A CPAP machine works by drawing in room air through a filter, using an internal blower (motor) to turn it into a steady stream of pressurized air, and delivering that air through a tube and mask into your airway. The pressure acts as a pneumatic splint that holds the collapsible upper airway open, stopping the pauses in breathing caused by obstructive sleep apnea [1].
This guide explains the machine itself: how it makes pressure, how that pressure is measured and set, and how the comfort features (EPR, ramp) and the humidifier work. For what CPAP therapy is at a high level, see what CPAP therapy is; to compare device types, see types of CPAP machines.
The airflow path, start to finish
Inside a CPAP, air travels a fixed path: room air enters through an air filter, the blower pressurizes it, the humidifier chamber adds moisture, the tubing carries it to the mask, and the mask delivers it to your airway [3]. Each stage maps to a part you can see and clean.
- Room air in: drawn through an air filter that keeps out dust and particles.
- Blower (motor): pressurizes the filtered air to your prescribed level.
- Humidifier chamber: the air passes over warm water and picks up moisture.
- Heated or standard tubing: carries the pressurized, humidified air to the mask.
- Mask: delivers the air into your nose, or nose and mouth.
- Upper airway: the pressure holds the airway open all night.
How does a CPAP machine create air pressure?
A CPAP machine creates pressure with an internal blower. Room air enters through the air filter, the blower pressurizes that air, and the device pushes it out through the outlet connector into the tubing at a controlled pressure [3]. If the blower fails, the airflow simply does not turn on [5]. A standard CPAP does not add oxygen or medication of any kind; it simply moves ordinary room air, only pressurized.
How does that pressure keep your airway open?
The pressurized air works as a pneumatic splint. In obstructive sleep apnea the soft tissues of the upper airway collapse and block breathing; the continuous positive pressure pushes against that collapse and keeps the airway open, so breathing does not stop [1]. This is why CPAP treats the mechanical cause of obstructive apnea rather than the symptoms. If your apnea is central rather than obstructive, the mechanism is different: see central sleep apnea.
How is CPAP pressure measured and set?
CPAP pressure is measured in centimeters of water, written cmH2O (shown as hPa on some screens, where 1 cmH2O is about 1 hPa). On ResMed devices the pressure is settable across a 4 to 20 cmH2O range in 0.2 cmH2O steps; a fixed CPAP holds one prescribed value all night [2]. Your exact pressure is set by your clinician from your sleep study, not chosen at home. A prescription is required to buy and set up any CPAP, which starts with a sleep apnea test.
What is EPR (Expiratory Pressure Relief)?
EPR keeps your full treatment pressure while you breathe in and lowers the delivered mask pressure while you breathe out, so exhaling feels easier [2]. On ResMed devices EPR is set to level 1, 2, or 3 cmH2O (the amount the exhale pressure drops), can run full time or during ramp only, and never lets the delivered pressure fall below 4 cmH2O [2]. Philips machines call their version Flex, adjustable from 1 to 3 [5]. Your provider sets the level, so there is no single default value.
What does ramp mode do?
Ramp starts therapy at a low, comfortable pressure and gradually rises to your prescribed pressure so it is easier to fall asleep. On ResMed devices ramp time can be Off, 5 to 45 minutes, or Auto; in Auto (AutoRamp) the device detects when you fall asleep and then increases pressure at 1 cmH2O per minute, or reaches the target within 30 minutes if it does not detect sleep onset [2]. The start pressure can be as low as 4 cmH2O [2]. Philips uses a standard ramp and a SmartRamp that holds a lower pressure until more is needed [5].
How does the CPAP humidifier work?
The humidifier is a water chamber that sits on a heater plate; the air from the CPAP passes over the warm water and picks up moisture before it reaches your airway, and the higher your pressure is set, the warmer the plate runs and the more water it uses [4]. A heated tube (ResMed Climate Control or Fisher & Paykel ThermoSmart) keeps that temperature and humidity constant down the hose and prevents rainout, the condensation that otherwise forms as warm air cools in the tubing [3][6]. ResMed Climate Control runs on Auto at 80°F (27°C), adjustable from 60 to 86°F, targeting 85% relative humidity, with a Humidity Level from 1 to 8 [3]. Use distilled water in a ResMed Standard tub (maximum 380 mL), fill only to the max line, and never add scented oils [3]. Good humidification is the first fix for CPAP dry mouth.
CPAP comfort features compared: ramp, EPR, and humidifier
These three features change how the air feels, not what pressure treats your apnea. Here is what each one does and its verified settings on ResMed devices [2][3].
| Feature | What it does | When it helps | Settings (verified) |
|---|---|---|---|
| Ramp / AutoRamp | Starts low and rises to your prescribed pressure; Auto detects sleep onset then rises 1 cmH2O per minute | Falling asleep at the start of the night | Off, 5 to 45 min, or Auto; start pressure as low as 4 cmH2O |
| EPR (Expiratory Pressure Relief) | Full pressure on inhale, lower pressure on exhale | Trouble breathing out against the pressure | Off/On; full time or ramp only; level 1, 2, or 3 cmH2O; never below 4 cmH2O |
| Heated humidifier + heated tube | Adds moisture; heated tube keeps humidity constant and prevents rainout | Dry nose, mouth, or throat; condensation in the tube | Humidity Level Off or 1 to 8; Auto tube temp 80°F (27°C), range 60 to 86°F; 85% RH |
What comes in the box with a CPAP machine?
