Each Indicator Strip has a standard black reference color -- the O in OK. When the complete penetration of the pack by pure pressure steam has occurred, the white K and arrow change to match the black letter O. Using strips guards against incomplete steam cycles due to sterilizer malfunction or human error. For use in steam sterilizer cycles of 250 degrees F (121 degrees C) , 272 degrees F (134 degrees C) or 286 degrees F (141 degrees C). Strips measure 4" x 9/16".