Slimline vs Standard... More Air?

[70sSanO]

I've read a number of posts on the Resmed Slimline (15mm) vs the Standard (22mm) but none of them addressed any real performance differences in terms of air flow.

I used a Standard 22mm hose for the first couple of months and then I went to a Slimline 15mm hose. My numbers have gotten worse, probably not the hose, but it made me wonder if the smaller diameter could reduce the amount (volume) of air that flows through the hose. And if that is an impact. If I set a pressure of 15cm does the volume of air delivered with that 15cm influence my therapy.

I realize that the hoses are not smooth and I didn't try to figure out the ID of them. But if I do the simple math "correctly" the volume of a 15mm hose 6ftr long is 19.7 cu in. The volume of 22mm hose is 42.4 cu in. That's a lot more air. Does the S9 make up for the volume or just pressure.

Although not as drastic, I guess it would be like a garden hose and a fire hose at the same pressure. Only one will deliver enough water to put the fire out.

John

[Alshain]

Did you change the machine settings for the slimline hose?

In theory the pressure should increase because of the decreased volume needed to be filled, so the setting for slimline in the machine should account for that. However also in theory, the machine should be compensating based on feedback pressure in the machine, so I don't really know why it needs the setting in the first place. In any case, it's there so make sure it's turned to slimline in the setup menu. It is possible your getting too much or too little pressure with the setting if it is incorrect.

[70sSanO]

Thanks for the info, but I am well aware of the Slimline/Standard menu selction and the S9 making adjustments to maintain the pressure based on the hose selected.

My question has more to do with the physics of the airflow where inlet pressure is adjusted to maintain a set outlet (CPAP) pressure at the mask.

Is it correct to assume that if the outlet pressure, at the mask, is 15cm regardless of whether a Slimline or Standard hose is used, does that mean there is a higher airflow (volume?) at the mask with a 22mm hose?

And if that is correct, is it possilbe that more volume of air coming into the mask may provide better therapy when it comes to preventing an obstructive apnea?

If pressure is the only factor, why not a 5mm or 10mm hose. Somehow the volume of air has to play some part.

Any takers?

John

[Guest]

Is it correct to assume that if the outlet pressure, at the mask, is 15cm regardless of whether a Slimline or Standard hose is used, does that mean there is a higher airflow (volume?) at the mask with a 22mm hose?

In a word, no.

Airflow is function of you breathing and leak (neither of which will necessarily change just because you changed the tube).

The pressure at the S9 may change when you change the tube due to the pressure drop along the tube, which will be different with different tubes, but that won't change the airflow.

There will be a minimum tube size which can be used to maintain a certain pressure at the mask for a given airflow which is why you couldn't use very small tube sizes.

[70sSanO]

In a word, no.

Thanks, but I was hoping for something a tad bit more technical.

Airflow is function of you breathing and leak (neither of which will necessarily change just because you changed the tube)

I have absolutely no background in flow dynamics, but I'm trying to understand this. There is a cetain volume of air at a particular pressure at the output of the hose. I may be all wet, but if I fill a 6ft Slimline hose with air, at any set pressure, I think that there is about 20 cu in of air in the hose. If I fill a Standard 6 ft hose, there is 42 cu in of air. When the air is pushed out of the hose that is the amount of you get. I think I understand your that a person only inhales a certain amount of air (breathing), but in trying to prevent an obstructive apnea does more air have any effect.

I'm not trying to be a jerk, personally the air out of the Slimline feels more piercing, if that is a corrrect description, than a Standard. I'll have top check my numbers now that I have gone back to the Standard. First night my AHI is 4.8 compared to some nights at 10 to 12. Over the past month I have had apneas that have lasted way too long. I even had one go on for 84 seconds. I may still have long ones, but last night I had one compared to 10-20 and it was 16 seconds long. Obviously it is only one hight and I will need more data. At this point in time I am just pondering any potential impacts due to hose diameter.

John

[Alshain]

In a word, no.

Thanks, but I was hoping for something a tad bit more technical.

