Filter by-pass valves are an essential part of filter cartridges, which, in most cases, are used in hydraulic assemblies. The main parameter of a by-pass valve is the opening pressure, which depends on the flow through the valve. The both parameters together are written in the Δp-Q-characteristic of the valve. In this thesis we were determining the opening pressures for already constructed valves, with two different media - air and hydraulic oil. The goal was to correlate how the media influences the opening pressure. The values were first analytically checked and also measured in real life scenario after. We could establish that a correlation between air and oil used is possible, but highly dependable on the shape of the by-pass valve itself.