whereas you'll only need ~2.5% for typical ocean diving.

 

[Altamira]

Yes, the Red Sea is more saline than most places I have dived, and is considered to be one of the most saline bodies of water in the world, not including lakes such The Dead Sea and Great Salt Lake..

 

[stepfen]

The Red Sea has a specific gravity of 1.028 – 1.035. "Typical" ocean saltwater has a specific gravity of 1.025.
So, yeah, you'll need ~3.5% of your total weight in lead in the Red Sea than in freshwater whereas you'll only need ~2.5% for typical ocean diving.

Hence the difference between normal sea and the Red sea should be just 1%, or between fresh water and Red Sea 3.5%. BTW I hear similar stories about the Mediterranean sea.

The OP claims that this 1% or 3.5% difference is 4kg which can't be true unless the OP dives with 400kg of lead in normal sea or 115kg in fresh water. Doesn't make sense to me...

For a "typical" 5-10kg weight belt the difference should be hardly noticeable (100-300gr or so)...

It is a bit late here and I am tired so my maths might be a bit off...

 

[stepfen]

I forgot to mention a possible explanation for the OP's experience. In a LoB I did in Egypt in November everybody was wondering about the exact same thing until somebody took a hand scale and actually measured the lead blocks the boat had. Before that everybody though they were 2kg a piece, while in fact they were 1.5kg... Hence for example you use 4 blocks of lead thinking you have 8kg while in reality you have only 6kg... You keep adding 2 blocks thinking you are now at 12 kg (4kg more than normal) while in fact you are now at 9 kg (only 1kg more than normal)

As usual just my 2c... I might be totally wrong...

 

[Esprise Me]

Somewhat related, I had a guide tell me that the ocean near Cabo San Lucas, Mexico was saltier than anywhere else on earth except the Dead Sea. He did mention the Red Sea as another high-salinity place where you'd need more weight. I remain dubious. I was wearing a thinner wetsuit than I used back home, so I couldn't make a direct comparison, but my buoyancy changed about as much as you'd expect based on the wetsuit.

 

[Eksilbergenseren]

It is a bit late here and I am tired so my maths might be a bit off...

I am afraid your math is way off.
Everything is more bouyant. Not just the lead.

You need to add 1% (or 3.5% if comparing to fresh water) to the total weight of the diver including gear and weights. If I am a litle over 100 kg fully equipped 3.5% is pretty close to 4 kg. If the 4 kg increase is compared to salt water else where 4 kg is a bit much, as it means you have a 300 to 400 kg diver, depending on salinity.

 

[victorzamora]

The OP claims that this 1% or 3.5% difference is 4kg which can't be true unless the OP dives with 400kg of lead in normal sea or 115kg in fresh water. Doesn't make sense to me...[/QUOTE]

I am afraid your math is way off.
Everything is more bouyant. Not just the lead.

So, yeah, you'll need ~3.5% of your total weight in lead in the Red Sea than in freshwater whereas you'll only need ~2.5% for typical ocean diving.

Your explanation of "light" weights is also very possible.

 

[stepfen]

I am afraid your math is way off.
Everything is more bouyant. Not just the lead.

You need to add 1% (or 3.5% if comparing to fresh water) to the total weight of the diver including gear and weights. If I am a litle over 100 kg fully equipped 3.5% is pretty close to 4 kg. If the 4 kg increase is compared to salt water else where 4 kg is a bit much, as it means you have a 300 to 400 kg diver, depending on salinity.

Sorry, we need to take it a bit easy here, as I said I might be wrong.
Assume that a diver is 75 kg and full equipment (without lead for now) weights another 20kg dry. Total 95kg.
Question no 1. Do we agree that the diver with equipment will be 95kg anywhere on planet earth??? I hope yes.

Now let's move in the water - fresh for now. The diver jumps in a fresh water lake with his equipment and say that he/she is 5kg positive. Question no 2. Do we agree that with 5kg of lead we will have: a) Total (diver + equipment + lead) weight 100kg, and b) neutral in fresh water???
Let's define here neutral as being neither floating neither sinking - hence since the diver and equipment (+ lead) weights 100kg he/she also needs to displace 100kg of fresh water to be neutral, so in total he/she have 0 buoyancy (hence neutral). Are we clear until here?

Now let's move to "normal" sea (specific gravity of 1.025) water. Assuming the same equipment and weights we have: Total weight is still 100kg (see Question 1). Total volume remains the same. What change is the weight of the amount of water the diver displaces. In fresh water he/she was displacing 100lt of FRESH water weighting 100kg (specific gravity of 1), now he/she still displaces 100lt of SEA water hence 102.5kg. Question no 3: Do we agree that now in the sea he/she needs to add 2.5kg of extra lead (7.5kg lead in total) to become neutral?

Now let's move to the initial weights of 5kg and go to the Rea Sea. Assume for now that he/she hasn't eaten yet enough falafels to gain weight hence total weight is still 100kg - at least until the first portion of falafels is served. The diver jumps in the water and again displaces 100lt of water, which now weight 103.5kg. Question no 4: Do we agree that now in the Red Sea the diver needs to add 3.5kg extra (8.5kg lead in total) to be neutral?

So to sum up: in fresh water the diver would need 5kg, in normal sea 7.5kg and in the Red Sea 8.5kg. Hence the difference between normal sea and red sea is just 1kg or ~13% increase. Yes I was wrong saying that:

For a "typical" 5-10kg weight belt the difference should be hardly noticeable (100-300gr or so)...

, (it is apparently 1kg which can be noticeable) but anyway the point still holds: 4kg extra as the OP claims doesn't seem possible and the reason is given above.

Cheers

 

[STIDVC]

Good replies, never expected such drastic difference in salinity in the oceans. definitely learned something today!