A new CPAP system includes the machine (blower unit), a humidifier water chamber, air tubing (standard or heated), a power supply and cord, a reusable filter, an SD card, and a travel case [3][5]. The mask is used with the system but is usually prescribed and bought separately, because fit is personal: see the best CPAP masks for how to choose one. Filters and the water chamber are consumables you replace on a schedule; see how often to change your CPAP supplies.
How do you set up a CPAP machine for the first night?
Setup follows the same order on most machines. Using the ResMed AirSense 11 as the example, the manufacturer steps are [3]:
- Place the device on a stable, level surface, lower than your head.
- Connect the power cord to the device and plug the AC adaptor into the wall.
- Connect the air tubing firmly to the outlet connector at the back.
- Remove the humidifier tub, fill it with distilled water to the max line (up to 380 mL), and close it.
- Insert the tub into the side of the device.
- Connect the free end of the tubing to your assembled mask, fit the mask, and press Start (or just breathe if SmartStart is on).
A respiratory therapist walks through the same unboxing and setup here:
"After you've unboxed your CPAP and plugged it into the wall, we're going to be putting on the humidifier. The manual has pictures that indicate where every piece goes."
Oswald Azua, Respiratory Therapist, Sleeplay | ResMed AirSense 10 CPAP Unboxing & Setup
For the adjustment period after night one, see our tips on getting used to a CPAP machine, and keep the airflow path clean with our CPAP cleaning guide.
Do CPAP, APAP, and BiPAP machines create pressure differently?
Yes, but only in how they schedule the pressure. A CPAP holds one fixed pressure, while an auto-adjusting APAP changes the pressure through the night [2]; a BiPAP uses a higher pressure to inhale and a lower one to exhale. The blower and the airway mechanism are the same; only how the pressure is scheduled differs. For the full comparison and which one fits you, see APAP vs CPAP and CPAP vs BiPAP.
Choosing a CPAP machine
Once you have a prescription, the right machine is the one whose comfort features match how you sleep. Browse current CPAP machines, or see our ranked best CPAP machines for expert picks. A few common starting points:
Frequently Asked Questions
How does a CPAP machine work?
A CPAP draws in room air through a filter, uses an internal blower to turn it into steady pressurized air, and sends it through a tube and mask into your airway. That pressure holds the airway open so breathing does not pause during sleep.
How does a CPAP machine create pressure?
An internal blower (motor) compresses filtered room air and pushes it out at a controlled pressure measured in cmH2O. No oxygen or medication is added; a standard CPAP moves ordinary room air, only pressurized.
What is EPR on a CPAP machine?
EPR (Expiratory Pressure Relief) keeps full pressure while you inhale and lowers the mask pressure while you exhale, making it easier to breathe out. On ResMed devices it is set to level 1, 2, or 3 cmH2O and never drops below 4 cmH2O.
What does ramp mode do on a CPAP?
Ramp starts therapy at a low pressure and gradually rises to your prescribed pressure so it is easier to fall asleep. ResMed AutoRamp detects sleep onset, then rises 1 cmH2O per minute, or reaches target within 30 minutes.
How does a CPAP humidifier work?
A water chamber sits on a heater plate; the airflow passes over the warm water and picks up moisture before reaching your airway. A heated tube keeps that humidity constant and prevents rainout, the condensation that forms in cooler tubing.
What pressure should a CPAP machine be set to?
CPAP pressure is measured in cmH2O and is settable across a 4 to 20 cmH2O device range. Your exact pressure is prescribed by your clinician from your sleep study, not chosen at home.
What comes in the box with a CPAP machine?
A new system includes the machine, a humidifier water chamber, air tubing, a power supply and cord, a reusable filter, an SD card, and a travel case. The mask is used with the system but is usually prescribed and bought separately.
Do CPAP, APAP, and BiPAP machines work differently?
They differ only in how they schedule pressure: CPAP holds one fixed pressure, APAP adjusts it automatically, and BiPAP uses a higher inhale and lower exhale pressure. The blower and airway mechanism are the same.
References
- Van de Perck E, et al. The effect of CPAP on the upper airway and ventilatory flow in patients with obstructive sleep apnea. Respiratory Research 2023 (PMC): CPAP acts as a pneumatic splint.
- ResMed AirSense 10 Clinical Guide: EPR levels and floor, AutoRamp behavior, pressure range 4 to 20 cmH2O.
- ResMed AirSense 11 User Guide (Americas): air path, box contents, six-step setup, Climate Control, distilled water and 380 mL tub.
- Fisher & Paykel: Why is heated humidification important in CPAP therapy? Chamber on a heater plate; distilled water; no essential oils.
- Philips Respironics DreamStation CPAP User Manual: blower, Flex 1 to 3, standard ramp and SmartRamp (mechanism reference only).
- Fisher & Paykel ThermoSmart: heated breathing tube keeps humidity constant and virtually eliminates condensation (rainout).
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