The instrument for measuring atmospheric pressure, the barometer, is calibrated to read zero when there is a complete vacuum; the pressure indicated by the instrument is therefore called absolute pressure. The term pressure gauge is commonly applied to the other instruments used for measuring pressure.
They see what's already there in a different way, and most of the time, they're aware that their altered perceptions are caused by the drug. After taking LSD, the effects -- known as a trip -- usually start within an hour and can last up to 12 hours, with a peak about halfway through the experience.
Because of its short half-life, LSD can be detected in the urine for up to eight hours. This detection window is shorter than that of many other drugs. For example, marijuana can be detected in the urine of heavy cannabis users for up to two months after last use. Upon consumption, LSD breaks down into 2-oxo-3-hydroxy-LSD, a major metabolite of the drug that can be found in urine.