Summarize this article:

Kinks and knots can throw off the level and result in an incorrect reading. Slide your hand over the entire length of the tube before you use it so you can be sure there are no kinks or knots. A tube that is old or worn may be more prone to knots and kinks so you may want to replace it over time. Leaving water in the tube for a long period of time can allow air bubbles to form, which can then throw off the water levels in the tube. You should also empty and refill the water level before you use it to ensure the measurements are correct. Exposure to heat and direct sunlight can cause the tube to get too hot, causing the liquid to expand once it touches the tube. This can then throw off your water levels and result in an incorrect reading. Store the tube for the water level in a cool spot indoors in your garage or home so it does not overheat.

Summary:
Make sure there are no kinks or knots in the tubing. Empty the water level after each use to prevent air bubbles. Keep the water level in a shaded, cool spot to prevent liquid expansion.