Take a length of clear plastic tubing, about two feet in length.
Fashion a cork or stopper that will fit snuggly into the outlet from the humidifier, and drill a hole through the cork that provides a snug fit for the tubing. Push the tube through the cork with enough length to allow the end to be under the water in the humidifier, Use rubber bands to attach the other end of the tube to a ruler with a centimeter scale, as shown below
That's all there is to it!
Fill the humidifier, and insert the cork into the outlet. When you turn the macihne on, the pressure will force the water up the tube. To make a measurement, simply place the 0 cm scale on the ruler at the level of the water in the humidifier, and read the height of the column.
In the photo I added some food coloring to make the water column stand out, but it's not really necessary. You can see that the column is showing 10 cm (100 mm).
Notes: 1) You may have to bend the tube near the cork to allow the water to flow (siphon) over the loop and fill the tube, especially if you are working at a low pressure.
2) Be careful to keep the open end of the tube sufficiently high above the machine, otherwise the CPAP pressure will blow the water out and ruin your carpet.
