Pressure - FAQs
The answers to some frequently asked questions about pressure and vacuum metrology are grouped below.
Search
- Science + Technology
- Commercial Services
- Educate + Explore
- Networks
FAQs
Pressure Units
-
No, they are not. The internationally recognised SI unit for pressure is the pascal, abbreviated to Pa, and this is the unit realised by the primary measurement standards in the world's national metrology institutes to provide traceability for pressure measurements.
-
When converting between pressure units consideration should be given to the number of significant figures to use, bearing in mind that many of the underlying conversion factors are not themselves exact and cannot be made so.
-
To convert one non-pascal pressure unit to an alternative non-pascal unit, multiply the 'starting' pressure value by the 'number of pascals' shown against its unit and then divide the product by the number of pascals shown against the second unit.
-
Pressure is not a base quantity but a derived quantity, with dimensions of length (L), mass (M) and time (T).
-
No, ultimately it is less accurate. Liquid depth can be determined from pressure measurements and the reason can be seen by looking at the fundamental (and simplified) equation for calculating the pressure at a particular depth in a liquid.
-
Your barometer is undoubtedly scaled in inches of mercury - properly abbreviated to inHg (with only the 'H' a capital letter).
-
There are no pressure units called just inches but there are inches of mercury - properly abbreviated to inHg (with only the 'H' a capital letter) and inches of water.
-
The SI unit of pressure is the pascal, symbol Pa, the special name given to a pressure of one newton per square metre (N/m2).
-
See the Pressure Units FAQs.
-
Many old pressure units have obvious practical and historical origins; for example, inches of water was the unit used where pressures were measured with a water column whose top surface was sighted against an inch scale.
Pressure Modes & Ranges
-
To avoid ambiguity, when referring to a pressure value it is important to specify its mode.
-
The terms high- and low- (and also medium-) vacuum are not very intuitive; they are used to describe the various pressure ranges known under the general banner of vacuum but are neither defined nor used consistently.
-
The letters are meant to stand for gauge-mode and absolute-mode respectively - where a gauge-mode pressure is one normally measured with respect to ambient pressure and an absolute-mode pressure is one measured with respect to zero pressure.
-
A hard vacuum is not well defined but it sometimes seems to be used erroneously to describe a pressure that is so low that extreme forces are involved.
-
Its definition is not precise but, as mentioned in the section above, it is commonly taken to mean pressures below, and often considerably below, atmospheric pressure.
-
There is no clear boundary between pressure and vacuum and the word vacuum simply refers to part of the pressure scale. Its definition is not precise but it is commonly taken to mean pressures below, and often considerably below, atmospheric pressure.
NPL Online Barograph
-
The instrument currently used to measure atmospheric pressure for the NPL on-line barograph is a good-quality, commercially available resonant pressure sensor.
-
Values of atmospheric pressure in Teddington, measured at about 10 metres above sea level, going back to 1 January 1998 and shown both graphically and digitally, can be found on the historical pressure page of the NPL on-line barograph.
-
To the best of our knowledge, the NPL on-line barograph is unique inasmuch as it provides a fully traceable pressure measurement, with a properly calculated uncertainty, and also allows historical data to be displayed both graphically and digitally.
Barometry
-
The most accurate barometers are indeed the mercury primary barometers used at national measurement institutes. Most barometers, though, are secondary instruments rather than primary ones and when considering these it is not correct to say that those based on a mercury column are invariably more accurate than those that are based on an alternative principle.
-
There are many different types of barometer but they fit into two broad categories - those containing mercury and those that do not.
-
The answer to this question depends on the type of barometer in question, particularly whether its pressure sensing mechanism is influenced by gravity or not, and what you are trying to get it to measure.
-
Mercury barometers must be transported with extreme care - the prime concerns are to prevent air from entering the vacuum space or the tube from being broken and leaking mercury.
-
Mercury barometers must be firmly supported, made vertical and given time to adopt ambient temperature before attempting to use them.
-
Some barometer measurement principles are listed below; there are bound to be others.
Pressure Balances
-
Yes it can in some circumstances but, where it does, it is fairly easy to ensure that its effect on the measurement uncertainties obtained is negligible.
-
A pressure balance is simply an alternative name for a piston gauge (more frequently spelt piston 'gage').
-
Pressure balances operate over a pressure range extending from about 3 kPa to 1 GPa. Any one piston-cylinder can only be used over a pressure range that typically varies from about 10:1 up to about 100:1.
Instrumentation
-
The diagram below shows the approximate operating ranges of some commonly available pressure and vacuum measuring instruments.
-
Transducers are voltage-output devices that can be used with simple signal conditioning but are more sensitive to electromagnetic interference. Transmitters are current-output devices and may have two or three wires.
-
Many members of Pressmet (EMAN's Pressure Measurement Special Interest Group) can supply, calibrate or repair such apparatus.
Accuracy, Uncertainty, Calibration & Traceability
-
Instrument calibration is often thought to be expensive but the information contained in the resultant certificate is usually worth considerably more. This is not always appreciated and, on receipt, many certificates are wastefully consigned to drawers, shelves or even wall plaques.
-
No, the difference is not very meaningful at all.
-
Probably not. The figures should be interpreted with caution, especially if an instrument has not been independently calibrated for some time.
-
The words accuracy and uncertainty are sometimes interchanged but the difference between them is significant and, in many applications it is vital.
-
The measurement uncertainties achievable with pressure gauges, particularly traditional circular 'dial' gauges, are often expressed in one of two ways - as a percentage of reading or as a percentage of full-scale reading and the differences can be very significant, particularly when working at pressures much lower than an instrument's full-scale.
-
Sometimes, the word calibration is misused to describe the process of altering the performance of an instrument to ensure that the values it indicates are correct within specified limits, strictly this is adjustment.
Miscellaneous
-
To calculate a pressure value using a liquid column - for example a mercury barometer - or a pressure balance it is necessary to know the gravitational acceleration at the location of the instrument. It can be determined by measurement on site, calculation or interpolation of measured values.
-
Pressure in a fluid, whether it be gas or liquid, varies with height. It doesn't matter whether the fluid is in pipework or more loosely confined such the atmosphere or the sea - just so long as there is gravitational attraction and something stopping free-fall (or a centripetal force - for those who are pedantic, in orbit or who twirl open cups of tea on horizontal cords).
-
Atmospheric pressure reduces with altitude for two reasons - both related to gravity.
Guides
-
The purpose of this guidance note is to explain why it is not possible to claim formal traceability for the pressure measurements made by a pressure balance, at anywhere approaching state-of-the-art levels of measurement uncertainty, when such traceability is based on locally obtained dimensional measurements of individual pistons and cylinders.
-
Calibration is the process of comparing a measuring instrument with a measurement standard to establish the relationship between the values indicated by the instrument and those of the standard.