A PRECISE AUTOMATIC PRESSURE REGULATOR BY LOUIS E. DAWSON
I n a previous publication1 the author described an automatic pressure regulator especially designed for pressures below that of the atmosphere, Certain necessary modifications were suggested to adapt such a regulator to the regulation of pressures above that of the atmosphere. The regulator here described differs slightly in form but involves the same principle as that previously described. In its present form it may be used for regulating pressures above as well as below atmospheric pressure. Also, improvements have been made in the adjustment of the regulator, which permit it to regulate pressures above as well as below atmospheric pressure to a very high degree of accuracy. N o adjustment of the apparatus is necessary to compensate for differences in atmospheric pressure when the pressure is measured with a manometer which has one end open to the atmosphere and the other connected with the system to be maintained under constant pressure. This holds true only when the effective pressure desired is the difference between the atmospheric pressure and a second pressure mechanically produced, for example, when using the pressure for filtration or for viscosity or plasticity measurements with variable pressure viscosimeters of plastometers. Under other conditions, as for example, when heating or distilling under constant pressure, the atmospheric pyessure chznges must be compensated for by changes in the setting of the regulator from time to time. With the regulators made by the author, pressures above that of the atmosphere, from about IOO cm. to 2 5 0 cm. of water and up to about jo cm. of mercury, were maintained without any fluctuation in the manometer levels. Pressures above and below these limits may be obtained by using longer or shorter regulators or for pressures below atmospheric by reversing the connections. These are being used in the Carbohydrate Laboratory with the Bingham and Green variable pressure viscosimeters and they may be used for other purposes which require little consumption of air where a variation of not more than 0 .I mm.2 of water pressure is desired. A regulator described by J. Rutten3 for use in vacuum distillation is similar in one respect to the author’s regulators. All have two tubes dipping into a reservoir of mercury, one for passage of air with mercury and the other for the return of the mercury to the reservoir. Description of the Regulator Figure I shows the general form of the regulator. It is made of average weight soft glass or Pyrex glass, the latter being preferable. It consists es‘Louis E . Dawson: Ind. Eng. Chem., 16 160 (1924) C. Bingham (“Fluidity and Plasticity”, p. 306 (1922))states that manometers may be read to o I mm., that mercury manometers can be used to as low as I O em. of mercury, and that water manometem can br used to as low as 50 cm. of water with sufficient accuracy for viscosity measurcments. SChem.,Weekblad, 1, 635-8 (1903-4); Proc. Akad. Wet. Bmsterdam, 6, 665-8 (1904). 2E.
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sentially of some form of trap or baffle (aj) connected by means of two vertical tubes of different diameters and lengths ( b and e) with a reservoir of mercury ( d ) . The regulator is connected to a tank, in which the pressure is to be maintained constant through tube ( f o r g), depending on whether the pressure is above or below atmospheric pressure. 1's action depends upon the passage of a controlled quantity of air through the tube of smaller diameter ( b ) . When enough mercury in the reservoir t o uncover the lower end of the smaller tube has been forced through both tubes into the trap, air begins to pass through with the mercury. The passage of this air alters the pressure in the tank. The quantity of air passing is then checked by a rise of the mercury in the reservoir ( d ) due to its return through the larger tube (c) under the changed pressure, thus more or less closing the end of the tube and controlling the quantity of air passing through the tube of smaller diameter. The trap ( a j ) should be so constructed that the mercury which is forced up with the air is rapidly returned to the reservoir. Tube (a) is made from glass tubing about 2 - I / Z cm. in diameter, and is about 8 cm. long. The tube ( j ) is connected into the side of ( a ) in such a way that the mercury strikes the top of (a)and has little chance of getting into the tube ( j ) ; in case it does it is allowed to rapidly return to the bottom of ( a ) . It is about I - I / Z cm. in diameter and about 8 cm. long. The reservoir ( d ) is about 2 - I / Z cm. in diameter and about 6 cm. long. The small tube ( b ) connecting the reservoir and trap extends about 3 cm. into the reservoir. Its lower end is flared open to about I - I / Z to z times its diameter. The diameter of this tube depends upon the source of pressure supplying the tank. With a very small capacity pump the tube probably needs to be less than z mm. internal diameter, but for most sources of pressure an internal diameter of about z or 3 mm. will be very satisfactory. The tube ( e ) , FIG. I through which