Airflow is function of you breathing and leak (neither of which will necessarily change just because you changed the tube)

I have absolutely no background in flow dynamics, but I'm trying to understand this. There is a cetain volume of air at a particular pressure at the output of the hose. I may be all wet, but if I fill a 6ft Slimline hose with air, at any set pressure, I think that there is about 20 cu in of air in the hose. If I fill a Standard 6 ft hose, there is 42 cu in of air. When the air is pushed out of the hose that is the amount of you get. I think I understand your that a person only inhales a certain amount of air (breathing), but in trying to prevent an obstructive apnea does more air have any effect.

I'm not trying to be a jerk, personally the air out of the Slimline feels more piercing, if that is a corrrect description, than a Standard. I'll have top check my numbers now that I have gone back to the Standard. First night my AHI is 4.8 compared to some nights at 10 to 12. Over the past month I have had apneas that have lasted way too long. I even had one go on for 84 seconds. I may still have long ones, but last night I had one compared to 10-20 and it was 16 seconds long. Obviously it is only one hight and I will need more data. At this point in time I am just pondering any potential impacts due to hose diameter.

John

No, it doesn't work that way. The differential pressure (i.e. your prescription number) is all that is relevant for our treatment. The same differential pressure can be achieved with different quantities of air, depending on how much space there is to fill. The mathematical equations behind it adjust based on the dimensions of the tube to ensure you are receiving the proper pressure. Having more or less air does not affect our treatment. Under your thought process, those people using a nasal mask are getting different treatment than those using a Full Face mask because they are bigger, this just isn't the case.

[kempo]

Someone correct me if I'm wrong but I believe the s9 has a setting for using a standard hose and a setting for using a slimline hose.

[Alshain]

Someone correct me if I'm wrong but I believe the s9 has a setting for using a standard hose and a setting for using a slimline hose.

Yes it does.

[Slartybartfast]

The product of pressure and volume delivered will be constant regardless of whether you use the slimline or standard hose.
Check out Poiseuille's equation for compressible fluids and you'll find the more than you want to know about. It's analogous to Ohm's law (sort of).

[70sSanO]

Having more or less air does not affect our treatment. Under your thought process, those people using a nasal mask are getting different treatment than those using a Full Face mask because they are bigger, this just isn't the case.

Now that does make sense.

Thanks!

John

[archangle]

I think Alshain's explanation is all wrong.

However, I do agree that the length, diameter, and roughness of the hose will effect air pressure.

First, let's consider fixed pressure CPAP. Assume the machine isn't told which particular type of hose you're using. The machine will produce a fixed pressure at the machine end of the hose. The mask at the other end of the hose has a fixed size group of holes that leak air out. Assuming no mouth or mask seal leaks, all the air going down the hose goes out through the mask vents.

Now, assume you have a fat hose. As air flows down the hose, a certain amount of pressure drop will occur. Subtract this pressure from the starting pressure, and you now have the pressure at the mask.

Take the same setup, but use a thinner hose. As the air flows down the hose, a larger amount of pressure drop will occur. You subtract a larger drop from the initial pressure, and you get less pressure at the mask.

That's why you need to tell the machine the type of hose you have.

It's actually a little more complicated than that, but the conclusion is still the same. A long hose will have more pressure drop than a short one. A hose with a rough or corrugated surface will probably cause more pressure drop.

It gets more complicated when the machine is trying to do EPR, or trying to sense apneas, hyponeas, etc., but the characteristics of the hose affect that even more. The machine is trying to regulate and sense the pressure at the mask by measuring the pressure and air flow at the machine end of the hose. The more restrictive the hose is, the more difficult it is for the machine to sense and control the pressure at the mask.

Cpap.com says "This tubing will ONLY work with the S9 Series CPAP Machines. On any other machine, the SlimLine Tubing will significantly reduce overall pressure at the mask and may produce erratic pressure outputs for Auto CPAP machines."

By the way, the important question is not "volume," it's "flow rate." Volume would be measured in liters. Flow rate would be liters per minute.

[70sSanO]

Check out Poiseuille's equation for compressible fluids and you'll find the more than you want to know about. It's analogous to Ohm's law (soft of).

I have looked at some of the info. Honestly it did hurt my head.

I thank everyone for allowing me to go down this path, as ridiculous as it may seem.

