Pipe sizes can be difficult to see from 3-5 m in height, tubular sheath thickness may vary, older concrete sewers can be obsolete, or there may be a sleeve installed therein. These factors can contribute to making the installation of the rat blocker difficult. We have therefore designed a measuring instrument that measures the distance between the top and bottom of the drain pipe. The distance between top and bottom is very important when installing a rat blocker, as a clearance of valve system is essential for the rat blocker to function. Before Installing the rat blocker, measure the distance between the top and bottom with the dimension gauge. The measured results are used to find the right rat blocker. In the case of obsolete sewers, the adjustable valve on a rat blocker from Nordic innovation can be adjusted up to 0.5 cm, which can provide it with air between the base and the valve, as is required for optimal operation. Using the tool before installing the rat blocker therefore ensures that the sewer technician saves time, that a more precise and easier installation is possible, and provides assurance for a proper clearance of the valve system.