A mercury pressure gauge is a type of

manometer Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressur ...

using

mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

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 Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressur ...

. 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 Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

. 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 A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly pascals. It is denoted mmHg or mm Hg. Although not an SI ...

). 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 A cathetometer is an instrument for measuring vertical distances in cases where a scale cannot be placed very close to the points whose distance apart is desired. The instrument consists essentially of an accurately graduated scale and a horizon ...

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 torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one "millimeter of mercury ...

. The mmHg is not an

SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...

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 Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

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 bottom of the third column on the left with a

capillary tube Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...

. The centre column is initially completely filled with mercury, as is the connecting capillary. The other two columns are partially filled. The top of both the main column on the right and the reservoir column on the left are connected together and to an inlet for the pressure to be measured. When the test pressure is applied, the mercury rises in both the left and right columns and falls in the centre column. The mercury at the top of the capillary breaks and a vacuum forms there. The pressure is then measured in the usual way by the difference between the heights of the right and centre columns. Since a new vacuum is formed each time a measurement is made, there is no problem with the vacuum becoming contaminated. Any bubbles that do form in the capillary are easily removed by inverting the gauge and shaking or tapping.

High pressure measurement

For extremely high pressures, the column can still be very high, even when using mercury. Gauges for measuring pressure in the range have been built.Hála ''et al.'', p. 220 A 23-metre-tall mercury column is difficult to read and suffers from inaccuracies caused by different parts of the column being at different temperatures. A more compact mercury pressure gauge suitable for high pressure was built by

Heike Kamerlingh Onnes Heike Kamerlingh Onnes (21 September 1853 – 21 February 1926) was a Dutch physicist and Nobel laureate. He exploited the Hampson–Linde cycle to investigate how materials behave when cooled to nearly absolute zero and later to liquefy helium f ...

, the discoverer of

superconductivity Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...

. This consisted of a series of mercury filled U-tubes connected together with inverted U-tubes. The inverted U-tubes contain compressed air at a pressure designed to bring the instrument into the pressure range of interest. The pressure is found from this instrument by summing together the difference in column heights in each of the U-tubes.

History

The parent of all mercury pressure gauges is the mercury

barometer A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...

invented by

Evangelista Torricelli Evangelista Torricelli ( , also , ; 15 October 160825 October 1647) was an Italian physicist and mathematician, and a student of Galileo. He is best known for his invention of the barometer, but is also known for his advances in optics and work o ...

in 1643. An early engineering application of the mercury pressure gauge was to measure pressure in steam boilers during the age of steam. The first use on

steam engine A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...

s was by

James Watt James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was fun ...

while developing the

Watt steam engine The Watt steam engine design became synonymous with steam engines, and it was many years before significantly new designs began to replace the basic Watt design. The first steam engines, introduced by Thomas Newcomen in 1712, were of the "at ...

between 1763 and 1775. This engine was a development of the popular

Newcomen atmospheric engine The atmospheric engine was invented by Thomas Newcomen in 1712, and is often referred to as the Newcomen fire engine (see below) or simply as a Newcomen engine. The engine was operated by condensing steam drawn into the cylinder, thereby creati ...

. A gauge for use on steam engines very similar to the later Kamerlingh-Onnes gauge was patented in 1858 by Thomas Purssglove. Like the Kamerlingh-Onnes device, it had multiple U-tubes connected in series. The connecting tubes were filled with an incompressible fluid. The instrument was formerly widely used in education, laboratories, and medical measurements as well as its industrial applications. However, the toxicity of mercury and the risk of spills, through broken glassware, has led to its decline. It is also easier to interface other types of sensor to electronic systems. By 1991 it had mostly been replaced by other technologies.

Use as a standard

Mercury gauges are commonly used as the

primary standard A primary standard in metrology is a standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are u ...

for pressure by national measurement standards laboratories. For instance, the

National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...

(NIST) in the US uses a gauge that is three metres tall and contains of mercury. For precision,

ultrasonics Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...

are used to measure the mercury column height. However, in 2019 the backup gauge was decommissioned after being out of service for years. It was so large that it could not be removed by normal means; a hole was cut in the ceiling to extract it. The decommissioning was part of an international move to stop using mercury in standards laboratories for environmental reasons. NIST will eventually also take the main mercury gauge out of service after a portable

photonic Photonics is a branch of optics that involves the application of generation, detection, and manipulation of light in form of photons through emission, transmission, modulation, signal processing, switching, amplification, and sensing. Though ...

device is installed to replace it.Lee

References

Bibliography

COWI COWI A/S is an international consulting group, specialising in engineering, environmental science and economics, with headquarters in Lyngby, Denmark. It has been involved in more than 50,000 projects in 175 countries and has approximately 7,300 em ...

Consulting Engineers and Planners
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''English Patents of Inventions, Specifications: 1858, 2675-2752'', patent no. 2739, filed 1 December 1858, issued 31 May 1859. * Singh, Sarbjit, ''Experiments in Fluid Mechanics'', PHI Learning, 2012 . * Stein, Benjamin, ''Building Technology: Mechanical and Electrical Systems'', John Wiley & Sons, 1996 . * Suski, J.; Puers, R.; Ehrlich, C.D.; Schmidt, J.W.; Abramson, E.H.; Sutton, C.M., "Pressure", ch. 3 in, Goodwin, A.R.H.; Marsh, K.N.; Wakeham, W.A. (eds), ''Experimental Thermodynamics (vol. 6): Measurement of the Thermodynamic Properties of Single Phases'', Elsevier, 2003 {{ISBN, 008053144X. * Zimmerli, Adolph
"An improved mercury U-gage"
''Industrial & Engineering Chemistry: Analytical Edition'', vol. 10, iss. 5, pp. 283–284, 1 May 1938. Pressure gauges