Manometric Manostat G. BRYAYT BACHMAN, The Ohio State University, Columbus, Ohio
1HE apparatus here described has been used success-
that the difference in height between the two mercury levels subtracted from the barometric pressure gives the pressure desired. Introducing more mercury into the manometer increases the degree of vacuum attained. Removing mercury has the reverse effect. The relay is set so that its armature operates against Lz. L1 may then be used as an auxiliary leak. As soon as the desired pressure is attained, S3 is closed to prevent changes in the atmospheric pressure from influencing the controlled pressure. If it is desired to operate a t pressures above atmospheric, the vacuum pump is replaced by an air-pressure pump and the relay is set to operate against L1. In this case Lf may be used as an auxiliary leak. The height of PI above the lower end of the meter stick determines the amount of pressure above atmospheric that may be controlled. Ordinarily this will not need to be more than 30 or 40 mm. if the object in view is to maintain a pressure of 760 mni. However, a t high altitudes or for other reasons it may be desirable to maintain an increased pressure of several hundred millimeters over the prevailing atmospheric pressure. In such cases PI must be fixed in the manometer tube a t a correspondingly higher level or a third contact introduced. As soon as the desired pressure is attained XBis closed.
fully by the writer for controlling pressures between one and 850 mm. It is particularly convenient for maintaining a given pressure either below or slightly above atmospheric pressure during distillations. The electrical circuit used is that of Hershberg and Huntress (1). 'The manometer, which is made of 8-mm. tubing, is provided
with :t 1-mm. stopcock at the bottom and an ordinary stopcock at the top of the open arm. There are also two platinum wire conta'cts, PI and Pt, on the open arm. Contact PI is conveniently locsted about 12 cm. from the bottom bend in the manometer tubing and should be sharpened to a fine point. The meter stick is set in such a position that the 10-cm. mark is exactly level with the tip of contact P I ,for convenience in making readings. Relay A (20100-ohm telephone relay) is fixed t o a carriage which is moved by means of a screw, from leak L1 t o leak Lz. The leaks consist of ordinary needle valves soldered at the open end to 2-cm. long brass rods through which fine holes 0.05 cm. (0.02 inch) have been drilled. Stopcock SI serves to admit or remove mercury from the manometer, stopcock Szt o close off the leaks completely if desired, and stopcock Sat o fix the volume of air in the otherwise open arm of the manometer.
Advantages of Manometric NIanostat
1
Except for the electrical control, which may be much simplified ( 2 , 3 ) ,the apparatus is relatively inexpensive. The glass parts may be easily put together by an amateur glass blower. All pressures from a few millimeters to 100 mm. or more above the prevailing atmospheric pressure may be controlled with an accuracy of hO.2 mm. or less. Changes in the controlled pressure due to changes in the atmospheric pressure are avoided. The settings are rapidly and easily made. A closed-arm manometer which will eventually become inaccurate with frequent operation is avoided. The manometer is easily drained, cleaned, and refilled through SI.
I
Chief Disadvantages Like all constant-volume pressure controls, the manometric manostat is subject to ambient temperature variations. These may be avoided or corrected for by surrounding the open arm of the manometer with a tube filled with water in which is immersed a thermometer according to the manner prescribed for burets used in gas analysis. The apparatus is subject to the disadvantages inherent in all manostats using mercury as the contact liquid (1). In the writer's opinion these disadvantages are not as great as those which arise from the use of other liquids, such as sulfuric acid which rapidly absorbs organic vapors and water and hence must be renewed frequently.
FIGURE1. DIAGRAM OF APPARATUS
Resistance R1 is a 40- t o 60-watt lamp, Rz a 25- t o 40-watt lamp, and Ra a 3- to 5-megohm resistance of 2-watt capacity. Condenser C is of 5 mfd. rating. T is a 71-A vacuum tube. All of the apparatus, exclusive of the manometer, may be easily mounted in a box 25 X 15 X 15 om. (10 X 6 X 6 inches) with the controls placed conveniently on the outside.
Literature Cited (1) Hershberg and Huntress, IND. ENG.CEEM.,Anal. Ed,, 5, 144,
344 (1933).
The operation of the apparatus is simple. If it is desired to maintain a pressure below atmospheric pressure, Sa is opened, the pump is started, and the amount of mercury in the manometer is adjusted by means of the leveling bulb so
(2) Jacobs, Ibid., 7,70 (1935). (3) McConnell, Ibid., 7,4 (1935). RECEIVED January 26, 1935.
201
