A free EN ISO 13849-1 Performance Level Calculator is available from the IFA (Institut fuer Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung) in Germany. This simple tool, developed in conjunction with the German ZVEI and VDMA organisations, consists of two discs; rotating the bottom disc reveals different values through windows cut in the top disc.
To use the Performance Level Calculator, first the bottom disc is turned until the desired mean time to dangerous failure (MTTFd) of one channel is displayed in the lower window (see diagram below). Then the desired category and diagnostic coverage (DC) are chosen, and the corresponding figures read through the upper window. The displayed value is then multiplied by the factor according to the legend on the right-hand side of the disc, which gives the average probability of a dangerous failure per hour of the safety-related control system. Colour coding indicates which factor has to be used and which PL is achieved. (Full instructions are printed on the reverse of the Performance Level Calculator.)
Follow the link for more information about the EN ISO 13849-1 Performance Level Calculator and to order a free copy.