snorkel after diving - No fly time risk?

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Water is nearly incompressible, so the volume of water changes very little as the pressure changes. This is also true of most of the body. What does compress is any gas filled areas of the body .... lungs, ears, etc.

Regarding blood pressure, I think you are confusing the absolute pressure of the blood with the differential pressure between different parts of the circulatory system.

Blood pressure readings are normally expressed in millimeters of mercury. 760mm of mercury is 1 atm, so a normal blood pressure of 120mmHg is about 0.16atm. On the surface, your arterial blood pressure is about 0.16atm (120mm Hg) above the ambient 1ata pressure. When you are down at 100' or 4ata, your arterial blood pressure will still be about 0.16ata relative to the ambient. The fact that your absolute blood pressure when at 100' is about 4.16ata doesn't matter.
 
Also, why doesn't your blood pressure change and kill you?
In addition, how does the inside of your body "magically" gain the pressure it experiences outside of your body? To change pressure, you either need a change in temperature, a change in volume, or a change in the amount of material itself. There isn't a temperature change, or you would either freeze to death or overheat, and there can't be a volume change in your entire body or you would die (the blood pressure question listed above), and your body obviously isn't adding or deleting itself. I'm just having a hard time reasoning how your internal pressure can change like that, at least scientifically.

Gas Laws don't hold for liquids. Your heart works like a 1st stage - it automatically adjusts for ambient pressure. What is the volume of 1 pound of water at 1 ATM? What is the volume of that same pound of water a 4 ATM? You could pour it in a balloon and see for yourself.
 
Gas Laws don't hold for liquids. Your heart works like a 1st stage - it automatically adjusts for ambient pressure. What is the volume of 1 pound of water at 1 ATM? What is the volume of that same pound of water a 4 ATM? You could pour it in a balloon and see for yourself.

Ah, so that's my problem. That's right, you can't apply gas laws to liquids because of the intra molecular forces. Duh. You'll have to forgive me, it's been a long time since chemistry.

So, let me get this straight: every time you snorkel down to say, 30 feet, are you facing the same effects as diving to 30 feet? Or is there a difference? I always thought there was a difference because in diving you are breathing compressed air, which is where the nitrogen problem comes about.
 
So, let me get this straight: every time you snorkel down to say, 30 feet, are you facing the same effects as diving to 30 feet? Or is there a difference? I always thought there was a difference because in diving you are breathing compressed air, which is where the nitrogen problem comes about.
You are facing some of the same effects. When free-diving, you are experiencing a similar increase in the pressure of the gas within your lungs, so there will be on-gassing of nitrogen. The amount of nitrogen absorption will be significantly limited because you will only be a depth briefly.
However, when free-diving you are not replenishing the air in your lungs with air from your regulator so your lungs decrease in size. The exterior of your body experiences the same increase in ambient pressure. Your middle ear and mask air spaces both want to compress, so must be equalized from the limited air in your lungs if you don't want to experience pain in your ears and mask squeeze.
 
Ah, so that's my problem. That's right, you can't apply gas laws to liquids because of the intra molecular forces. Duh. You'll have to forgive me, it's been a long time since chemistry.

So, let me get this straight: every time you snorkel down to say, 30 feet, are you facing the same effects as diving to 30 feet? Or is there a difference? I always thought there was a difference because in diving you are breathing compressed air, which is where the nitrogen problem comes about.

The pressure of the gas in your lungs is ambient pressure just like the gas inside your 2nd stage. Only your tank, ist stage, and hoses (up to the LP seals) see elevated pressures. If the pressure in yourt lungs ever rises above ambient (like when ascending with the airway closed), the overpressure will do severe damage to your lungs.
 
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