AUGUST 1947
617 LITERATURE CITED
(1) Friedmann, T. E., and Kendall, A. I., J. B i d . C h m . , 82, 47 (1929). (2) McCready, R. M . , Swenson, H. A., and Maclay, W. D., IND. ENG.CHEM.,ANAL.ED., 18, 290 (1946). (3) MacIntire, W. H., and Willis, L. B., J. I d . Eng. Chem., 7, 227 ( 1915).
(4) Reid, J. D., and Weihe, H. D., IND.ENG.CHEM.,ANAL.ED.,10, 271 (1938). ( 5 ) Roberts, E. J., and Ambler, J. A., Ibid., 19, 118 (1947). (6) Wells, P. A., May, 0 . E., and Senseman, C. E., Ibid., 6 , 369 (1934). AQRICULTURAL Chemical Research Division Contribution 198.
Vacuum Pressure Regulator F. W. MELPOLDER The Atlantic Refining Co.,3144 Passyunk Ave., Philadelphia, Pa.
DESCRIPTION OF REGULATOR
PRESSURE regulator was developed to monitor pressures in the range of 1 to 1000 microns of mercury in vacuum systems comprised predominantly of noncondensable gases. I t was desirable that the regulator indicate absolute pi essures rather than relative pressures such as those mcasured by the Pirani or ionization gages ( I ) , since calibrations of these latter gages are different for various gases and frequently the composition of the gas i n vacuum syqtems is unknoir n. This regulator is an adaptation of the \IcI,eod compression gage, to which automatic controls were added to move mercury alternately up and down the gage in cycles and to operate the vacuum confro1 valve For accurate prrqsure measurement the gas under compression in the gage must conform t o the gas lan s and consequently condensable gases should be removed in a liquid nitrogen trap located between the gage and vacuum system The regulator has been in satisfactory operation in the laboratory for the past t n o years as the pressure controller on a short-path assay distillation unit at 1micron prrssure
VACUUM CONTROL
A McLeod gage of conventional design was fitted with sealed-inglass tungsten contacts a t positions A , B , and C. At position D a short length of 30-gage tungsten wire was inserted in the top of capillary tube E and cemented in place with thermoplastic cement. The operating pressure of the gage was deter -’ ined by calculation of the proper dimensions of bulb F , capillary tube E , and length of the wire contact inside the capillary tube at D . Only the tungsten contacts at A and D required careful positioning. The control circuit shown in the schematic wiring diagram operates on 115 volts 60 cycles. The vacuum control solenoid valve, single-pole SI, having a 0.5-inch pdrt opening, is the controlling valve between the pumps and the vacuum system manostat. KOspecial requirements are necessary for providing a tight seat in this valve, since gas flow through small orifices a t low pressures is relatively slo~v. Two other solenoid valves, Sz and Sd,control the air flow in and out of of the McLeod mercury reservoir. -411 three solenoid valves, SI, Sn,and S3,are opened xvhen energii.ed. Attached to S?and S I are needle valves which restrict the flow of air, thus allowing a smooth rise and fall of mercury in the gage. -4separate vacuum pump war used to pull mercury down in the gage through SI. The volume of the ballast bottle was determined by the total volume of the vacuum system, diameter of port opening in t h e vacuum control solenoid valve, 81, and speed of the pumps. The ballast volume must be large enough to prevent surging or rapid change in pressure. In this case a ballast volume of 5 liters was satisfactory. OPERATION OF REGULATOR
3 SWl
115v 60-
VACUUM
REGULATOR UNIT
R I ,R2. United-Cinephone electronic switches having t h e following numbered post
connections : 1, 3. Power leads 4. Normally closed switch position 5. Center pole o f single-pole double-throw switch 6. hormally open switch position 7, 8. Trip connections which moves switch arm from position 4 to 6 when closed R3. Struthers-Dunn midget latch-in relay, normally spring-closed (as shown) h u t opened w h e n energized and latched i n t h e open position R1. kdvance midget power relay, one pole normally spring-open, one pole normally spring-closed. Relay ehown i n spring-held position SWi. Toggle switch double-pole single-throw SWz. Toggle switch double-pole double-throw SW2. Toggle switch single-pole single-throw S W I . Toggle switch single-pole double-throw, center off posi tion
The solenoid valves, SI,Sz, and Sa,are controlled manually with sw;tches S y s and SW4 upon starting the distillation until the pressure in the manostat is reduced below 10 mm. of mercury. As soon as the pressure is sufficiently lowered SWz is placed in the “automatic” position. Mercury in t h e McLeod gage is then automatically alternately raised and lowered between contacts A and B , showing the progressive decrease of pressure in the system. When the pressure reaches the desired point, mercury in bulb F rises in capillary tube E and touches contact D , thereby actuating relays R, and RBand closing Si. This valve remains closed until a cycle occurs in which mercury fails to touch contact D, owing t o increase in pressure of the system. Relays Rz and Ra a t this instant transmit a momentary impulse to the “trip” coil of RB,causing the contacts t o close which open S1. The “trip” coil in Ra is permitted to actuate the relay only when the mercury fails to touch D. Although the regulator monitors the pressure only once a minute, no significant change in pressure resulted during this interval. The maximum pressure fluctuation a t 1-micron pressure was found to be 0.1 micron. LITERATURE CITED
(1) Pirani, M., and Neumann, R., ElectToiiic Eng., 17, 277 (1944); 17, 322, 367, 422
(1946).
