Diving for Dummies Series. Part III: Henry’s Law for Halfwits
PART III: HENRY’S LAW FOR HALFWITS !
Article by Stuart Jacques, 2005
William Henry was a mancunian1, born in the early 1800’s. That’s the boring history over
with! He came up with this one:
“The amount of any gas that will dissolve in a liquid, at a given temperature, is a function of
the partial pressure of that gas in contact with the liquid, and the solubility coefficient of the
gas in the particular liquid.”
Phew! What does all that mean? To help understand this, go down to the shops and get
yourself a bottle of lemonade. Inside the bottle, you’ll see the lemonade (the liquid) at the
bottom and above it, just under the cap, there’s a gas. This gas just happens to be carbon
dioxide and it is pressurised. Henry’s Law says that because a liquid is in contact with a
gas, that gas will dissolve into the liquid. The higher the pressure of the gas, then more
gas will dissolve. That’s how your lemonade becomes fizzy, pressurised carbon dioxide
has dissolved into the lemonade!
To understand how Henry’s Law affect us in diving (rather than how we end up burping
after drinking lemonade!) first you must understand a little about human “fizzyology”!
We need air to breathe. Simply speaking air is made up of 79% nitrogen and 21% oxygen.
With me so far? You should be! Air is drawn into the lungs, the oxygen and nitrogen in
the air travels through the lungs and ends up passing in the bloodstream where it is
pumped in the blood around the body by the heart (read your diving manual).
So, we end up with a situation inside us a bit like the lemonade bottle example above.
Replace, for a moment, the lemonade with your blood and the carbon dioxide with air you
breathe. The air we breathe is pressurised, right? It has to be for us as divers else we
couldn’t breathe it in and the deeper we go the higher the pressure of this air (or gas) has
to be. The interface between the lemonade and the carbon dioxide is in our lungs. The
deeper we go the greater the pressure of the air we have to breathe and the more air is
dissolved into our bloodstream, get it?
We can be really clever now we understand a little bit more and divide the “air we breathe”
into it’s separate constituent parts, that is the nitrogen and the oxygen and we can refer to
the “partial pressures” of each constituent (refer to Part II: Dalton’s Law for Dummies). We
can look at the amount of nitrogen and the amount of oxygen that dissolves into our
bloodstream.
It’s the dissolved nitrogen gives us as divers the problems. Lots of it dissolves into our
bloodstream as there’s lots of it in the air we breathe and the amount absorbed increases
as we go deeper for longer. Imagine you are diving at say, 30 metres. The ambient
pressure here is 4 bar. This means that you breathing air at 4 bar supplied by your
regulator (you have to be else you couldn’t breathe!). That’s four times the pressure of the
air you were breathing at the surface! 0.79 x 4 bar = 3.16 bar of nitrogen is beginning to
enter your bloodstream (compared to 0.79 x 1 bar = only 0.79 bar of nitrogen entering your
bloodstream at the surface). That’s a big increase and all this nitrogen travels around your
1
From Manchester!
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Diving for Dummies Series. Part III: Henry’s Law for Halfwits
body and eventually is “absorbed” by the tissues in your body to which your bloodstream
services.
“FAST” & “SLOW” TISSUES
Some body tissues “absorb” this nitrogen very quickly, some more slowly. In diving we
can track the amount absorbed by different parts of the body dividing the body up into
areas or “compartments” which all absorb the gas at similar rates. We call the tissues
which absorb the nitrogen very quickly “fast tissues” and the ones that take longer, “slow
tissues”. Fast tissues such as the heart, lungs and abdominal organs (in fact any with a
good blood supply) fill up or become “saturated”2 very quickly (sometimes within minutes).
The slower tissues such as fats, cartilage and joints can take hours (and may never
become saturated using SCUBA equipment as you just can’t stay breathing the high ppN2
for long enough as, before this can happen, your air runs out!). This is where the term
“Saturation Diving” comes from in the Offshore Industry as the divers spend a very long
time underwater where their whole body can become “full up” of nitrogen at depth just as
we all are “saturated” with 0.79 (bar) nitrogen at the surface.
Your Repetitive Group Designation Letter in your chosen decompression tables is one way
in which you can track how much nitrogen is in your system. Long bottom times at deep
depths allocate you a “high” letter in the alphabet denoting your body’s high level of
absorbed nitrogen. On most diving decompression tables as time goes by you will tend
towards the start of the alphabet indicating that the amount of nitrogen absorbed over time
is reducing but what’s going on here? How did you get rid of the nitrogen you’ve just
absorbed in your dive?
“OFF-GASING”
Let’s go back to the lemonade bottle example. Let’s unscrew the cap! What happens?
The lemonade spills out of the bottle and all over your front, right? Why? Well let’s look at
what you’ve just done. By unscrewing the cap you allowed the pressurised carbon dioxide
(the gas just under the cap) to escape. Henry (remember him?) told us that the amount of
gas which will dissolve in a liquid depends on the pressure of the gas in contact with that
liquid. You’ve just reduced the pressure of the gas in contact with the liquid. Now there’s
too much gas in the lemonade and it comes out of solution – it’s just Henry’s Law in
reverse. The faster you unscrew the top the faster you alter the pressure of the gas in
contact with the liquid and the faster the gas comes out of solution. That’s why you open
the screw cap slowly to allow the gas to come out of solution slowly and then you don’t get
a wet T-Shirt! If you do see gas coming out of the lemonade too quickly you close the cap,
right? Wait a bit then slowly open it again. You might repeat this 2 or 3 times, right?