the mercury returns, is about 8 or I O mm. in diameter. I t may be advantageous, although seldom necessary, to have the tube (9) extend into (j)through a double seal, with holes at ( h ) t o take care of any sudden spurts of air and mercury when changes in adjustments of pressures are carelessly made. The apparatus is mounted on a board (Figure z ) pivoted a t the top. For the purpose of regulating the pressure a t different values, the regulator can be supported at various angles from the vertical by means of a metal strip ( r ) (Figure z ) , with one end fastened to a pivot a t the bottom of the board, and the other end passing through a bracket (s) (Figure z ) with a set screw. For pressures above that of the atmosphere, the tube (f) (Figure I ) is connected with the tank in which the pressure is to be maintained constant. For pressures below that of the atmos-
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phere, the regulator is connected through (9) to the tank. Except with pressure regulators for pressures between that of the atmosphere and z j o cm. of water above, the upper end of the trap a t (9) (Figure I ) is left open to the atmosphere. With pressures above that of the atmosphere between 50 cm. and 2 5 0 cm. of water, there is likely to be a fluctuation of about I mm. of levels in the water manometer. In order that the regulator shall maintain a pressure be-
tween these limits with no fluctuation of the levels in the water manometer, the regulator is conr,ected through the tube (9) (Figures I and 2) to a tube about 8 or I O mm. in diameter with a right angle bend at the bottom. The tube is allowed to dip into a jar of water to a depth of about 35 cm. (u) (Figure 2). The air coming from the regulator bubbles through the water, which stabilizes the slight fluctuations produced by the regulator. Over a period of 24 hours or more there may be a slight drop in pressure in the system to be maiptained under constmt pressure, owing t o evaporation of water from the jar through which the air bubbles. The amount or rate of decrease in pressure will depend upon the total area of the water, or the diameter of
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the vessel at the surface of the water, to be lowered by the evaporation. With a vessel of large diameter the rate of decrease in pressure will be much less than with a vessel of small diameter. Operation and Adjustment The apparatus is about half filled with mercury and when being adjusted for the first time it is placed in the vertical position for the maximum pressure regulation, Pressure is carefully applied and the mercury withdrawn through the stop-cock ( e ) (Figure I ) until the mercury is about 3 cm. above the end of the tube ( b ) in the trap (a)when the air is passing through. When the regulator first starts operating the air may go through in pulses, but it should begin to pass steadily through in less than I minute. For most efficient work, the effect of the surface tension of the mercury a t the flared end of the tube ( b ) must be minimized by allowing an uninterrupted but not excessive stream of air to pass through the small tube ( b ) of the regulator. When thus operating there is no variation in the manometer readings. This also applies in the case of the regulator for vacuum previously described.l Entire System under Pressure with a Source of Variable Pressure Above Atmospheric Pressure. The source of pressure used by the author varies from 20 to I I O pounds per square inch above that of the atmosphere. With the following arrangement these variations are kept from causing fluctuations of pressure in the tank that is to be kept under constant pressure. The system is connected to the source of compressed air through the valve (a)(Figure 3). The air passes through a trap (e) made of a larger piece of pipe and fittings, which is filled with glass wool to hold back dust, oil, and water that might clog the capillary tube ( d ) . The volume of air passing through the system is reduced by the capillary tube (d). This tube is 9 cm. long and has a bore 0.4 mm. jn diameter. It may be by-passed by a valve (z) to allow quick filling of the system. The glass capillary tubes are supported in 1/4inch pipe-nipples, provided at both ends with brass unions, by which they are connected into the pipe line. The capillary may be fastened in the pipe with rubber stoppers, cut short and forced into the openings of the unions, and then the capillary pushed through the perforations in the rubber stoppers. They may also be fastened in the pipe by means of sealing wax, litharge and glycerine, or other cements. When the source of pressure fluctuates over a wide range, a pre-regulator and 5-gallon tank are needed to supply the tank to be maintained under constant pressure with a nearly steady current of air. The pre-regulator used by the author has the tube ( b ) (Figure I), about 45 cm. long by 3 mm. inside diameter, and operates with a total difference in level of mercury of about 49 cm. This pre-regulator may be mounted in the vertical position for all pressures up to about 45 cm. mercury in the main tank ( h ) . 'LOC.
cit.
