Meniscus stability influenced by fluid flow patterns inside a medium thickness-parallel mould walls was analyzed using water modeling techniques including dye tracer injection, PIV and ultrasonic measurements on real time of meniscus levels. Two different submerged entry nozzles (SEN) were compared, the first is under current operation at the caster which yields a very high level of turbulence and the second is a computer aided design SEN which is aimed to reemplace the first one. The experimental results indicate that the first SEN 1 induces a single-unstable large flow roll which promotes large bath oscillations and generation of vortex flows on the bath surface. Fluid speeds at meniscus level reach magnitudes as large as 0.6 m/min and in the wall mould as high as a 0.95 m/s which are large enough to attain "shell washing conditions". Meanwhile SEN 2, new design, yields a very stable meniscus with speeds as large as 0.45 m/s at the meniscus in a stable-double flow roll. The analysis of entrapment and entrainment mechanisms of slag leads to conclude that the first SEN will induce slag entrapment under almost all operating conditions while the second SEN will induce slag entrapment only under very specific conditions of high casting speed of 3.68 m/min.