There is no need to tap the screen...
The NPL online barograph has been recording the pressure in Teddington, UK, since 1 January 1998.
The barometric pressure data is derived from an on-line electronic pressure transducer. The measurement uncertainty of this on-line transducer is approximately ±0.3 hPa. The unit hectopascal (hPa) is numerically identical to the millibar (mbar) - see Pressure Units
The pressure values shown are those at the point of measurement, that is approximately 10 metres above mean sea level. If a hole were dug to sea level, the pressure at the bottom would be about 1 hPa greater than at the top. Note that, when estimating a pressure value from the graphs, the uncertainty will be considerably higher than that of the measuring transducer because of limitations in screen resolution.
When using the pressure values, particularly for checking another barometer, it is important to appreciate that the differences in altitude, distance from Teddington and weather conditions may seriously limit the validity of the comparison.
A list of frequently asked questions relating to the online barograph and barometers is given below.
- The instrument currently used to measure atmospheric pressure for the NPL online 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.
- 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.
- 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.