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The air enters the j-gallon tank ( e ) , and is maintained at approximately constant pressure by means of the pre-regulator (f). From the tank ( e ) the air passes through a capillary tube (g), which reduces the volume of air passing into the tank ( h ) . This capillary is 18 cm. long and has a bore 0.75 mm. in diameter. It is by-passed by the valve (y) t o allow rapid filling of the tank ( h ). The tank ( h ) is a 30-gallon household galvanized-iron range boiler. The pressure in this tank is kept constant by means of the regulator ( j ) mounted on a board. The regulator used by the author has the tube ( b ) (Figure I ) 42 em. long and 3 mm. inside diameter. The board is pivoted at the top and may be supported a t various angles from the vertical as already described
h
e.
e.
00.
p
FIG.3 Dingrammatical arrangement of system for regulation of pressure when the Souice of pressure fluctuates.
(Figure 2 ) . A pipe line is run from the tank ( h ) (Figure 3) to the point ( k ) where the pressure is to be used. A mercury manometer (m) for measuring the pressure is connected to the same line. For pressures above that of the atmosphere, from 50 cm. of water up to about 15 cm. of mercury, a short regulator is used. It has a tube ( b ) (Figure I) about 15 em. long and is connected to the system through the valve (n) (Figure 3). For the pressures which are to be measured on the water manometer, which is connected to the system through the valve (0) (Figure 3), B tube dipping into a jar of water is connected to the short regulator at (9) (Figures I and 2 ) . The air then passes from the short regulator through the tube dipping into the water and bubbles out into the atmosphere through the water. Figure 2 shows how the short and long regulators are mounted and
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arranged so that they may be set for regulation at any pressure within the limits of these regulators. The only adjustments necessary in changing the pressure in the system from one value to another is t o chaslge the degree of inclination of the regulators by shifting the point where ( r ) (Figure 2 ) is clamped a t (s), both regulators being connected with a short strip ( t ) . For pressures beyond the range of the water manometer, the valve (0) (Figure 3) is closed. For pressures beyond the range of the small regulator, the valve (n) is closed. No valve is needed on the mercury manometer (m) or the large regulator ( j ) .
A reducing valve, such as is used for reducing the pressure of compressed gases in cylinders, may be used instead of the first capillary ( d ) , the pre-regulator (f) and the 5-gallon tank ( e ) . This is probably a more expensive arrangement, however. With this reducing valve it is necessary also to change the adjustment of the valve control when changing from one pressure to another in the tank ( h ) . Below Atmospheric Pressures. For pressures below that of the atmosphere the system described in the preceding section may be used after making the following modification: The valve ( a ) (Figure 3) opens to the source of vacuum. The pre-regulator and the regulators connected to tank (h) are connected to the system through the tube (9) (Figure I ) instead of tube (f). Modification of System with Source of Air Pressure Constant When the source of air pressure is practically constant, the control of the volume of air entering the system to be maintained under constant pressure is effected by including capillary tubing of a suitable size and length in the piping between the valve ( a ) (Figure 3) communicating with the source of pressure and the tank ( h ) , instead of the capillaries, pre-regulator, and 5-gallon tank. Device providing for the Accurate Setting of Regulation at a Definite Value When it is necessary to maintain a pressure above atmospheric to within mm. of a desired definite value, adjustment by varying the angle of inclination of the mercury regulator alone is too coarse. The following simple arrangement provides for a fine setting after an approximate adjustment of the mercury regulator has been made. A vertical tube, about I cm. in diameter, of convenient length, and bent at right angles within 3 cm. of the bottom, is connected a t the top of the tube (9) (Figure I ) of the mercury regulator and inserted in a cylinder 5 em. or more in diameter containing a column of water. The air from the mercury regulator passes through this vertical tube and bubbles up through the column of water into the atmosphere. The fine changes in setting a t the pressure desired are made by varying the height of I
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the column of water. The height can be conveniently controlled by providing the cylinder at the bottom with a T-tube, one branch of which is connected to a water supply and the other to a drain.
It may be advisable t o eliminate any danger of slow mercury poisoning by conducting the air coming out of any mercury regulator into a hood or the outdoor atmosphere. Carbohydrate Laboratory, Bureau of Chemistry, U.S . Department oj Agriculture, Washington, D. C.
