closing force. To limit the door's closing speed, most door closers use oil-filled hydraulic dampers. This provides soft closing. Other types use a friction-based mechanical speed control mechanism. The absence of an oil damper makes mechanical closers more resistant to drastic temperature changes. Some closers incorporate a back-check facility which prevents the door from being opened too fast; this is useful for exterior doors where there is a danger of wind catching and blowing it open, potentially damage to the door, nearby objects, or people near it. The speed at which the door closer closes the door may be adjustable by up to three adjustment valves. These valves often adjust the sweep speed and the latch speed of the door and some closers are optioned with a delayed action valve. The latch speed is the speed that the door travels in the last 10 to 15 degrees of it closing cycle and is often set faster than the "sweep" so that the door can properly latch closed. The sweep speed is the speed which the door travels along the majority of its travel, before reaching the final 10 to 15 degrees, and is often set slower than the latch speed. For openings where a much longer close time is desired, a delayed action closer may be appropriate. The delayed action valve slows the sweep speed dramatically for roughly the first half of the sweep range. Door closers which provide this two or three-stage action and close doors at a determined rate are called 'controlled' door closers. Automatic An automatic door closer, more often called a "door opener", opens the door itself, typically under the control of a push button, motion detector or other device, and then closes it as well, employing a motion or proximity detector to determine when it is safe to close the door. These types of door mechanisms can also be used for security purposes, keeping the door locked on an electromagnetic device (mag-lock) until either a code is entered into a key pad or a swipe card is presented. High security areas may opt for a biometric system, using a retinal scanner or some sort of electronic finger print device in place of the key pad or card method. Control of closing force and speed Hydraulic and pneumatic closers often feature dampers that offer control over the closing force and speed. Hydraulic closers are known for