A mercury pressure gauge is a type of manometer using mercury as the working fluid. The most basic form of this instrument is a U-shaped glass tube filled with mercury. More complex versions deal with very high pressure or have better means of filling with mercury. Description The instrument consists of a glass U-tube half-filled with mercury. One end is connected to the vessel whose pressure is being measured. The other may be either left open or sealed. If it is left open, the pressure measured is relative to air pressure, which is variable. If it is sealed, the pressure measured is the absolute pressure. The tube is sealed during manufacture with the sealed end containing a vacuum. Mercury is a useful material to use in a manometer because of its high density. This means that a much shorter column is needed compared to water. For instance, the pressure represented by a column of of water is just under of mercury (mmHg). The pressure is determined by measuring the difference in height between the reference column and the column connected to the item under test. Calibration marks are usually provided to aid in this measurement and in laboratories a cathetometer might be employed for accuracy. When relative pressure is being measured the difference may be negative, meaning the test pressure is below the reference pressure. The ubiquity of this instrument led to mmHg becoming a common unit of measure of pressure. It is also related to another unit of pressure, the torr. The mmHg is not an SI unit but is still sometimes found in use, particularly in medicine. In SI units, is approximately . Filling with mercury The initial filling of a sealed gauge with mercury can be problematic. One method involves fusing the glass of the gauge to a vessel of mercury, pumping out the air and boiling the mercury. After filling, the gauge is then cut away again. Further, the vacuum in the gauge eventually deteriorates due to slow diffusion of gases through the mercury, making the device inaccurate. In 1938, Adolph Zimmerli (1886–1967) invented a gauge that overcame the filling problems, at least for pressures below ambient pressure. Zimmerli's gauge consists of three relatively wide columns. Referring to the diagram, the columns in the centre and on the right function as a standard U-tube gauge. Additionally, the top of the centre column is connected to the