National Physical Laboratory

How do I use a Fortin or Kew Pattern mercury barometer? (FAQ - Pressure)

Operating Sequence

Mercury barometers must be firmly supported, made vertical and given time to adopt ambient temperature before attempting to use them. The correct sequence for measuring pressure with a Fortin or Kew Pattern barometer is:

  • make vertical
  • adjust the cistern mercury surface to within a small distance of the fiducial point (Fortin barometer only)
  • tap the barometer (to achieve a stable meniscus)
  • set the fiducial point (Fortin barometer only)
  • set the vernier
  • measure the barometer's temperature (read thermometer and add calibration correction)
  • read the vernier
  • apply the calibration correction (eg from Certificate of Calibration)
  • apply the temperature and gravity corrections (from tables or formula)
  • apply a height correction if pressure value is required at a different height

Making Vertical

Fortin barometers should be used with their axis of rotation vertical - such that a setting of the mercury surface to the fiducial point remains correct after axial rotation through any angle. Kew Pattern barometers should have their scale edge made vertical.

Fortin Barometer

Setting the Fiducial Point (Fortin barometers only)

Schematic of Fortin barometerThe lower mercury surface in a Fortin barometer has to be set to a datum level before adjusting its vernier (see schematic diagram) and the accuracy of pressure measurement depends crucially upon proper setting. To achieve this the mercury surface should first be lowered until it is clearly below the fiducial point (F). After tapping the barometer, the level-adjusting screw (S) should be turned slowly until the thin background of light between the mercury surface and the point just disappears, while viewed along a horizontal line of sight. To confirm proper setting, observe the point of contact at an angle of elevation of about 30° and determine whether or not the fiducial point is making an indentation in the mercury.

When the setting is correct there should be no more than the slightest dimple where the fiducial point makes contact with the mercury surface. If the surface is bright and the setting correct, the tip of the point will appear to coincide with its reflected image in the mercury surface. Correct setting is achieved when the mercury surface is raised to the fiducial point but not lowered - if the mercury surface overshoots the point it should be lowered and then raised again.

A dirty mercury surface, poorly shaped fiducial point or a partially clogged porous plug (P) can make proper and repeatable setting very difficult. If the barometer is being used in an application where particular measurement uncertainties are needed the barometer should be re-conditioned.

Setting the vernier

With suitably bright background illumination, and taking care to correctly align the front and back setting edges, the vernier cursor (V) should be lowered until it appears on the summit of the mercury meniscus. Proper setting can be aided by making small 'pecking' head movements to ensure that both cursor edges just meet the meniscus.

Measuring temperature

Mercury barometers have a temperature coefficient of nearly 0.2 hPa per °C so it is important to ensure that their thermometers respond to ambient temperature changes in the same fashion as the barometer. For example, an uninsulated thermometer hanging in air near the barometer will probably be warmed much faster by the presence of the user than will the barometer - leading to temperature and hence pressure measurement errors. Traditionally, barometer thermometers are mounted with their bulbs within the frame of the barometer so that the thermal time-constant of each is about the same. An alternative is to mount the thermometer in a short tube of mercury having the same cross-sectional area as the barometer's tube, and shielded with the same materials as the barometer's frame.

Calculation of pressure from Fortin barometer readings

Fortin Calculation


  • R is the barometer reading
  • c is its calibration correction (ie determined from its calibration)
  • g is the acceleration due to gravity at the level of the barometer in m/s2 (see section)
  • b is the coefficient of expansion of mercury (taken to be 0.000 181 8 °C-1 for meteorological purposes)
  • a is the coefficient of linear expansion of the scale (taken to be 0.000 018 4 °C-1 for brass)
  • t is the temperature of the barometer in °C

Calculation of pressure from Kew pattern barometer readings

Kew Pattern Calculation

where the symbols are as above and:

  • V/A is a geometrical factor in millimetre units, the value of which should be inscribed on the barometer. (V is the total volume of mercury in the barometer and A is the effective area of cross section of the cistern.)
  • f is a unit conversion factor:
    • 0.133 3 for a kPa scale
    • 1.333 for a mbar scale
    • 1.000 for a mmHg scale
    • 0.039 37 for an inHg scale
Last Updated: 25 Mar 2010
Created: 9 Aug 2007


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