Some patients read to me their CPAP prescription over the phone before they fax it over. They say the numbers and letters as if it's all Greek to them and usually follow up by saying something along the lines of ""so I guess that means something to you"". Yes, it actually does mean something to me. The CPAP pressure that a customer reads from their prescription tells me how I need to set their CPAP machine before I ship it out, and I usually explain that while on the phone with customers. But I figured it's better for CPAP users to have a really good understanding of CPAP pressure, so that in turn they have a better understanding of their CPAP machines and so that they can easily interpret a prescripition in the future. In this article I'll set out to describe CPAP pressure in its entirety, I'll show you how to build a simple water manometer so that you can verify that your CPAP machine is calibrated correctly, and I'll fill you in on the different pressure settings you might encounter on various CPAP machines.

What's in a Prescription?
Many CPAP prescriptions indicate a particular pressure setting to which the machine should be set in order to deliver adequate CPAP therapy to the patient. The pressure setting is usually determined during a sleep study and generally represents the highest pressure required to prevent average apnea events you encountered throughout the course of your study. The pressure is simply a number to which the CPAP machine is set so that the machine will blow enough air to keep your airway open throughout the night. But what exactly does this number mean? Let's take a look at an example pressure setting:

20 cm H2O

This is an example of the most basic type of CPAP prescription, although 20 cm happens to be a very high pressure setting. The number is simply a distance given in centimeters - in this example 20 cm (which is roughly equal to 8 inches). The second part of the pressure is the chemical formula for a molecule of water - 2 atoms of hydrogen and one atom of oxygen stuck together in a covalent bond. The pressure is read aloud as, ""20 centimeters of water"". Now the question is, ""what does 20 centimeters of water actually mean?"".

Simply put, 20 centimeters of water means how much air pressure it takes to move a column of water up a distance of 20 centimeters. In order to show you this I've made a simple water manometer that I've hooked up to a CPAP machine. Next we'll take a look at how to build the water manometer and we'll use it to check the calibration of a CPAP machine at various pressure settings and altitudes.

Building Your Own Water Manometer
Materials Required to Build a Water Manometer

  • 4 or 5 foot length of clear vinyl tubing which can be bent into a U-shape, 3/4"" diameter or at least close to the diameter of your CPAP tubing connector (the diameter doesn't matter except in as much as you want the CPAP tube to fit onto the manometer tube and you don't want any air to escape)
  • Plywood board approximately 3 feet long and 2 feet wide (although you can always build a much smaller manometer)
  • Meausuring device - either a ruler or custom measuring scale you can affix to the board next to the tube
  • Several clamps and screws to attach the tubing to the board
  • Screw driver
  • Water
  • Food coloring (if you'd like to easily see the water level)

    Note: my water manometer is really big. You could try to build a travel size water manometer!
Assembling the Manometer
  1. First, attach your clear tubing to the plywood board using the clamps. The tubing should be positioned in a U-shape on the board so that you can fill the tube half full of water without any water spilling out either end. One end of the tubing needs to be accessible so that you can attach your CPAP tube to it. Each side of the tube should be pretty much vertical.
  2. Next, make sure your CPAP hose fits securely onto the manometer tube. I used a 3/4"" clear tube which I purchased at Ace Hardware, and the fit was slightly loose. I put some tape around the end of the manometer tube to make a better fit.
  3. Fill the tube with water. You'll need enough water to show up to 10 or so centimeters of vertical movement (which would represent a total pressure of 20 cm - 10 cm down and 10 cm up). You can pour the water directly from a water bottle. You won't need much water at all. Add food coloring for contrast.
  4. Affix your measuring scale to the board with a zero marker at the water level.
  5. Attach your CPAP machine and tube to the end of the manometer tube on the opposite side from where the measurement scale is positioned.
  6. Once the CPAP machine is hooked up to the manometer, turn the machine on and watch the column of water rise, marking the new level of the water. The distance the water traveled multiplied by two shows you the CPAP pressure. You have to multiply by two because with this arrangement you're interested in the difference in height between the two columns of water. For example, when one column goes up 10 cm the other goes down 10 cm for a difference in height of 20 cm. On my paper measurement scale each 1 cm increment is marked as 2 cm of pressure to account for this.
That's all there is to a water manometer. Once you have the materials it only takes a few minutes to build.

Testing the Manometer at Different Altitudes
Have you wondered what effect the altitude control has on your CPAP machine? With your own water manometer you can easily see the effect on the delivered pressure as you change altitude settings. When I first powered on a REMstar Plus CPAP machine to test the pressure at 20 cm H2O I noticed the column of water was only showing about 19 cm H2O. When I changed the altitude setting to 2 instead of 1 the water manometer showed 20 cm. This makes a lot of sense because my elevation here in Spokane is approximately 2,450 feet above seal level making the number 2 altitude setting more appropriate.

Buying Your Own Water Manometer
If you're not a DIYer, you can purchase a manometer. They come in a few different varieties including a water manometer like the one I built, a dial gauge, or a digital gauge. If you're at all curious about your CPAP pressure you can order a manometer and check it out. It would be especially interesting to observe any pressure differences as you travel to different altitudes. Your machine will always tell you, for example, that it's blowing 10 cm but at different altitudes this isn't accurate unless your machine adjusts for altitude automatically or unless you manually adjust the altitude control on your CPAP machine.

The Different CPAP Pressures
There are a few different pressure settings to consider during CPAP therapy.

  • Standard pressure: the constant pressure at which your CPAP machine will blow air once it has ramped up to full pressure
  • Ramp start pressure: the pressure at which your machine will start each night before it ramps up to full pressure
  • Inhalation pressure (for BiPAP or bi-level CPAP machines): the pressure your CPAP machine delivers when you inhale
  • Exhalation pressure (for BiPAP or bi-level CPAP machines): the pressure your CPAP machine delivers when you exhale
Now you know pretty much all there is to know about pressure settings on CPAP machines, including how to check your CPAP pressure and how to ensure your CPAP pressure is what it needs to be at various altitudes. If you decide to build your own CPAP water manometer, feel free to send me some photos!

Andrew Senske

CPAP-Supply.com is a leading online retailer of CPAP equipment. Located in Spokane, WA CPAP-Supply.com has been serving thousands of customers around the world since 2001. Founded on a belief that patients are their own best primary care providers, CPAP-Supply.com understands the importance of educating patients and customers on both the effects of and treatment for obstructive sleep apnea. For more information visit CPAP-Supply.com or call toll free 1-888-955-2727.

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