building. Peltier effect refrigerators The Peltier effect uses electricity to pump heat directly; refrigerators employing this system are sometimes used for camping, or in situations where noise is not acceptable. They can be totally silent (if a fan for air circulation is not fitted) but are less energy-efficient than other methods. Ultra-low temperature refrigerators "Ultra-cold" or "ultra-low temperature (ULT)" (typically −80°C or −86°C) freezers, as used for storing biological samples, also generally employ two stages of cooling, but in cascade. The lower temperature stage uses methane, or a similar gas, as a refrigerant, with its condenser kept at around −40°C by a second stage which uses a more conventional refrigerant. Well known brands include Forma and Revco (both now Thermo Scientific). For much lower temperatures, laboratories usually purchase liquid nitrogen (−196 °C), kept in a Dewar flask, into which the samples are suspended. Cryogenic chest freezers can achieve temperatures of down to −150°C, and may include a liquid nitrogen backup. Other refrigerators Alternatives to the vapor-compression cycle not in current mass production include: Acoustic cooling Air cycle Magnetic cooling Malone engine Pulse tube Stirling cycle Thermoelectric cooling Thermionic cooling Vortex tube Water cycle systems. Architecture Many modern refrigerator/freezers have the freezer on top and the refrigerator on the bottom. Most refrigerator-freezers—except for manual defrost models or cheaper units—use what appears to be two thermostats. Only the refrigerator compartment is properly temperature controlled. When the refrigerator gets too warm, the thermostat starts the cooling process and a fan circulates the air around the freezer. During this time, the refrigerator also gets colder. The freezer control knob only controls the amount of air that flows into the refrigerator via a damper system. Changing the refrigerator temperature will inadvertently change the freezer temperature in the opposite direction. Changing the freezer temperature will have no effect on the refrigerator temperature. The freezer control may also be adjusted to compensate for any refrigerator adjustment. This means the refrigerator may become too warm. However, because only enough air is diverted to the refrigerator compartment, the freezer usually re-acquires the set temperature quickly, unless the door is opened. When a door is opened, either in the refrigerator or the freezer, the fan in some units stops immediately to prevent excessive frost build up on the freezer's evaporator coil, because this coil is cooling two areas. When the freezer reaches temperature, the unit cycles off, no matter what