CPAPtalk.com

Wulfman... wrote:

WindCpap wrote:The vent rate is not proportional to pressure. More like vent rate squared is proportional to pressure.

NO! WRONG!

Page 7 of the following manual (for your mask).

http://www.resmed.com/us/dam/documents/ ... er_eng.pdf


Den

.

Look at the curve. It is not a straight line. Proportional means that the relationship is linear, and therefore a straight line.

OkyDoky wrote:I used propotional as the # 3 definition at dictionary.com
3. of, relating to, or based on proportion; relative.
Every mask has a venting pressure curve flow that shows how it increases.
Here is the P10

Even by this definition, a proportional curve is a straight line. If the curve you show was proportional, the flow rate at 20 cmH20 would be 100 lpm. I am not arguing with you, only correcting an inaccuracy.

Pressure Flow Proportional Prediction More Accurate Prediction based on pressure being roughly proportional to square of flow.
4 20 20 20
8 29 40 28.28427125
12 37 60 34.64101615
16 43 80 40
20 49 100 44.72135955

The increases are relative not a mathmatical constant.

The definition of proportional is that the relationship is defined by a mathematical constant if you are talking about numbers.

http://dictionary.reference.com/browse/proportional

Proportional can only be taken as a more loose relationship if you are talking about non-quantifiable things.

WindCpap wrote:

Wulfman... wrote:

WindCpap wrote:The vent rate is not proportional to pressure. More like vent rate squared is proportional to pressure.

NO! WRONG!

Page 7 of the following manual (for your mask).

http://www.resmed.com/us/dam/documents/ ... er_eng.pdf


Den

.

Look at the curve. It is not a straight line. Proportional means that the relationship is linear, and therefore a straight line.

Doesn't look like much of a "curve" to me. It's "straighter" than "curvier". And, possibly at the higher pressures, there's a "Law of Diminishing Returns" coming into play based on the amount of air that can exit through the vents.


Den

.

Wulfman... wrote:

WindCpap wrote:

Wulfman... wrote:

WindCpap wrote:The vent rate is not proportional to pressure. More like vent rate squared is proportional to pressure.

NO! WRONG!

Page 7 of the following manual (for your mask).

http://www.resmed.com/us/dam/documents/ ... er_eng.pdf


Den

.

Look at the curve. It is not a straight line. Proportional means that the relationship is linear, and therefore a straight line.

Doesn't look like much of a "curve" to me. It's "straighter" than "curvier". And, possibly at the higher pressures, there's a "Law of Diminishing Returns" coming into play based on the amount of air that can exit through the vents.


Den

.

Got to love how you said I was wrong, I proved with numbers that I am right, and you still say I am wrong. "Law of Diminishing Returns" has absolutely nothing to do with fluid mechanics. At air speeds less than 1/3 the speed of sound, the square of the flow rate is roughly proportional to the pressure difference through most openings. "Straighter" than "curvier"...really.

Would it have made you happier if I had used the word relational instead of proportional?

WindCpap wrote:The vent rate is not proportional to pressure. More like vent rate squared is proportional to pressure.

I agree with your statement. Over the range in the example, the function is far from a straight line. A simple function that best fits is,

Y = 9.1906(X^0.5577)
correlation = 0.9998

where X = CPAP pressure in cm H2O, and Y = mask design vent flow in lpm.

Someone check my math.

Thank you, all!

A lot to think about and try here.


MWD

WindCpap wrote:

Wulfman... wrote:

WindCpap wrote:

Wulfman... wrote:

WindCpap wrote:The vent rate is not proportional to pressure. More like vent rate squared is proportional to pressure.

NO! WRONG!

Page 7 of the following manual (for your mask).

http://www.resmed.com/us/dam/documents/ ... er_eng.pdf


Den

.

Look at the curve. It is not a straight line. Proportional means that the relationship is linear, and therefore a straight line.

Doesn't look like much of a "curve" to me. It's "straighter" than "curvier". And, possibly at the higher pressures, there's a "Law of Diminishing Returns" coming into play based on the amount of air that can exit through the vents.


Den

.

Got to love how you said I was wrong, I proved with numbers that I am right, and you still say I am wrong. "Law of Diminishing Returns" has absolutely nothing to do with fluid mechanics. At air speeds less than 1/3 the speed of sound, the square of the flow rate is roughly proportional to the pressure difference through most openings. "Straighter" than "curvier"...really.

