Fortin barometer

Detailed sketch of a Fortin barometer. Uncorrected reading shown is 79.02 cm. 

Pressure Measurement and the Flow System.—For moderately precise g.l.c.-determinations of activity coefficients [as obtained from equation (3)], the outlet pressure is usually considered as being atmospheric pressure and can be measured on a normal Fortin barometer. The inlet pressure is usually determined by difference using a manometer placed in the carrier-gas flow line. An excellent review by Ambrose gives details of such pressure measurements. 

Mercury barometers of this type, with a reservoir of mercury, are known as cistern barometers. A common type is the Fortin barometer, in which the mercury is held in a leather bag so that the level in the reservoir can be adjusted. The height is read from a scale along the side of the tube in conjunction with a vernier scale that can be moved up and down. Corrections are made for temperature. 

The simplest instrument for measurement of pressure with moderate accuracy by determination of the height of a mercury column is the fixed cistern (Kew type) barometer models are available on which heights from a nominal zero up to 1000 mm can be read. The error associated with the reading, however (and with that on the normal Fortin barometer used in the laboratory), is not less than 0.15 mm because of the effect of capillarity in the relatively narrow tube, and because of the uncertainties in determining the height of the mercury meniscus above its level in the cistern, and in the temperature. Reduction of the error requires refinement... 

The value of the pressure calculated in one of the ways just described is the pressure at some reference point in the instrument, e.g. at the level of the mercury in the reservoir of a Fortin barometer, or at the base of the piston in a piston gauge. If, when a barometric measurement is made, the height of the reservoir differs from that of the point at which the pressure needs to be known, a correction for the hydrostatic head of air may be required at atmospheric pressure, for example, a difference in height of 1 m corresponds to about 12 Pa. Tables are available for making barometric corrections, but in apparatus for vapour-pressure measurement the situation is complicated by the fact that some parts of the system may contain a gas (probably nitrogen or helium) and some parts will contain the vapour (which is of different density) the corrections are small and may be calculated by treating both gas and vapour as ideal gases, i.e. the... 

When sight is taken on the meniscus, the simplest optical system is similar to that used in the Fortin or Kew barometer the meniscus is illuminated from the back through a diffusing screen, and adjustable blackened opaque screens in front and behind are set about 1 mm above the meniscus so that light does not fall on its top surface and a sharply defined silhouette is seen. A simple mechanical device for this operation, in which the source of illumination and the screen are attached to an endless... 

Pressures may be obtained from readings of Fortin and Kew type barometers by means of tables of corrections " the appropriate correction... 

In many cases a Fortin-type barometer (Fig. M-13) is suitable. In this case the merexuy in the well is exposed to the atmosphere with the other end evacuated and sealed. All barometer readings shotdd be corrected for temperature, local gravity, and capiUary effect. Atmospheric pressure can also be measured by an aneroid barometer, which is a special type of elastic gauge. It is sometimes used in place of a manometer-type barometer because of the ease of transportation. 

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