Purchasing this option will notify WeatherYourWay to switch the measuring tube(s) in this same order to the metric version. The metric measuring tube is exactly the same size as the imperial version...but the scale shows millimeters in 0.2 millimeter increments to 25.4 millimeters. Please select the correct quantity of this item for the number of measuring tubes you need switched to the metric scale! The TROPO gauge measuring tube includes both imperial and metric units, so this option is valid for the original CoCoRaHS gauge ONLY!