22 mm vx 15 mm tubes

[OMyMyOHellYes]

Most newer machines have a setting for 22 vs 15 mm tubing.

Why?

The pressure out the front of the machine is a set pressure regardless of the size tube it is going into. If the machine reads a pressure of say 12 cm H2O, then it will pump a 15mm diameter tube up to 12 cm H20 and it would pump up a 22 mm diameter tube to 12 cm H2O.

So what am I missing?

OMM

[TyroneShoes]

The setting is for the resultant pressure to the patient. This is calculated by the pressure out of the xPAP minus the loss through the tube due to the Venturi effect. The pressure is calibrated by simulating a hose on the output of the xPAP, and the cm of water raised is measured downstream from the hose, so that the pressure can be set at the xPAP (slightly higher) to provide the therapeutic pressure to the patient.

A skinnier hose has more resistance, therefore it outputs less resultant pressure unless compensated for by raising the xPAP pressure, so there are settings for different sized hoses, and even different masks.

Blow through a skinny straw. Blow through a fat straw. The skinny straw causes more turbulence which resists the pressure more, meaning you have to blow harder to get the same pressure at the output end of the straw.

The pressure will be similar, just slightly different. But what is much different is the speed of the air. When air goes through a restriction, it speeds up (pressure remaining the same), so a larger diameter hose means less noise, and potentially less CO2 venting noise.

In free atmosphere, this is what accounts for lift on an airplane wing; the air is forced to go further (and therefore faster) over the top rounded surface of an airfoil than it goes over the flat bottom surface, and so the pressure decreases correspondingly on the top surface, and the plane rises, in accordance with Bernoulli's Principle. Another example of this law of fluid dynamics is when you turn on the hose. The pressure stays essentially the same, but if the end of the hose is restricted by your thumb creating a smaller opening, the water speeds up.

[OMyMyOHellYes]

I get that, but I am asking more if this is kind of a PII thing (preoccupation w/ inconsequential increments). How much pressure change is induced by changing from a 22cm tube to a 15cm tube of, say, six ft~two meter length all other factors being held equal? Or, probably more common, changing from a six foot 22mm tube to a 9 foot 22mm tube?

I see the diameter being more of a flow rate issue than a pressure issue. And in the PAP application, we are not talking an open flow tube, but more of a closed vessel with a controlled exhaust - more like the inside of a pressurized aircraft's fuselage.

Has anyone got the scoop on how much difference, using a standard six foot~two meter hose, that a 15mm tube vs a 22 mm hose would make? Is it 11.98 cm vs. 12.00 cm of H2O or more like 11.03 cm vs. 12.00 cm of H2O?

If you are setting a CPAP pressure, and aiming for the 95th percentile pressure, we are looking at averages over time and are not that precise anyway.

And if using an auto adjusting machine, wouldn't it be less relevant in that the machine is responding more to events than just pressure?

OMM

[surferdude2]

I agree and will add my thoughts on the subject for consideration.

The manufacturer obviously takes pressure measurements at the mask cavity on a before and after basis while attaching the various hoses and masks that are approved for the flow generator in question. From those measurements, it is determined how much additional pressure will have to be added to the output pressure, as measured at the flow generator, to give the proper therapy pressure at the mask when using any particular user selected combination of mask and hose. i.e., When you have the flow generator set to deliver 8 cm H2O pressure to your mask, it may actually be furnishing 9 cm H2O pressure at the outlet port of the generator. It isn't necessary for the user to know that since all that is needed is to select the mask type, hose type, humidifier (whether off or on) and dial up the desired pressure wanted at the mask. The firmware on the flow generator control board does the rest.

In a perfect world, the output pressure measured at the flow generator would be the same as is measured at the mask but we must consider the friction losses when the air volume is moved through the hose and humidifier and also consider that the losses will be higher if a greater volume is moved through the hose (as when a FF mask is used versus a nasal mask). Actually the friction losses vary with both volume and pressure but the small Delta P we are dealing with is insignificant.

Strangely enough (or perhaps not), the air volume being aspirated by the user doesn't get much (if any), consideration in these calculations since it amounts to an equal positive and negative quantity that cancels itself out.

None of these fudge factors would be necessary if we actually had a sensor in the mask that measured pressure but that isn't practical at the present time for home users.

[justMongo]

We're looking at this like it's an impedance effect. It think it's more of a transmission line effect.
Since our flow is a plus/minus flow waveform, the 15mm v 22mm hose will have different transmission velocity effects.
(There is also the FOT waveform to consider.)

