Thermostat

THERMOSTAT, a cabinet in which a specific, pre-set temperature is maintained. The thermostat has gained the widest distribution in microbiological practice for the cultivation of microorganisms. Since the majority of pathogenic microbes have an optimum growth temperature close to 37°, thermostats are usually set to this temperature. For the cultivation of saprophytes, the temperature is set to 22°. A thermostat is usually constructed with double walls, between which water is poured for uniform heating. The upper wall of the cabinet has a ventilation opening and an opening for a thermometer indicating the temperature of the thermostat. On the side wall, a water level indicator and a tap for draining water are installed. For the purpose of maximum insulation, the thermostat has double doors—an inner glass one and an outer metal one. The Roux thermostat has a somewhat different design, representing a cabinet with single walls, heated by hot air from copper tubes passing along the walls of the thermostat. The system of heating devices depends on the sources of heat. Electric thermostats are heated by the incandescence of nickel wire, laid either directly in the walls of the thermostat (the Saratov plant model) or wound onto asbestos cylinders in the form of heating candles covered with a metal casing, which are inserted into specially constructed sockets (German models Krister, Lautenschläger, and others). Gas thermostats are heated by special gas burners equipped with automatic shut-off valves, which close the gas supply if the burner goes out. In expeditionary work, thermostats heated by kerosene lamps are used. In large institutes, entire thermostatic rooms are often constructed, having double walls with a heat-insulating lining (cork) between them. An essential part of the thermostat is the device that maintains a constant temperature in the thermostat by means of automatic regulation of heating, the so-called thermoregulator. Depending on the source of heat, thermoregulators are of various designs. The temperature of German-model electric thermostats is regulated by means of the automatic switching on and off of an additional heating candle. Electric thermostats from the Saratov plant are regulated by means of a so-called thermostatic pad, constructed according to the following scheme: inside the thermostat, a brass pad is attached, hermetically sealed and containing ether. Upon heating, the ether vapors expand the pad, which, through a series of levers, disconnects the poles of the heating device and stops the supply of electric current. For gas thermostats, mercury thermoregulators of the Reich system are most frequently used.