The regulating valve mechanically meters out the gas at a pressure of roughly and a rate of about per minute. Typically there are two high pressure hoses attached to the valve, which are connected at the other end to aspirators. These are usually cylindrical, hollow aluminum tubes with sliding cylindrical or internal flapper doors that open when high pressure gas is applied, and close when the gas stream subsides and the internal slide back pressure reaches about 2.5 - 3.0 psi. They work on the Venturi principle, and draw outside air into the evacuation unit at a rate of about 500:1. A gas cylinder can fill a slide with about of air to a pressure of about in about four to six seconds.