pressure tubing length question

I have a question that I believe that I already know the answer to, but I want to post this as proof to a relative.

My dad uses a VPAP Adapt SV. Several years ago, one of our cats started chewing on the pressure tubing (the clear tubing) line at night, causing leaks that the machine immediately detected. It indicated (correctly) that there was low pressure in the line. When my dad saw the holes, his solution was to cut off the affected line of pressure tubing and put the attachment cuff in a new spot. This kept the remaining pressure tubing intact (i.e. w/o holes), but the line is now shorter, at around 4 ft, whereas the air tubing is still 6 ft. It works intermittently, but often warns that the circuit is invalid.

Again, I believe I already know the answer to this, but I'll ask anyway: do the air tubing and the pressure tubing have to be the same length in order for the sensor in the machine to correctly determine the pressure and, therefore, to recognize the circuit as valid again? My dad thinks that the fact that the pressure tubing is shorter shouldn't make a difference. I disagree, so I'd appreciate some input. I suspect that his machine hasn't been working well for years because of this tubing length problem, and I don't like how groggy it's made him. Just give me the word and I'll buy the 6 ft pressure replacement line in no time.

dema312 wrote:I have a question that I believe that I already know the answer to, but I want to post this as proof to a relative.

My dad uses a VPAP Adapt SV. Several years ago, one of our cats started chewing on the pressure tubing (the clear tubing) line at night, causing leaks that the machine immediately detected. It indicated (correctly) that there was low pressure in the line. When my dad saw the holes, his solution was to cut off the affected line of pressure tubing and put the attachment cuff in a new spot. This kept the remaining pressure tubing intact (i e w/o holes), but the line is now shorter, at around 4 ft, whereas the air tubing is still 6 ft. It works intermittently, but often warns that the circuit is invalid.

Again, I believe I already know the answer to this, but I'll ask anyway: do the air tubing and the pressure tubing have to be the same length in order for the sensor in the machine to correctly determine the pressure and, therefore, to recognize the circuit as valid again? My dad thinks that the fact that the pressure tubing is shorter shouldn't make a difference. I disagree, so I'd appreciate some input. I suspect that his machine hasn't been working well for years because of this tubing length problem, and I don't like how groggy it's made him. Just give me the word and I'll buy the 6 ft pressure replacement line in no time.

I think he'll be fine as long as there are no leaks in his "repair job".
Most of the problems arise when using very long hoses and auto-adjusting machines.
But, 6' hoses are fairly cheap (about $10.00 from online sellers) and everyone should have a few spares on hand for emergencies.


Den

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I can't answer the technical question, but as hoses are so inexpensive relative to everything else, why not just get him 1-2 new ones of the right type and present them to him? Would he turn them down?

Wulfman... wrote:This kept the remaining pressure tubing intact (i e w/o holes), but the line is now shorter, at around 4 ft, whereas the air tubing is still 6 ft. It works intermittently, but often warns that the circuit is invalid.

What kind of tubing are we talking about, that warns that the circuit is invalid, and from the statement it souunds like the total tubing lenght is now 10 feet long.

It does make a difference. The shorter length of the tubing causes the pressure to hit the sensor "sooner". Any time he replaces the circuit or replaces the mask he needs to check the circuit by running the check circuit function. If he gets an error message, the vpap asv may not function properly.

An interesting machine - it has 2 hoses - one as the supply line for air to the mask and a small hose feeding back to the unit to measure the pressure at the mask.
link to user manual = http://www.resmed.com/us/assets/documen ... er_eng.pdf

Most interesting setup that I have never run across before - perhaps the tube length does have an effect in this machine. For my machine I would expect it to have limited to no effect in actual pressure to remove a foot or two. With this machine, I am not so sure.

With such a length difference in the two tubes, be sure that the pressure tube is not getting kinked, or there will be no feedback on the pressure

This must be a very old machine.

Reducing the tubing length increases the pressure at the mask because the frictional losses in the tubing are lower (engineers use Moody diagrams to figure this out, example here: http://upload.wikimedia.org/wikipedia/c ... iagram.jpg, gory technical details here: http://en.wikipedia.org/wiki/Moody_chart) I haven't done the calculation, however I estimate reducing a 6' hose to 3' will increase the pressure at the mask by a cmH2O or two over what the CPAP machine thinks it is. In general, however, the diameter of the hose will have a larger impact on pressure at the mask than the length of the hose, which is why CPAP machines require the hose size to be input as part of their setup but not the length.

Cats? In the bedroom? At night? Ya gotta do something about that. The rest will follow.

Get it? The "rest" will follow?