404
INDUSTRIAL AND ENGINEERING CHEMISTRY
yield of nitrogen would have to be rather accurately determined and controlled for each type of protein sample. LITERATURE CITED
(1) Ashton, J. Agr. Sci., 26, 239 (1936). (2) Bradstreet, R. B., IND. ESG. CHEM., ANAL. ED., 12, 657 (1940).
Vcl. 17, No. 6
(3) Gilbertson, L. I., and King. G. B., J. Am. Chem. SOC., 58, 180 (1936). (4) Lauro, M. F., IND.EXG.CHEM.,ASAL. ED.,3, 401 (1931). (5) Osborn, R. A, and Krasnitz, A , J . Assoc. Oficial Agr. Chem., 16, 110 (1933). (6) Sandstedt, R. M., Cereal Chem., 9, 156 (1932). (7) Sreenivasan, A., and Sadasiran, V., ISD. ENG.CHEM.,A N ~ L . ED.,11, 314 (1939).
A n Automatic G a s Circulating Pump School
J. H. SIMONS, T. J. BRICE, A N D W. H. PEARLSON of Chemistry and Physics, The Pennsylvania State College, State College, Pa.
A
i\' .1UTOhIATIC gas circulating pump for use on a vacuum
system has been devised and used with satisfactory results in this laboratory. The design of the pump and the controls is shown in the diagram, The pump consists of two small one-way valves and two 50-cc. bulbs arranged as shown. Gas is drawn in through valve A by lowering the mercury in C, thus reducing the pressure inside the pump below that of the system; the lowering of the mercury is accomplished by applying a vacuum on bulb D. I n this step B acts as a check valve. When the mercury has been lowered sufficiently so that C is empty, the cycle is reversed by letting air into D and allowing the mercury to flow back into C. Gas is forced out through B while A acts as the check valve. The pump used by the authors made a complete cycle once a minute a t 200-mm. pressure. This period could be varied by changes in the dimensions of the various parts of the apparatus. The levels of the mercury in A and B are adjusted by means of the reservoirs below them. The pump can be made to operate a t pressures as loiv as 3 to 4 mm. by suitably adjusting these levels. This is the pressure required to overcome the resistance of the mercury check valves. If other check valves were used this minimum might be reduced. Residual gas in the pump can be let out,into the system by drawing the mercury down into the reservoirs. The pump was designed to operate with a continually varying pressure inside the system. This is accomplished by constructing the right arm of the U-tube of small-bore tubing; pressure variations register on this arm while the maximum height in C' remains practically unchanged. The height between the bottom of this small-bore tubing and the top of C should exceed the mercury equivalent of one atmosphere pressure for safe operation a t pressures down to a few millimeters. The minimum operating pressure in millimeters of mercury is equal to or greater than the height of the bottom of C above the upper contact on 8, plus the resistance of one check valve, and the lowest pressure reached by the operating vacuum pump. The maximum operating pressure is equal to or less than the atmospheric pressure, minus the resistance of one check valve, minus the height of the top of C above the lower contact on D. The operating range of pressures for any set of fixed dimensions or level of bulb D is the difference between the maximum and minimum operating pressures and is equal to atmospheric pressure minus the sum of the distance between the top and bottom of C, the distance between the top and bottom contacts on D, the lowest pressure of the operating pump, and twice the resistance of one check valve. For safety of operation, so that mercury from C does not enter the valve chambers, the minimum pressure encountered in millimeters of mercury plus the height of the top of C above the rest level of the mercury in the small-borc tubing should be equal to or greater than atmospheric pressure. For operating a t pressures down to a few millimeters the upper contact on D must be below the bottom of C by a t least the lowest pressure of the operating pump plus the resistance of one check valve. The maximum operating pressure under these conditions is equal to atmospheric pressure minus the height between the lower contact on D and the top of C. Of course, the lower D is the shorter the period of operation but the lower the maximum pressure. For operating a t pressures above t,his maximum D must be raised relative to C. I n this case the minimum pressure a t which the pump will operate will increase by the amount that the upper contact of D is raised above the highest level that it could have for operating down to a few millimeters. The height of D can be made adjustable by connecting it to the glass system by a flexible rubber tubing.
The operation of the pump is made automatic by an electrical device based on the fact that it requires less force to hold an iron core in a solenoid than to lift it into this position against the force of gravity, particularly as the vacuum below the air leak provides an additional resistance to be overcome in lifting the Folenoid core.
PUPlP
IlOVDC
When the mercury starts rising in the right arm of the C-tube, it completes a circuit through the sealed-in contact a t the bottom of bulb D. The current through the solenoid is sufficient to support the weight of the rod, but not sufficient to lift the rod and overcome the vacuum force, so the air leak remains closed. The mercury continues to rise until it makes contact with the lead a t the top of D. A larger current then flows through the solenoid, and the rod is pulled off the air leak. This contact is immediately broken, since air rushes into D and the mercury starts to fall. The air leak does not close, since the current through the lower contact is sufficient to hold the rod in the solenoid. The mercury continues to fall until the lower contact is broken. A t this point no current flows through the solenoid, the rod falls closing the air leak, and the cycle is repeated. In the or:ginal apparatus an ordinary aspirator supplied the vacuum and an air leak of 7-mm. tubing was used. The stop on the air leak was made of 0.6-cm. (0.25-inch) iron rod and weighed 15 grams. The open end of the glass tube was ground flat. A cork was fitted to the lower end of the plunger rod by means of a centrally bored hole. The rod entered this hole partway through the cork. The hole wm enlarged a t the lower end, and a rubber disk cemented to the bottom face. An air cushion was thus provided above the rubber disk. A direct current solenoid of approximately 400 turns was used, which required a current of 2.3 amperes to overcome both forces on the rod and 1.0 ampere to maintain the weight of the rod. Resistances E and P were approximately 50 and 30 ohms. Alternating current could be used by using a n alternating current solenoid; the design could be further modified by using relays to cut down the current through the mercury.
