If a syringe plunger is squeezed with a closed nozzle, the resistance will rapidly rise. In a gas spring the volume is quite large compared to the diameter of the plunger and the gas which may be dry air or nitrogen is pre-compressed. Hence if a 1 square inch plunger (radius approx 0.56 in) is used with a container with an internal pressure of 30 pounds per square inch (207 kPa), a thirty pound-force (130 N) spring will result. If the volume of gas is large, little increase in strength will result as the rod is pressed in. The gas volume can be altered to change this parameter. The standard gas equation is used to calculate the difference. Pressure x volume / temperature (must be kelvins or degrees Rankine) = pressure x volume / temperature. If the internal plunger has a diaphragm, which extends to the side of the gas tube it will not move. If a fine hole exists it will be a slow dampened spring as used on heavy doors and windows. If there is no diaphragm apart from the washer to stop it flying out it will be a quick gas spring as used on air rifles and recoil buffers. Adjustability can be achieved by reducing the gas volume and hence increasing its internal pressure by means of a movable end stop or allowing one tube to slide over another. The rod may be hollow by use of clever seals or can be multiple small-diameter rods. A small amount of oil is normally present. The gas may be introduced by Schrader-type valve,using a lip seal around the rod and forcing it to allow gas in by external over pressure or a shuttling O ring system. Gas springs of high pressure contain a very large amount of energy and could be used as a power pack. In emergency use the gas may be introduced via a gas generator cell as used in air bags. Gas springs are used to operate the main valve on Formula 1 racing cars.