There are a few reasons why the tube for a CPAP (continuous positive airway pressure) machine may collect water:

Humidifier: If the CPAP machine has a built-in humidifier, water from the humidifier can condense in the tube. This is more likely to happen in colder temperatures or if the humidification level is set too high.

Leaks: If there are any leaks in the seal between the mask and the face, or leaks in the tube, air can escape and cause condensation to form.

Length of tube: The longer the tube, the more likely it is to collect water due to the increased surface area inside the tube.

Travel: When traveling with a CPAP machine, changes in temperature and humidity can cause condensation to form in the tube.

It's important to drain the tube of any water, in order to ensure the air delivered is at the correct pressure and also in order to avoid mold from growing, which can be inhaled. If you have a humidifier it may help to adjust the humidity level, and to check for leaks, also can be helpful to use a hose cover to reduce condensation.

"Rainout" is the term used to describe the condensation of water in the CPAP tubing, and it can be caused by a variety of factors. Here are some ways to help prevent rainout:

1. Use a heated hose: Heated hoses, also called climate-controlled hoses, help to keep the temperature of the air in the tube above the dew point, which can prevent condensation from forming.

2. Move your Machine: Moving your machine, below your mask and head, may not entirely prevent rainout. But at least any water droplets will flow down the hose, away from your mask. Doing this may improve comfort, when eliminating rainout completely is not possible. 

3. Adjust the humidity level: If your CPAP machine has a built-in humidifier, try adjusting the humidity level to see if that helps reduce the amount of condensation in the tube.

4. Use a tube cover: Insulating the CPAP tube can help to keep the air inside warm and prevent condensation from forming. There are insulating covers available to buy that can go around the tube.

Eg. Zippered Tubing Wrap

https://www.thecpapstore.ca/product/zippered-tubing-wrap

5. Keep your room temperature stable: Keep your room temperature stable when you use your cpap, especially if you live in a cold climate, it can prevent condensation.

6. Keep the tube as short as possible: The longer the tube, the more likely it is to collect water due to the increased surface area inside the tube.‍

It's important to note that some amount of condensation in the tube is normal and not necessarily a problem, as long as it does not create discomfort or interfere with the proper functioning of the machine. 

If you have tried these recommendations and you continue to experience rainout, consider discussing with your healthcare provider or CPAP supplier, as it could indicate another issue with the device or other settings that may need to be adjusted.