Observations and reflections on manometric calibrations with air: Methods for indolent mercurophobes

Abstract Methods approaching the ultimate in simplicity, accuracy, reproducibility, and rapidity have been described for the calibration of manometric vessel and capillary volumes with either known or unknown volumes of air, withdrawn from or added to the test manometer system by means of a precalibrated reference standard manometer system (or other equivalent precalibrated device) connected by tubing to the test manometer. Two fundamental steps of procedure are employed, in accordance with Boyle's law, to obtain measurements applicable in two simultaneous equations for two unique conditions of simplicity, the one at constant pressure, the other at constant volume. In the first step, after initial equilibration, the standardizing volume of air is transferred between standard (∗) and test manometer with releveling to constant initial pressure (Δ P = 0), yielding a determination of v , the capillary volume per specified length, from no more than simple inverse proportionality between the observed capillary fluid length changes ( v = v ∗ L ∗ L ). In the second step the manometers are closed off at their stopcocks and the confined gases brought back to their initial space volumes (Δ V = 0), yielding a determination of the vessel gas space volume v g , from no more than simple inverse proportionality between the observed capillary fluid pressure changes ( v g = v g ∗ h ∗ h ). v ∗ and v g ∗ are previously known from the standard manometer, L ∗ and h ∗ are obtained with it, and L and h with the test manometer. Our most recommended procedures for measuring manometric capillary ( v ) and vessel ( V ) values (Eqs. (1′) and (5) with known volumes of air, see Table I, and Fig 1, and Eqs. (1′ u ) and (5 u ) with unknown volumes) require no (numerical) knowledge of barometric pressure, watervap or pressure, absolute temperature, “dead space” of connecting tubes, solubility of air in water, etc., apart from assurance of maintenance of their constancy during manipulations. These factors and other second- and third-order effects (difference in draining of capillaries, changes in dissolved air with pressure changes, differences of temperature outside and inside the thermostat) that enter into applied manometry, are in principle rendered negligible by the differential methodology of our procedure of calibrating a test manometer with another (precalibrated) manometer. The accuracy and reproducibility of these air calibrations can be adjusted, depending upon the will and the skill of the investigator, either to the maximum attainable in operating manometry (say ± 0.2%), or to an accuracy as good or better than that employed in 95 % of all manometry, namely ±1 %. Methods for the minimization or actual elimination of thermobarometric (TB) changes, an important factor largely neglected in previously described methodology of air calibration, are outlined. The outer manometer arms are not left open to the external atmosphere, but are connected with a large vessel (e.g., a 20-l. carboy) whose pressure can be manipulated (as by a large connected syringe) so that any effects of pressure or temperature variations can be readily restored to precisely zero in the TB manometer, and simultaneously thereby in all other manometers connected in parallel. The uses and advantages of such a carboysyringe device in not only calibration, but even more importantly and generally, in many aspects of operative manometry, are indicated. No manometric laboratory should be without such a device (see Fig 1). The proposed methodology is recommended to all indolent mercurophobes and aerophiles, for whom the gravity of the problem of mercury calibration is herewith reduced by the levity of air, and a little air of levity.