Weighings in air to weighings in vacuo


Buoyant Effect of Air. Weighing operations performed m vacuo are not affected by buoyancy forces. An object in air, however, is subject to a buoyancy force that is equal and opposite to the gravitational force on the mass of air the object displaces (10). If the equal arm balance of Figure 1 is in balance with a test weight of mass, in one pan, and material of mass, m, in the other, m = m if they have the same density. If the densities are different, then the buoyancy forces acting on each pan affect the result. Taking moments about the center pivot point gives  [c.331]

Iodine is a dark-coloured solid which has a glittering crystalline appearance. It is easily sublimed to form a bluish vapour in vacuo. but in air, the vapour is brownish-violet. Since it has a small vapour pressure at ordinary temperatures, iodine slowly sublimes if left in an open vessel for the same reason, iodine is best weighed in a stoppered bottle containing some potassium iodide solution, in which the iodine dissolves to form potassium tri-iodide. The vapour of iodine is composed of I2 molecules up to about 1000 K above this temperature, dissociation into iodine atoms becomes appreciable.  [c.320]

Conversion of Weighings in Air to Weighings in Vacuo 2.83  [c.75]

TABLE 2.14 Conversion of Weighings in Air to Weighings in Vacuo  [c.157]

The table which follows gives the values of k (buoyancy reduction factor), which is the correction necessary because of the buoyant effect of the air upon the object weighed the table is computed for air with the density of 0.0012 m is the weight in grams of the object when weighed in air weight of object reduced to in vacuo = m + m/1000.  [c.157]

TABLE 2.14 Conversion of Weighings in Air to Weighings in Vacuo Continued)  [c.158]

Calibrating a balance, however, does not eliminate ah sources of determinate error. Due to the buoyancy of air, an object s weight in air is always lighter than its weight in vacuum. If there is a difference between the density of the object being weighed and the density of the weights used to calibrate the balance, then a correction to the object s weight must be made. An object s true weight in vacuo, Wy, is related to its weight in air, Wa, by the equation  [c.105]

In the premix operation, a uniform slurry of all components, except the oxidizer, is prepared. The premixes may contain cross-linking, wetting, opacifying, and antifoaming agents, plasticizers, metallic fuels, catalysts, and curing compounds. Automated techniques ensure formulation uniformity and reproducibihty. The polymer and other large-volume fluids required are pumped from the storage tanks to weigh tanks and then to the premix vessels. These may be up to 5000 L in capacity and equipped with turbine-driven agitators designed for the specific materials being handled. The secondary Hquid components, including a portion of the curing agent, are weighed, added, and mixed at a controlled temperature after first purging the premix vessel with nitrogen. The necessary soflds other than the oxidizer are screened and added, followed by further mixing under nitrogen, and finally under vacuum to remove entrapped gases. Batch mixers are temperature controlled and designed to deaerate the viscous mass while imparting a shear action to ensure thorough and rapid incorporation of soflds. They include relatively conventional horizontal mixers such as the sigma-blade dough mixer used in making nitrocellulose propellants, mixers with heavy-duty bear claw blades, and ribbon mixers. Vertical change-can planetary mixers are commonly used to meet requirements for increased mix capacity. Mixing times and temperatures are tightly controlled to maximize mix uniformity and minimize viscosity changes without accelerating the cure reactions to the stage where the pot life is excessively reduced. The mix temperature increases as a result of work input on the viscous mass and the exothermicity of the initial cure reaction. The required quantity of the premix is transferred to the mixer bowl, which is moved into position and assembled to the mixer. Mixing is begun after purging with nitrogen. When the process control conditions have been attained, the oxidizer is added followed by the curing agent. Mixing then proceeds under vacuum.  [c.49]

Calibrating a balance, however, does not eliminate all sources of determinate error. Due to the buoyancy of air, an object s weight in air is always lighter than its weight in vacuum. If there is a difference between the density of the object being weighed and the density of the weights used to calibrate the balance, then a correction to the object s weight must be made. An object s true weight in vacuo, Wv, is related to its weight in air, W, by the equation  [c.105]

Discharging the Scale. Where a gate is used to discharge a hopper scale, very Httie control can be exercised over the rate of discharge. In some cases, particularly where the various scales of a simultaneous batching system discharge onto a single conveyor belt, it is desirable to control the discharge rate so as to premix the materials. This can be accompHshed by using a suitable feeding device in place of the gate. Also, using a controllable feeder as the discharge device, it is not necessary to empty the hopper or tank completely since material can be accurately weighed out of the hopper, as well as into it. In the case of materials that cling to the sides of a hopper, use of this device provides a definite advantage. In the case of Hquids, more rapid discharge of a definite amount can be accompHshed because the last material does not have to be discharged fully with diminishing pressure head this method is also very useful in the case of very viscous Hquids. Of course, it cannot be used when different materials are accumulated in a scale hopper because the composition of the material remaining in the hopper would vary.  [c.339]

A round flask (250 c.c.) is fitted with an air-condenser. The phosphorous pentachlorlde is introduced from the bottle and weighed by difference. The operation must be conducted in the fume-cupboard. The benzoic acid is then added, and the air-condenser attached to the flask. The action begins almost immediately, and clouds of hydrochloric fumes are evolved. The whole contents become liquid and consist of benzoyl chloride (b. p. 200 ), phosphorous oxychloiide (b. p. 107°), and unchanged pentachloride. Most of the oxychloride may be removed by distilling in vacuo on the water-bath. The remainder is fractionated at the ordinary pressure and collected at 190-200°. Yield, 20 —25 giams.  [c.208]


See pages that mention the term Weighings in air to weighings in vacuo : [c.442]    [c.181]    [c.343]    [c.1015]    [c.248]   
Langes handbook of chemistry (1999) -- [ c.0 ]