John

[Alshain]

I think Alshain's explanation is all wrong.

However, I do agree that the length, diameter, and roughness of the hose will effect air pressure.

First, let's consider fixed pressure CPAP. Assume the machine isn't told which particular type of hose you're using. The machine will produce a fixed pressure at the machine end of the hose. The mask at the other end of the hose has a fixed size group of holes that leak air out. Assuming no mouth or mask seal leaks, all the air going down the hose goes out through the mask vents.

Now, assume you have a fat hose. As air flows down the hose, a certain amount of pressure drop will occur. Subtract this pressure from the starting pressure, and you now have the pressure at the mask.

Take the same setup, but use a thinner hose. As the air flows down the hose, a larger amount of pressure drop will occur. You subtract a larger drop from the initial pressure, and you get less pressure at the mask.

That's why you need to tell the machine the type of hose you have.

It's actually a little more complicated than that, but the conclusion is still the same. A long hose will have more pressure drop than a short one. A hose with a rough or corrugated surface will probably cause more pressure drop.

It gets more complicated when the machine is trying to do EPR, or trying to sense apneas, hyponeas, etc., but the characteristics of the hose affect that even more. The machine is trying to regulate and sense the pressure at the mask by measuring the pressure and air flow at the machine end of the hose. The more restrictive the hose is, the more difficult it is for the machine to sense and control the pressure at the mask.

Cpap.com says "This tubing will ONLY work with the S9 Series CPAP Machines. On any other machine, the SlimLine Tubing will significantly reduce overall pressure at the mask and may produce erratic pressure outputs for Auto CPAP machines."

By the way, the important question is not "volume," it's "flow rate." Volume would be measured in liters. Flow rate would be liters per minute.

Well it depends on how the machine works. Since I haven't built one I don't know for sure but there are couple of ways it can work. It can measure flow rate directly, however this is inefficient because flow rate is not an indication of the pressure at the other end of the device regardless of how well you know what equipment there is. This is where differential pressure comes in... based on the fact your prescription is a DP (some units of water column, i.e. cm H2O) it would make me believe it is an orifice measurement. This is the type of measurement I work with every day, except we measure methane, not nitrogen (it's all gas). However in this application it seems like orifice measurement wouldn't be very efficient. Orifice measurement uses differential pressure to measure flow, while the calculation can be reversed, it doesn't seem like this would be very effective.

The most efficient way (in my opinion) is pressure feedback. Imagine your blowing up a beach ball with an air compressor, but it's a smart air compressor. As the beach ball inflates to near full, the compressor backs down on the output (of course it has to know how much pressure is too much). The walls of the beach ball press in on the air to create the pressure feedback. Likewise, the entire CPAP chamber, hose, and mask can be that beach ball. In this case the amount of air makes no difference because it's detecting pressure feedback. As the mask bleeds off CO2 it has to keep pumping more air in to maintain the pressure feedback. The only point at which the tubing matters is when it's first pressuring up. After that, the pressure should remain pretty close to constant across the entire sealed area and how much air is there is irrelevant, because it's the pressure as you inhale that clears the air passage way. To me, I would think this is would be the most efficient way to ensure the pressure is maintained, but this uses a static pressure, which is typically measured in static pressure units (like PSI, your tire gauge is a pressure feedback device) and not measured in water column.

FYI: Pressure feedback is how the SmartStart feature works too. It detects increased pressure from your breathing and realizes you must want to sleep now.

SO, The real question is how does the machine work? My best guess at this point without knowing someone who works for ResMed or Respironics, would be that at one time in history they used an orifice measurement and people got used to the water column numeric system, so now that the tech has improved, they are simply running some kind of equivalency conversion to get the same basic number.

Regardless of ALL of this speculation, if the machine is set up properly, it should give you the same treatment regardless of what tubing or mask your using. Setting it up properly means choosing the right mask and tubing in the setup menu. The machine is not going to suddenly drop or raise your pressure for different equipment or ResMed wouldn't be able to sell it very well.

[chunkyfrog]

Bernoulli's Law?

[Alshain]

Bernoulli's Law?

Yes, that's what orifice metering is based on.