We discuss the current quantization obtainable in SINIS turnstiles. Various error processes, for example environmentally assisted and higher order tunneling as well as quasiparticle exitations in superconducting leads, may limit the achievable quantization. We argue that by careful design of the device, relative uncertainty of less than 10 ^-7, required by metrological applications, is feasible. We also present a scheme for error detection of the turnstiles.