[surferdude2]

Yes, maybe we need to consider the SWR on the hose. Isn't it always the seemingly simple things that are more complex than we realize. This small electric fan housing with an internal processor control board can actually deliver up all this data to us with no wires connecting to the mask. Simply amazing! Now if someone could amaze me similarly with a mask that actually is comfortable and does the job well.

Best regards,

Dude

[TyroneShoes]

... I am asking more if this is kind of a PII thing ...

I see the diameter being more of a flow rate issue than a pressure issue...

Has anyone got the scoop on how much difference, using a standard six foot~two meter hose, that a 15mm tube vs a 22 mm hose would make? Is it 11.98 cm vs. 12.00 cm of H2O or more like 11.03 cm vs. 12.00 cm of H2O?

If you are setting a CPAP pressure, and aiming for the 95th percentile pressure, we are looking at averages over time and are not that precise anyway...

It apparently is a PII thing for at least one of us, and its possible that you might have answered your own question.

But the xPAP is designed to deliver a certain pressure, and not a certain flow rate. Pressure is an easily-controlled parameter, while flow rate can be all over the place due to numerous factors.

---

I agree and will add my thoughts on the subject for consideration.

The manufacturer obviously takes pressure measurements at the mask cavity on a before and after basis while attaching the various hoses and masks that are approved for the flow generator in question. From those measurements, it is determined how much additional pressure will have to be added to the output pressure, as measured at the flow generator, to give the proper therapy pressure at the mask when using any particular user selected combination of mask and hose. i.e., When you have the flow generator set to deliver 8 cm H2O pressure to your mask, it may actually be furnishing 9 cm H2O pressure at the outlet port of the generator. It isn't necessary for the user to know that since all that is needed is to select the mask type, hose type, humidifier (whether off or on) and dial up the desired pressure wanted at the mask. The firmware on the flow generator control board does the rest.

In a perfect world, the output pressure measured at the flow generator would be the same as is measured at the mask but we must consider the friction losses when the air volume is moved through the hose and humidifier and also consider that the losses will be higher if a greater volume is moved through the hose (as when a FF mask is used versus a nasal mask). Actually the friction losses vary with both volume and pressure but the small Delta P we are dealing with is insignificant.

Strangely enough (or perhaps not), the air volume being aspirated by the user doesn't get much (if any), consideration in these calculations since it amounts to an equal positive and negative quantity that cancels itself out.

None of these fudge factors would be necessary if we actually had a sensor in the mask that measured pressure but that isn't practical at the present time for home users.

What he said.

Or was that what I said?

[vsheline]

Most newer machines have a setting for 22 vs 15 mm tubing.

Why?

Hi OMM,

Especially on machines which may need to deliver high pressures, such as on bilevel machines where the pressure may go higher than 20 cmH2O, use of the 22 mm tubing is highly recommended.

The amount of pressure drop across the hose will be smaller for the larger diameter hose, and I assume that use of the larger diameter hose yields more accurate therapy when the airflow rates are large because any error in estimating the pressure drop across the hose will be proportionally smaller when using the larger diameter hose. I think this will be especially important whenever the flow is changing quickly, such as when Leak is varying quickly or when a bilevel machine kicks in with a large pressure increase.

[eseedhouse]

We're looking at this like it's an impedance effect. It think it's more of a transmission line effect.
Since our flow is a plus/minus flow waveform, the 15mm v 22mm hose will have different transmission velocity effects.
(There is also the FOT waveform to consider.)

Well, even 20 breaths a minute is an extremely low frequency even in audio terms, around 1/3 hz. A 32 hz organ pedal has a wavelength of around 32 feet, so . That would imply a wavelength of > 300 ft. or about 100 meters. It seems to me that five foot, or even a ten foot hose is going to be so far below that wavelength that the resistance to airflow should be the dominant factor at those frequencies.

I don't think that's long enough to consider as a transmission line. I think resistive impedance would swamp out any capacitance.

But I'm by no means an engineer.

[AshSF]

A 15mm tube adds 323 cc of dead space in the breathing circuit. A 22mm tube adds 695 cc.

Given that our anatomical dead space is ~500cc. A mask adds anywhere from 100cc (nasal pillows) to 250cc (FFM). the tube is on top of that. This may have some consequences for your ventilation metrics, Rebreathing CO2 fraction, EtCO2 etc.

I speculate that, for a fixed leak rate, the CO2 washout will be lesser for a 22mm tube than a 15mm tube. I don't think it will matter too much for most people but for some, it may mean a slight headache in the morning for same AHI/RDI numbers.