The Mercury Manometer

The Mercury Manometer.—For pressures not exceeding 130 kPa the pressure-measuring instrument will usually be a mercury manometer, by means of which the pressure is related to the fundamental quantities of length ii.e. the height of the mercury colmnn) and the density of the mercury. In amplification of what follows, one introductory and two comprehensive articles on the principles of barometry will repay study. 

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 

In planning an experiment it is best to make the diameter of the manometer tube sufficiently large for any residual uncertainties to be negligible, and study of the tables of capillary depression suggests for precise work a diameter of not less than 20mm Thomas and Cross have described a manometer for measurement of differential pressures up to 100 mmHg in 

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 

For measurement of low pressures where the height of the mercury column is within the compass of standard micrometer screws, the positions of the menisci may be determined by observation of the contact of probes attached to micrometers, the spindles of which are sealed to the apparatus by means of 0-rings. When the mercury surface is located by contact in this way, the design of the probes is important if sticking of the mercury to the probe (or a jump in the mercury surface as the probe approaches) 

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The beauty of the mercury manometer is that you can assess its accuracy by simply looking at it. If the mercury meniscus is at zero when there is no pressure in the cuff and the column moves smoothly with inflation and deflation it is accurate and can be used as the gold standard for pressure measurement. All other devices must be calibrated against a... 

Pump air into the system until the mercury manometer reads standard say 180. Then record the pressure that the aneroid reads. Do this throughout the range to be tested. Aneroid should be 3 mm Fig. 

If using an electronic calibration standard, it is connected in place of the mercury manometer. You should test only one device at a time. 

This chapter discusses some key features for BP measurement and management in the office and the home and stresses the continued use of the mercury manometer as recommended by the newest AHA guidelines. A method to validate home and office device accuracy is detailed. Finally a stepwise combination of combinations approach to BP control in the difficult patient is reviewed, which can be used in the in- and outpatient setting. 

Spring balances are still used in certain research investigations when adsorption equilibration is very slow, e.g. for the study of hysteresis phenomena. For this purpose, it is advisable to replace the mercury manometer by a modem pressure gauge. However, in recent years spring balances have been largely superseded by electronic microbalances. The essential features of an electronic, null adsorption microbalance are indicated in Figure 3.11. 

For more precise work, other devices may be used. The most familiar is the closed-tube mercury manometer (see Chapter XVIII). Clearly, the mercury manometer is convenient only for pressures that do not exceed —1000 Torr (1.33 bar). Furthermore, manometer readings are time consuming and do not provide a convenient analog or digital output signal. Finally, concerns about health hazards associated with mercury (see Appendix C) have led to a reduction in the use of mercury manometers. 

Beckman Coulter Z1 operates in the 1 to 120 pm size range and 1 to 60 pm using ampoule insertable aprture tubes. Metered volumes include 0.10 ml, 0.50 ml and 1.00 ml. The mercury manometer is replaced with an oil displacement pump. 

The first method is a direct method and employs an apparatus which is diagrammatically represented in Figure 16. The entire apparatus is enclosed in a constant temperature bath. The liquid whose vapor pressure is to be determined is placed in vessel A and the system is evacuated to remove all air from both sides of the mercury manometer. When all the air has been removed, the two stopcocks B and C are... 

The general procedure is given in Section 25-A. The reaction vessel should be covered to exclude light and the mercury manometers closed except when the diborane is measured. These two precautions lessen the chance of extensive product decomposition. 

IMPORTANT It should be obvious that the other side of the mercury manometer must be similarly pressurized or evacuated. High or low pressure experiments should only be performed by those well acquainted with the system and have run numerous experiments at atmospheric pressure. 

The isoteniscope measurement contains three major sources of error. Two of these sources are independent of pressure. The mercury manometer accuracy of H torr is set as a standard normal distribution about the "true" pressure. The question of sample purity is addressed in the ASTM methodology, with a maximum upward bias of 1 torr on the vapor pressure assured by using this technique. A log-normal distribution with a mean to 0.1 torr is chosen as the representative model based upon suggestions by Osborn and Scott (14). Since these errors are simultaneous in nature and independent of sample pressure, they are taken to be additive. 

To understand the calculations you must have a clear picture of the pressures exerted on the right and left mercury columns at the dashed reference line in FIGURE 11.1. The right side of the mercury manometer is open to the atmosphere. This side is the lower side in all of your measurements and serves as a basis for the dashed reference line. The only pressure exerted on the right mercury column is atmospheric pressure which is equal to the barometric pressure and can be designated Patm is then only a... 

Find the pressure of the gas in the mercury manometer shown if atmospheric pressure is 747 torr. 

It has been suggested that the adsoiption of mercury vapor could affect adsorption of nitrogen and to overcome this problem Dollimore et al. [122] devised a doser unit incorporating a pressure transducer to replace the mercury manometer. Bugge and Kerlogue [123] simplified... 

Nitrogen is admitted to a burette (5 in this case) and the mercury manometer pressure reading is taken. The gas is compressed to the next burette volume and the new reading taken. As with the helium the volume is reduced to the volume at STTP. Usually 2 or 3 readings are noted but more may be taken if desired (Table 2.3). Volumes should be reproducible to 0.02 mL. 

If a dead-weight tester is used instead of the mercury manometer (See 16.2), apply the calibration factor in kilopascals (pounds-force per square inch) established for the pressure gage to the uncorrected vapor pressure. Record this value as the calibrated gage reading and use in Section 7 in place of the manometer reading. 

A 1.7 Dead-Weight Tester—k dead-weight tester can be used in place of the mercury manometer (A 1.5) for checking gage readngs above 180 kPa (26 psi). 

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