Low instable oil prices at the stock market and an increasing water cut force engineers to constantly improve and redesign their methods in order to optimize efficiency. The use of efficient artificial lift systems in old oil fields is a step forward to improve the value chain from reservoir to customer. To this day Sucker Rod Pumps are one of the most used artificial lift systems in the oil industry. These pumps represent a cost efficient and simple way to increase oil production. One the one hand this system is easily adjustable to chaning operating conditions and on the other hand has a good cost-effectiveness compared to other systems. Due to the high number of installed units it is essential to have a technical efficient and failsafe construction of the lift system. A deep understanding of the procedure of the pumping operation is vital for the continuous optimization. This thesis presents the results of the research on the topic of volumetric efficiency for different operating conditions performed at the Pump Test Facility of Montanuniversitaet Leoben. The different results of the test are illustrated in order to gain detailed understanding of internal losses durind the pumping operation. Slippage strongly depends on the differential pressure across the pump and the pumping speed. Five different plungers were tested with three different types of liquids. The experimental results were then compared with existing models. The results were then compared to existing slippage models. The comparison between those models clearly indicate the underestimation of the amount of slippage. The reasons mentioned above show the necessity to develop a new equation. The result of several different theoretical approaches was than fitted to describe the measurements performed at the Pump Test Facility in Leoben. The new model differs in result and approach compared to the already existing models. The new model helps engineers to calculate slippage up to 38bar pressure difference across the pump.