Yep. I'm sure that if you made the vent holes bigger you could get more airflow going out of them at higher pressures and that line would flatten out even more.
But, as the pressures get higher, there's only about so much that can exit the mask vent.
Besides, those are just some "ideal" numbers from their labratories to compare "reality" (mask LEAKAGE) to.
Having looked at different mask "vent flow rate" charts, there really isn't much difference in them across the board.
If I'm using a pressure of 12 or 13 and my sleep reports show "about 40", that's close enough for me. If I'm a little over or under them.......oh, well.......each night is different.


Den

.

And, if a person is using a range of pressures it can be a real challenge......determining the average pressures and how much actual "leakage" is over and above the "vent flow rate".
After a certain number of years of use and monitoring, you just look at the numbers and go "that's close enough" (or not).


Den

.

Wulfman... wrote:Besides, those are just some "ideal" numbers from their labratories to compare "reality" (mask LEAKAGE) to.

You do understand the chart is about design vent rates? Your comment indicates you don't - those are not "ideal" vent rates. They are design vent rates.

Wulfman... wrote:Having looked at different mask "vent flow rate" charts, there really isn't much difference in them across the board.

Well, yes, that is because the manufacturers designed the vents that way.

Wulfman... wrote:If I'm using a pressure of 12 or 13 and my sleep reports show "about 40", that's close enough for me. If I'm a little over or under them.......oh, well.......each night is different.

No, each night is not different - the same pressures will yield the same vent rate on any night.

You are confusing vent rate with total leak which is vent rate plus unintentional leak.

ChicagoGranny wrote:

Wulfman... wrote:Besides, those are just some "ideal" numbers from their labratories to compare "reality" (mask LEAKAGE) to.

You do understand the chart is about design vent rates? Your comment indicates you don't - those are not "ideal" vent rates. They are design vent rates.

Wulfman... wrote:Having looked at different mask "vent flow rate" charts, there really isn't much difference in them across the board.

Well, yes, that is because the manufacturers designed the vents that way.

Wulfman... wrote:If I'm using a pressure of 12 or 13 and my sleep reports show "about 40", that's close enough for me. If I'm a little over or under them.......oh, well.......each night is different.

No, each night is not different - the same pressures will yield the same vent rate on any night.

You are confusing vent rate with total leak which is vent rate plus unintentional leak.

Right. And, I'm trying to interject some "reality" here. Nobody sleeps the same night after night and the "leak rates" from the reports are going to vary......sometimes greatly. Of course these "vent flow rates" are their "design rates" for us to be able to judge (if we have data-capable machines and use software to monitor our therapy) if we have excessive leaking or not.

Edit:
And, this subject becomes even more convoluted when using ResMed machines which deduct the anticipated vent flow rate based on the mask type entered in the setup menu.


Den

.

Proportional: y=mx
Linear: y=mx+b

That's the mathematical definition. Some people may misuse the term or define it poorly in colloquial usage.

The graph is not proportional. The flow at 8 cmH2O is not 2x the flow at 4 cm. The graph is fairly linear, but not quite. I'm tempted to try to fit a curve to the data, but since the airflow has to drop to zero at zero pressure, I think I don't have enough data points at low pressure to mean much.

I would have thought the flow would be proportional to pressure, but apparently not. The flow increases more slowly than proportionally. Apparently air through an orifice is not proportional to pressure. This page is a bit interesting for airflow through an oriface. http://www.aircompressorworks.com/airfl ... fices.html

I'm guessing it's a question of turbulent airflow being nonlinear. I guess I gots myself edumacated today.

It would be neat if it were possible to have the flow rate be constant with pressure, because that would waste less air and water at higher pressures, but I don't see a reasonable way to do that in a mask without having something like a pressure setting on the mask itself.

ChicagoGranny wrote:

WindCpap wrote:The vent rate is not proportional to pressure. More like vent rate squared is proportional to pressure.

I agree with your statement. Over the range in the example, the function is far from a straight line. A simple function that best fits is,

Y = 9.1906(X^0.5577)
correlation = 0.9998

where X = CPAP pressure in cm H2O, and Y = mask design vent flow in lpm.

Someone check my math.

In other words, the square of the flow is roughly proportional to the pressure (or the square root of the pressure is roughly proportional to the flow).

Out of curiosity, what did you do to get the correlation?