Air, which was first used in diving tanks, contains roughly 20% oxygen, and 80% nitrogen.
The trouble with nitrogen in this situation is that nitrogen is a fairly heavy gas, and is soluble in blood at high pressure. Long term use of nitrogen can cause a strange sense of euphoria, or well being called nitrogen narcosis. This is a bit like being drunk, and makes the diver unable to assess dangers. Just as working drunk on the surface is not a good idea, working drunk at the bottom of the sea can be extremely hazardous.
Divers working at depth would have nitrogen dissolved into their blood, and it becomes necessary to decompress (surface) very slowly, to allow the nitrogen to come back out of the blood to be breathed out. Divers who come up too quickly have a huge RISK of the nitrogen being released from the blood too quickly and forming bubbles in the bloodstream. This causes blockages, as the bubbles, although tiny, cannot pass through the fine capillaries. The condition is called the Bends, is very painful, and life threatening.
One way to avoid all these problems is to avoid the nitrogen. Divers who work at depth or for long periods use a mixture of 20% Oxygen and 80% Helium. Some recreational divers use a slightly cheaper mix of Helium, Oxygen and air. It still has nitrogen, but less than air.
Helium is used for a number of reasons - It is light, cheap, and does not dissolve in blood the same way that nitrogen does. Being inert it cannot be toxic to the diver or corrosive to equipment.
Nigel Skelton
"Air" tanks are used by divers at various depths of water. This higher pressure dissolves more "air" the deeper the diver goes. If the diver ascends too quickly, the equilibrium of the dissolved "air" in the lungs does not have sufficient time to equilibrate with the pressure at the decreasing depths. Like opening a carbonated beverage, if this happens in the divers blood, a painful hang-up of gases occurs, causing bubbles to form in the blood stream. The name of this event is called "the bends" (you can Google this for details). Of the atmospheric gases, nitrogen is the least soluble but highest concentration (about 80% by volume), so this is the gas that tends to form "bubbles" first.
In diving tanks, pure oxygen is mixed with helium rather than with nitrogen (which is removed by purification). Helium being almost totally insoluble in water (blood) does not tend to form these bubbles. Dissolved and gaseous oxygen is rapidly EXCHANGED so it does not present such a problem. In very deep dives the ratio of oxygen to helium is optimized to even minimize this problem even with oxygen.
Vince Calder
For normal, no decompression dives helium is not used in scuba tanks, just normal air. Technical divers who go beyond recreational depths EMPLOY the use of trimix. Trimix is a mixture of oxygen nitrogen and helium. Under pressure nitrogen and oxygen will build up in the BODYwhich is not a problem at normal recreational limits. Beyond 100 feet nitrogen will start to cause a condition called nitrogen narcosis that causes decreased motor skills and a euphoric state. Beyond 190 and 220 feet oxygen will become toxic,resulting in sensory distortions and seizures. Helium is used to dilute the oxygen and nitrogen to reduce these affects. Helium is the gas of choice to use because it is an inert gas, is thinner, therefore more compressible than air, and its narcotic properties are negligible in comparison to nitrogen. In a nutshell it allows a diver to go deeper for longer periods of time.
Answer from Matthew Faulkner, dive instruction and PhD student in Immunology the Durdik lab
Jeannine M. Durdik
Professor and Vice Chair of Biological Sciences
UNIVERSITY of Arkansas
As a diver goes deeper, the pressure increases and gasses breathed are forced into body tissues (that, or the diver gets squashed). Helium is used to replace the Nitrogen found in normal air because at high pressures enough Nitrogen is forced into the body tissues to have a narcotic effect. Nitrogen narcosis or "raptures of the deep" affects the ability to think clearly and that can lead to serious problems and even death. Stories abound of divers offering their regulators (the breathing mouthpiece) to passing fish or similar inappropriate behaviors. There are occasions where, using plain compressed air, people have dived deep and died because of the poor judgments they make under the effect of Nitrogen narcosis. Helium is a Noble gas and is non-toxic at all pressures.
Also, Helium is much lighter than Nitrogen and bleeds out from the tissues that much faster and its use reduces decompression times. with Helium there is less chance of getting the bends (originally called caissons disease).
Robert Avakian
http://www.newton.dep.anl.gov/askasci/gen06/gen06508.htm
The trouble with nitrogen in this situation is that nitrogen is a fairly heavy gas, and is soluble in blood at high pressure. Long term use of nitrogen can cause a strange sense of euphoria, or well being called nitrogen narcosis. This is a bit like being drunk, and makes the diver unable to assess dangers. Just as working drunk on the surface is not a good idea, working drunk at the bottom of the sea can be extremely hazardous.
Divers working at depth would have nitrogen dissolved into their blood, and it becomes necessary to decompress (surface) very slowly, to allow the nitrogen to come back out of the blood to be breathed out. Divers who come up too quickly have a huge RISK of the nitrogen being released from the blood too quickly and forming bubbles in the bloodstream. This causes blockages, as the bubbles, although tiny, cannot pass through the fine capillaries. The condition is called the Bends, is very painful, and life threatening.
One way to avoid all these problems is to avoid the nitrogen. Divers who work at depth or for long periods use a mixture of 20% Oxygen and 80% Helium. Some recreational divers use a slightly cheaper mix of Helium, Oxygen and air. It still has nitrogen, but less than air.
Helium is used for a number of reasons - It is light, cheap, and does not dissolve in blood the same way that nitrogen does. Being inert it cannot be toxic to the diver or corrosive to equipment.
