March 2009
Manometer makeover
Manometers, instruments for measuring the pressure of gases and vapors, also serve as the reference to which gauges are calibrated. Manometer tables provide a reference for measuring devices using various liquids at various conditions.
The last published version of the ISA2 Manometer Tables recommended practice was in 1978. That is why the ISA2 committee is ready for updates and is assembling a review committee to revisit the standard and convert it to a technical report. As it stands now, the document is a “compilation of manometer tables that has historical value,” said Edward Holden, committee chair and project manager at Shaw Power Group in Centennial, Col. “Before the advent of high-powered computers and high-speed calculating, tables provided a quick and accurate resource,” Holden said. The goal is to replace the current format with computer modeling, “but the data contained could be relevant to a lab or testing group that would like a reference basis,” he said. “This information does not change over time, and the standard is undergoing a transition to a technical paper that will not be subject to the ISA standards reaffirmation process.”
The current document presents abbreviations and fundamental conversion factors commonly used in manometry, recommended definitions of pressure in terms of a column of mercury and water. It also sees use for a large number of liquids and tables of pressures indicated by, or equivalent to, heights of columns at various temperatures. These data facilitate and standardize the use of manometers and U-tubes as direct pressure-indicating instruments or in the calibration of pressure recorders and controllers.
“Having reaffirmed manometer tables will be useful as a base reference for all forms of pressure related measurements, including level, flow, and the like,” said Ian Verhappen, committee member and director of industrial networks at MTL Instrument Group in Edmonton, Alberta, Canada. “Pressure-based measurements can handle about 80% of all process measurements,” he said, which is “why they are important.” “Because of their accuracy and simplicity, liquid manometers are used in field and shop calibration applications,” said Donald R. Gillum in his book, Industrial Pressure, Level, and Density Measurement. “As they have no mechanical moving parts, they require no calibration, and excessive use causes no loss of dependability or accuracy. Well manometers see use primarily for shop calibration of process instruments and shop standards. Because shop calibration is inconvenient for some process instruments, portable manometers for field calibration are available,” he said.
Holden said “companies that do basic low-pressure and flow measuring” will benefit most from using the new document, which will allow users to incorporate data into software development for analysis and measurement. The new document will also help the industry as a reference and standard for software development, he said. Small diverse groups that will look over the existing data will work best for this committee, Holden said. The committee is looking for members who can “confirm the accuracy and add any value to the content, then support a publication as a technical report,” he said. Holden expects the standard to hit the street at the end of this year.
Ellen Fussell Policastro (efussellpolicastro@isa.org) writes and edits Standards.
Manometer, pressure primerThree types of pressure measurement are common in process applications and in the calibration of process instruments: 1) positive or gage pressure—those above atmospheric; 2) negative pressures or vacuums—those below atmospheric; and 3) differential pressure—the difference between the two pressures. You can measure differential pressures by connecting one leg to each of the two pressures. The higher pressure causes the fluid level to be depressed, while the lower pressure causes the fluid level to be elevated. You measure the true differential pressure by the difference in height of the fluid level in the two legs. Sometimes it is convenient to use one manometer to measure two different pressures. Such is the case in the calibration of a pneumatic differential-pressure instrument. You can use a dual-tube manometer for this purpose. The instrument input range, which is normally in inches of water, connects to one tube for measurement; the instrument output pressure, which is normally psig, connects to the second tube and is read on an associated scale. When using water as the fill fluid, a 10-inch fluid height will measure 0.360 psi. Another tube using mercury as the fill fluid will measure 4.892 psi for a 10-inch level displacement. The range of the two measurements is 13.57:1, which happens to be the ratio of the specific gravities of mercury and water. Electronic manometers have largely replaced glass tube manometers for low-pressure applications. The term inches of water, derived from the use of glass tube devices, is still commonly used for low-pressure application even with electronic manometers. These manometers measure pressure in inches of water to a few thousand psi. SOURCE: Donald R. Gillum, Industrial Pressure, Level, and Density Measurement, Second Edition, ISA, 2009. |
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