In several industries, specifically in the petroleum industry, single and multiphase flows through restrictions are commonly found. The restrictions are installed for multiple purposes, e.g., to control the flow and measurement. This research focuses on an experimental study of two-phase (air-water, air-oil) flows through orifice plates, with different restriction sizes (β) to better understand the effects of viscosity in three flow patterns: elongated bubble, slug flow, and annular flow. The experimental setup allows us to visualize the flow with a high-speed camera and measure the differential pressure signature, allowing a detailed discussion on the influence of Reynolds, Froude, and aspect ratio (β) for each flow pattern. This research demonstrates that with simple data analysis (PDF, PSD) of a low-frequency pressure drop sensor installed in the of singularity, it is possible to determine not only the flow pattern upstream the restriction, which has been done before in the literature, but also to determine several properties of the flow upstream and downstream, such as the intermittent factor of the flow and the oscillation wave downstream of the restriction, among others. This paper shows that any restriction properly instrumented can be used to determine the flow pattern in a system, which is usually an unknown variable. The analysis shows how the transition of flow patterns can be observed based on both probability density functions and the power spectrum density of the differential pressure signature.