Temperature control

Temperature control is a process in which the temperature of an object is measured and the passage of heat energy into or out of the object is adjusted to achieve a desired temperature.

A thermostat is a simple example for a closed control loop: It constantly measures the current temperature and controls the heater's valve setting to increase or decrease the room temperature according the user-defined setting. A simple method switches the heater or cooler either completely on, or completely off, and an overshoot and undershoot of the controlled temperature must be expected. A more expensive method varies the amount of heat or cooling provided by the heater or cooler depending on the difference between the required temperature (the "setpoint") and the actual temperature. This minimizes over/undershoot. This process is called PID and is implemented using a PID Controller.

Energy Balance

An object's temperature increases when heat energy moves into it, increasing the average kinetic energy of its atoms. Heat energy leaving an object lowers its temperature. Heat moves from one place to another (always from a higher temperature to a lower one) by one of three processes: conduction, convection and radiation. In conduction, energy is passed from one atom to another by direct contact. In convection, heat energy moves by conduction into some movable fluid (such as air or water) and the fluid moves from one place to another, carrying the heat with it. At some point the heat energy in the fluid is usually transferred to some other object by means conduction again. The movement of the fluid can be driven by buoyancy (natural convection) or by fans or pumps (forced convection). In radiation, the movement of the atoms make electromagnetic connections directly with other atoms. The electromagnetic forces caused by one atom's vibrations cause sympathetic vibrations in other atoms, sometimes over astronomical distances.

An object in contact with the surrounding world can lose or gain heat energy by means of any or all of the three methods. An object can receive energy from some of its surrounding atoms and give up energy to others, depending on their relative temperatures. If more energy is received, its temperature increases. If the amount of energy coming in and going out are exactly the same, the object's temperature stays constant- the energy balance is in equilibrium. It is the goal of any temperature control system to adjust the surroundings of the object so that it is in equilibrium at the desired temperature.