Incubators are constant temperature air chambers or water baths that provide controlled temperature environments. Incubators must maintain a uniform and constant temperature at all times in all areas, that is, they must not vary more than ± 0.5°C in the areas used. Provide incubators with open metal wire or perforated sheet shelves so spaced as to assure temperature uniformity throughout the chamber. Leave a 2.5 cm space between walls and stacks of dishes or baskets of tubes. Hot-air sterilizing ovens are constructed to give a uniform and adequate sterilizing temperature of 170 ± 0.2°C and are equipped with suitable thermometers. Autoclaves are used for moist heat sterilization. They are normally operated at 15 lb/in2 in steam pressure for 15 min, producing a temperature inside the autoclave of 121.6°C (250°F) at sea level. A pH meter must be accurate to at least 0.1 pH unit to determine the pH of the prepared media.