Sound familiar?
Let’s look at what’s happening inside us when we ascend from a dive. We have a high
pressure gas (that is the air from the regulator) in contact with our bloodstream/tissues
which, over the duration of the dive, have been absorbing that gas. Some tissues (the fast
ones) could be totally saturated with the gas, whilst others only partially “fill up” and others
2
No more can be absorbed
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Diving for Dummies Series. Part III: Henry’s Law for Halfwits
(the very “slow” ones) may be hardly touched – it depends on the depth and duration of
your dive.
We now start an ascent and by doing so we reduce the pressure of the gas in contact with
our bloodstream – in effect we unscrew the lemonade bottle cap! The gas (let’s just
consider the nitrogen for the moment) in our system has to come out as the pressure of
the nitrogen in contact with the blood has just been reduced - exactly the same as the
carbon dioxide coming out of the lemonade). Using safe ascent rates and practices, which
we’ll teach you in your diving course, will cause the nitrogen simply (and safely) to pass
back in to lungs and escapes as you exhale BUT if we ascend too fast the nitrogen may
come “out of solution” and form bubbles in the tissues/bloodstream. THIS IS BAD NEWS!
DECOMPRESSION ILLNESS (DCI)
Bubbles3 are not supposed to be in our bloodstream. The gas is supposed to be in
solution ie absorbed. As soon as bubbles form – a diver could be in big trouble! Now
some bubbles (called microbubbles) are always present after every dive. In fact you can
wire yourself up to a machine called a “Doppler” and listen to them going around your
bloodstream) but we are concerned here mainly with the larger bubbles – the ones that
cause Decompression Illness.
The bubbles formed by opening the screw cap too quickly are transported by the
bloodstream around the body and can damage or strip the internal lining of the blood
vessels but it’s where they end up that causes the real problems. For example, if the
bubble attempts to pass down a blood vessel for which it is too big (remember these
bubbles get bigger by expansion of the gas the shallower you get!) it will block off the flow
of blood down that vessel and, whatever is at the end of it will be starved of blood (and
therefore oxygen!). Tissues do not like being starved of O2 – they die! Bubbles also have
a nasty habit of clinging together because of surface tension (if you don’t believe me have
a look in the top of your next pint of lager!). Can you remember the last time you were sat
in a chair with your legs crossed – you eventually had to move because you got pins &
needles, right? The pressure on your leg was preventing blood from getting down your
legs and tissues further down were being starved of oxygen. Your body tells you to move
as “you are killing part of me!” Exactly the same happens with the bubble, same
symptoms etc but unfortunately you cannot get into the blood to remove the bubble – you
have to be recompressed (in a chamber) – quickly! – before bits start to die!
Recompression in a chamber reduces the size of the bubble(s) again, gets them back into
solution again and hopefully you can get it out of the bloodstream in the normal way to “fix”
the patient.
The bubbles can go anywhere really, some get stuck in arms or legs (sometimes it’s more
comfortable for the casualty to hold the limb in a “bent” position hence the term “the
bends”), others get stuck in the ears affecting balance (the “staggers”). Bubbles affecting
the nervous system can cause real trouble and bubbles affecting blood supply (or the
stripping of the vessel lining) to the brain for example, can be fatal.
3
the posh/medical word for “bubbles” is emboli
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Diving for Dummies Series. Part III: Henry’s Law for Halfwits
So you can see the signs and symptoms of DCI can be extremely varied – you’ll learn
more about these in your diving course. The treatment however is the same: try not to
breathe any more nitrogen (use readily available O2 instead) and get recompressed quick
(to reduce the size of the bubbles)
SAFE ASCENTS
So, how do we avoid all these problems? We now know that we have to keep the bubbles
“in solution” and we also know that to do this we must ascend slowly. In effect we need to
open the screw cap slowly but just think back to opening the lemonade bottle for a
moment. Sometimes even opening in slowly wasn’t good enough, you had to close the
cap, wait a bit, then open it again to prevent getting a wet T-shirt! It depended upon how
“fizzy” the lemonade bottle was right? Put another way it depended upon how much
carbon dioxide gas was absorbed in the lemonade.
You do exactly the same when you are diving. On some dives where you are heavily “gas
loaded” with nitrogen and you start to ascend (that is open the cap), you must stop your
ascent (close the cap), wait for a while and then continue your ascent (open the cap
again). This opening, waiting and then closing the cap again prevents the bubbles coming
out of the bottle, right? Recognise them yet? They are called decompression stops and
you need more stops for longer times depending upon how much gas you must offload.
You’ve probably noticed that the lemonade goes flat eventually? This is because there’s
no pressurised gas in contact with the liquid anymore – all the carbon dioxide in the
lemonade “escapes” over time. Once more, exactly the same happens inside you, that is,
eventually all the nitrogen escapes from your system – the majority of it escaping whilst
you are back at the surface. Using the surface interval chart in your decompression tables
show you the rate at which you are “losing” the nitrogen. Eventually you are free of the
nitrogen load and known as a “clean diver” – mmm, a clean diver, not in my lifetime ☺
Safe diving!
~END~
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