A N A L Y T I C A L EDITION
April, 1944
275
Nomographic Chart for Correcting Weights to Vacuum H E N R Y C. THACHER, JR., C.W.S. Development Laboratory, Massachusetts Institute of Technology, Cambridge, Mass.
+
I
40
A
LTHOUGH the correction of weights in air to in vacuo is a very common operation in many fields of precise work, the use of the tables available in handbooks is rather cumbersome when the correction must be applied frequently. Moreover, in many instances greater precision may be required than is obtained by using tables based on constant air density. The correction factor, K , by which the air weight of an object munt be multiplied to give its weight in vacuo is a function of the density of the object, the density of the weights used, and the density of the air. The density of the air, in turn, is a function of the barometric pressure, the temperature, and the relative humidity. The effects of pressure and temperature upon the density of the air are relatively great, while the influence of humidity is considerably less, diminishing the effective pressure by 0.3783 times the partial pressure of water vapor. The accompanying nomograph facilitates the correction of weighings with brass weights to vacuum. All variables except relative humidity of air have been included within the limits indicated below. A constant relative humidity of 50y0 has been used. Temperature, 10' to 3.5' C. Barometric prpsure, 730 to 790 mm. of mercury Density of object. 0.7 to 4.0 Density of brass weights, 8.4
Inasmuch as the effect of humidity of the air is small, limited variations in relative humidity will have only a slight effect on results. The nomograph therefore offers a more rapid means of computing correction factors and a t the same time permits a higher degree of precision, since it is based on constant air humidity rather than on constant air density. In constructing the chart, it has been assumed that dry air obeys the ideal gas law, and that the effect of humidity upon air density is given by subtracting 0.3783 times the partial pressure of water vapor from the observed barometric pressure. Where d, is the density of the air; d, is the density of the object, and d, is the density of thr weights, the vacuum correction factor, K , is
given by K = dm(dw .-. da)/dw(dm - $ 0 ) . When d,*/dm is neglieble in companson with d,, this reduces to the commonly used equation, K = 1 d.(l/d, l/d,),. which has been used in the construction of this chart. USE OF THE CHART. To use the chart, draw a line from the observed Centigrade temperature on scale t to the observed pressure on line P . From the intersection of this line with line a draw a second line to the density of the object on line d. Then factor K will be found a t this second line on scale K , and the vacuum weight of the object may be obtained by multiplying the observed weight by this factor. For example, suppose that an object of density 0.70 when weighed in air a t 50% relative humidity, 770-mm. barometric pressure, and 20' C., weighs 1OO.OOO grams. Then drawing a line connecting 770 mm. and 20" C., and a second line from 0.70 on the d scale to the intersection of the first line with the a scale, we find K equals 1.00158, and the vacuum weight of the object is 100.158. Or again, when with an object of density 1.5 weighing 1O.oooO grams in air a t 50 per cent relative humidity with barometer reading 760 mm. and temperature 25" C., K is found to be 1.00064, the true weight of the object is 10.0064 grams.
-
The chart may be mounted on a piece of cardboard, and indexes made of loops of thread of suitable length with rubber bands or light springs to give sufficient elasticity. If such loops of thread are placed around the chart, they can be moved a t will, and make it unnecessary to draw lines on the chart.
Joining Plastic Tubing to Glass R I C H A R D KIESELBACH, Bakelite Corporation, Bound Brook, N. J.
B
ECAUSE of the present scarcity of rubber and copper, thermoplastic tubing is becoming increasingly popular for laboratory use. Where chemical inertness is desirable, it is far superior to the materials it replaces. The conventional method of joining plastic tubing to glass involves the use of a short length of rubber tubing. This method is usually satisfactory, where a temporary joint is required. For more permanent connections, or where the use of rubber is objectionable, a neat, strong, vacuum-tight joint can be made in the following manner: Draw out the end of the glass tubing to a gradual taper, cutting it off a t the point where its outside diameter is slightly less than the inside diameter of the plastic tubing. Fire-polish the end and let cool. Then heat the tapered section uniformly in a Bunsen flame for about 2 seconds, and quickly force it into the end of the plastic tubing for a distance of at least 1 cm. (Care must be taken to avoid overheating the glass, since it will then char the plastic.) While it is still hot, press the curled end of the plastic tubing to the glass with the fingers, to give the joint a smoother appearance. Allow the joint to cool thoroughly before putting it into service.
A properly made joint of this kind will be found t o be as leakproof as the tubing itself, and able to withstand a surprising amount of mechanical stress.
