make a bigger loop if you need to measure more wind. Putting smaller amounts of water will not work since the wind is pushing against a lighter load, it will just blow the water out of the tube as before. If you need to measure a pressure of 15 inches (for example) your loops should be 20 inches tall with the water level at just below the middle of the loop's height. And (as a reminder) you read the difference between the water at both the lowest and highest point (or you can measure the difference from the zero-point and multiply by two).
Rick in VA