STERIS Steraffirm? Control Tubes are 45 mm by 5 mm borosilicate glass tube sealed at both ends. One end is color coded to facilitate identification of the unboxed indicator. The tube contains approximately 200 ?l of heat-sensitive red liquid which changes color from red to green at various times and temperatures that are integrated by the device. The control tubes are calibrated to respond to specific temperature and time parameters during a dry or moist heat sterilization cycle. They undergo a permanent color change when the proper parameters are achieved at the location of each control tube. A color guide in the instruction provides an instant visual check of the results. Since the control tubes consist of sealed vials containing heat-sensitive solution, chemicals from the indicator do not normally come into contact with adjacent items during the cycle.