umbilical air pressure?

When commercial divers receive air(?) through the umbilical cord pumped down from the boat is it at the pressure of depth when they breathe it? If so, is it a gradual change in pressure as it is pumped down?

An umbilical loses air pressure the deeper the diver descends. 2.24 psi every foot they descend in seawater or 14.7 psi per 33 feet. To compensate the tender can increase the pressure as they descend. This is not a big issue if the diver is using a freeflow hat but if it is a hat like a Kirby Morgan or a Bandmask where the diver is using a second stage reg attached to his mask the pressure needs to be adjusted. Most of the second stage regs used in this application have an adjustment knob to allow the diver to adjust how the reg breathes so he can tweak it when diving. An umbilical will have a pneumofathometer hose that is open at the end at the level of the diver. This serves a function similar to a capillary depth guage that is transferred to an depth guage and allows the tender to determine the depth of the diver by loooking at the guage. Also there may be a comm wire to use for underwater communications, a rope and sometimes a waterhose to run warm water into a suit.

james croft:

An umbilical loses air pressure the deeper the diver descends. ....

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This is a good question and answer, but just to clarify a fine point.

The umbilical doesn't actually lose pressure as the diver decends, but the relative pressure inside the hose drops compared to the ambient water pressure which increases with depth.

The internal pressure is has to be maintained at or just slightly above the ambient water pressure around the diver or he would be dealing with a suit squeeze, and no air would flow to him.

For practical purposes the air inside the hose can be considered static and therefore the pressure would be uniform throughout the length of the hose, however if there were pressure gauges tapped in every 33 vertical feet each gauge would read 1 ATM less than the one above it. It seems paradoxical, but while the pump hand is reading 60psig up on deck, the diver at about 125fsw is reading almost zero on his gauge.

Thanks for the answers-- Let me re-word the question. The diver is already at his deepest depth. The air is being pumped down the hose. Is that air being gradually compressed as it travels down the hose? Also, we know that the air in a BC for example, compresses when you descend, thus you must add air to keep the volume the same due to the air being denser. If the air in the hose is gradually being compressed as it is pushed deeper, what do they do to fill up the extra space in the hose?

The absolute pressure in the hose is, more or less, the same from the compressor to the diver. At the diver the hose couples to the hat with a one way valve, which keeps the diver from being compressed into the hat if there were to be a failure of the compressor or the how were to get cut topside.

Were your BC so large as to stretch from the bottom to the surface it would inflate from surface to bottom as the pressure in the bag went up until it was full, and yes the pressure at the top of the bag would have to be about the same as at the bottom.

Thalassamania:

a one way valve, which keeps the diver from being compressed into the hat if there were to be a failure of the compressor or the how were to get cut topside.

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Not *that's* a mental image...

That's not a "mental image" in the olden days, before they put one way valves on hard hats there were divers who were, "buried in their hat."

TMHeimer:

The air is being pumped down the hose. Is that air being gradually compressed as it travels down the hose??

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Not really the air is compressed at the compressor on the deck and is a uniform pressure all the way down the hose, as has been said it must be 14.7psi per atm or else water or the divers lips will enter the hose at the bottom.

TMHeimer:

Also, we know that the air in a BC for example, compresses when you descend, thus you must add air to keep the volume the same due to the air being denser. ?

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again not really, it is not because the air is denser per se, it is because the water is crushing the air in the BC and to achieve the same volume and thus bouyancy you must put more air in.

TMHeimer:

If the air in the hose is gradually being compressed as it is pushed deeper, what do they do to fill up the extra space in the hose?

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I believe the space is filled with more air.