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.
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. 100 mm of water is just under 7.4 mm of mercury (mmHg).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
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. 1 mmHg is approximately 133 Pa .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,
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.
A gauge that overcame the filling problems was invented by Adolph Zimmerli (1886–1967)in 1938, at least for low pressures (that is, pressures below ambient pressure – partial vacuums). 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. 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.
For extremely high pressures, the column can still be very high, even when using mercury. Gauges for measuring pressure in the range 20–30 standard atmospheres (15,000–23,000 mmHg) have been built. 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, the discoverer of superconductivity. 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.
The parent of all mercury pressure gauges is the mercury barometer invented by Evangelista Torricelli 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 engines was by James Watt while developing the Watt steam engine between 1763 and 1775. This engine was a development of the popular Newcomen atmospheric engine. 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.
Mercury gauges are commonly used as the primary standard for pressure by national measurement standards laboratories. 225 kg of mercury. For precision, ultrasonics 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 device is installed to replace it.For instance, the National Institute of Standards and Technology (NIST) in the US uses a gauge that is three metres tall and contains
Pressure measurement is the analysis of an applied force by a fluid 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 pressure and vacuum. Instruments used to measure and display pressure in an integral unit are called pressure meters or pressure gauges or vacuum gauges. A manometer is a good example, as it uses the surface area and weight of a column of liquid to both measure and indicate pressure. Likewise the widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.
Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.
A pitottube, also known as pitot probe, is a flow measurement device used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy. It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure liquid, air and gas flow velocities in certain industrial applications.
The torr is a unit of pressure based on an absolute scale, defined as exactly 1/760 of a standard atmosphere. Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).
A vacuum is space devoid of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is considerably lower than atmospheric pressure. The Latin term in vacuo is used to describe an object that is surrounded by a vacuum.
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Professor Heike Kamerlingh Onnes, 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 for the first time, in 1908. He also discovered superconductivity in 1911.
The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average pressure at sea level. By the barometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.
A sphygmomanometer, also known as a blood pressure monitor, or blood pressure gauge, is a device used to measure blood pressure, composed of an inflatable cuff to collapse and then release the artery under the cuff in a controlled manner, and a mercury or aneroid manometer to measure the pressure. It is always used with a means to determine at what pressure blood flow is just starting, and at what pressure it is unimpeded. Manual sphygmomanometers are used with a stethoscope when using the auscultatory technique.
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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 133.322387415 pascals. It is denoted mmHg or mm Hg.
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Inch of mercury is a unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.
In fluid mechanics, pressure head is the height of a liquid column that corresponds to a particular pressure exerted by the liquid column on the base of its container. It may also be called static pressure head or simply static head.
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The Pirani gauge is a robust thermal conductivity gauge used for the measurement of the pressures in vacuum systems. It was invented in 1906 by Marcello Pirani.
Frans Michel Penning was a Dutch experimental physicist. He received his PhD from the University of Leiden in 1923, and studied low pressure gas discharges at the Philips Laboratory in Eindhoven, developing new electron tubes during World War II. Many detailed observations of gas ionization were done with colleagues, finding notable results for helium and magnetic fields. He made precise measurements of Townsend discharge coefficients and cathode voltage fall. Penning made important contributions to the advancement of high resolution Mass spectrometry.
The article reviews the evolution of continuous noninvasive arterial pressure measurement (CNAP). The historical gap between ease of use, but intermittent upper arm instruments and bulky, but continuous “pulse writers” (sphygmographs) is discussed starting with the first efforts to measure pulse, published by Jules Harrison in 1835. Such sphygmographs led a shadowy existence in the past, while Riva Rocci's upper arm blood pressure measurement started its triumphant success over 100 years ago. In recent times, CNAP measurement introduced by Jan Penáz in 1973 enabled the first recording of noninvasive beat-to-beat blood pressure resulting in marketed products such as the Finapres™ device and its successors. Recently, a novel method for CNAP monitoring has been designed for patient monitoring in perioperative, critical and emergency care, where blood pressure needs to be measured repeatedly or even continuously to facilitate the best care for patients.
Arterial blood pressure is most commonly measured via a sphygmomanometer, which historically used the height of a column of mercury to reflect the circulating pressure. Blood pressure values are generally reported in millimetres of mercury (mmHg), though aneroid and electronic devices do not contain mercury.