Nigel Skelton
"Air" tanks are used by divers at various depths of water. This higher pressure dissolves more "air" the deeper the diver goes. If the diver ascends too quickly, the equilibrium of the dissolved "air" in the lungs does not have sufficient time to equilibrate with the pressure at the decreasing depths. Like opening a carbonated beverage, if this happens in the divers blood, a painful hang-up of gases occurs, causing bubbles to form in the blood stream. The name of this event is called "the bends" (you can Google this for details). Of the atmospheric gases, nitrogen is the least soluble but highest concentration (about 80% by volume), so this is the gas that tends to form "bubbles" first.
In diving tanks, pure oxygen is mixed with helium rather than with nitrogen (which is removed by purification). Helium being almost totally insoluble in water (blood) does not tend to form these bubbles. Dissolved and gaseous oxygen is rapidly EXCHANGED so it does not present such a problem. In very deep dives the ratio of oxygen to helium is optimized to even minimize this problem even with oxygen.
Vince Calder
For normal, no decompression dives helium is not used in scuba tanks, just normal air. Technical divers who go beyond recreational depths EMPLOY the use of trimix. Trimix is a mixture of oxygen nitrogen and helium. Under pressure nitrogen and oxygen will build up in the BODYwhich is not a problem at normal recreational limits. Beyond 100 feet nitrogen will start to cause a condition called nitrogen narcosis that causes decreased motor skills and a euphoric state. Beyond 190 and 220 feet oxygen will become toxic,resulting in sensory distortions and seizures. Helium is used to dilute the oxygen and nitrogen to reduce these affects. Helium is the gas of choice to use because it is an inert gas, is thinner, therefore more compressible than air, and its narcotic properties are negligible in comparison to nitrogen. In a nutshell it allows a diver to go deeper for longer periods of time.
Answer from Matthew Faulkner, dive instruction and PhD student in Immunology the Durdik lab
Jeannine M. Durdik
Professor and Vice Chair of Biological Sciences
UNIVERSITY of Arkansas
As a diver goes deeper, the pressure increases and gasses breathed are forced into body tissues (that, or the diver gets squashed). Helium is used to replace the Nitrogen found in normal air because at high pressures enough Nitrogen is forced into the body tissues to have a narcotic effect. Nitrogen narcosis or "raptures of the deep" affects the ability to think clearly and that can lead to serious problems and even death. Stories abound of divers offering their regulators (the breathing mouthpiece) to passing fish or similar inappropriate behaviors. There are occasions where, using plain compressed air, people have dived deep and died because of the poor judgments they make under the effect of Nitrogen narcosis. Helium is a Noble gas and is non-toxic at all pressures.
Also, Helium is much lighter than Nitrogen and bleeds out from the tissues that much faster and its use reduces decompression times. with Helium there is less chance of getting the bends (originally called caissons disease).
Robert Avakian
http://www.newton.dep.anl.gov/askasci/gen06/gen06508.htm
لماذا يتنفس غطاس البحار خليطا من الهيليوم والأكسجين
بدلا من الهواء الطبيعي؟!
تتكون معظم أنابيب الهواء من نيتروجين وأوكسيجين بنسبة أربعة إلى واحد . والنيتروجين غاز لا يتمتع بأي نشاط كيميائي يمر في الجسم أثناء التنفس دون أن يسبب أي تغيير كيميائيي أو تأثيرات لاحقة .
إلا أن ضغط الماء يزداد كلما ازداد العمق ، وللمحافظة على تنفس الغواص بشكل طبيعي يجب أن يزداد ضغط مزيج الهواء الذي يتنشقه . وعلى عمق نحو ثلاثين مترا ، مثلا، سيحتاج الغطاس إلى تنشق هواء يزيد ضغطه أربعة أضعاف الضغط الجوي ، وإذا كان هذا المزيج هواء عاديا ، يجب أن تقوم أنسجة الشحم في جسم الإنسان بامتصاص النيتروجين بسرعة اكبر مما يظهر في بقية أنسجة الجسم . وبما أن الدماغ والجهاز العصبي المركزي يتكونان من الدهن والشحم بنسبة 60 بالمئة . لذا يؤثر فيهما الغاز بشكل قوي ، ويعيق حركتهما العادية . وتكون النتيجة تخدير الغواص بالنيتروجين ، وهي حالة مشابهة كثيرا للثمل الناتج من شرب مادة مسكرة .
والخطر الآخر المرتبط بتنشق مزيج النيتروجين في أعماق البحار هو مرض نقص الضغط ( الانحناءات). فإذا نزل الغطاس بسرعة كبيرة الى عمق البحر ، سيتمدد النيتروجين الموجود في أنسجة الدهنية ويكون فقاعة في الدماغ أو الحبل الشوكي أو المفاصل ، ويسبب عددا من الأعراض المرضية من ضمنها الشلل والألم الشديد.
ويمكن للغطاسين تجنب هاتين الحالتين عبر تنشق مزيج الهيليوم والأوكسجين بدلا من الهواء. والهيليوم أيضا غاز خامل غير نشيط ، لا يتفاعل مع أنسجة الجسم ونسبة امتصاصه أقل من نسبة امتصاص النيتروجين . إلا أنه ، يولد حرارة أكبر وأسرع من النيتروجين الأمر الذي يؤدي إلى فقدان حرارة جسم الغطاس نسبة الي درجة حرارة الماء المحيط به، لذا فإن الغطاس الذي يتنشق مزيج الهيليوم والأكسجين مضطر آلي ارتداء بزة غطس ساخنة . وأخيرا هناك سيئة أخرى لغاز الهيليوم هو تأثيره في الصوت حيث يصبح صوت متنشقه كصوت دونالد داك .. وذلك لفترة مؤقتة .
إعداد م/عارف سمان