Design and Operational Characteristics of Cartesian Manostats ROGER GILRIONT, The E m i l Greiner Co., New York, N . Y . The purpose of this study was primarily to verify experimentally the theoretical equations devised for operation of Cartesian diver-type manostats. In addition, it was desired to present to the field some of the more important modifications of design for specific applications of pressure and vacuum control. Excellent correlation was shown to exist between theory and experiment, so that the devised equations could now be used with more confidence. Laboratory and industrial applications of Cartesian-type manostats have been increasing considerably in the past few years and the findings of this study should be of great value to workers in the field. The wide popularity of Cartesian manostats is due to the combination of their simplicity, accuracy, and inexpensiveness.
S
I S C E the discussion of the theory and operation of a Cartesian diver type of manostat in previous papers (3, Q), considerably more information has been made available, especially on practical designs and the experimental verification of the theory. DESIGNS
The Cartesian diver was first applied to automatic pressure control in 1936 by 4.H. Holtzinger and J. W. Heyd a t the Pennsyl. vania State College (IO). This was an electrically operated manostat, in which an inverted test tube containing trapped air floated freely in a larger tube partially filled with mercury. The outside tube was connected to the closed system and the rise or fall of the inner tube served t o make or break electrical contacts, controlling a solenoid valve hrough which gas was allowed t o bleed to the vacuum pump when the pressure in the system became too high. Designed for a particular pressure, it was not readily adjusted to operate a t any other pressure; moreover, because the float tended to stick t o the walls of the container, an electromagnetic tapper was used continuously to realize its full sensitivity. Plambeck ( I O ) incorporated the idea of using the buoyant force of the float t o operate a valve directly, as shown in Figure 1, but to obtain a practical sensitivity increased the volume of the float.
To set the device for the desired pressure, the apparatus was disconnected from the vacuum line, the stopcock was opened to allow mercury to flow into the beaker and then closed, and the vacuum was turned on. When the pressure in the system was down to the control point, the stopcock was cautiously opened, and mercury was allowed to rise into the outer tube until the open end of the float was sealed off, causing it to rise and cut off the pump. As pressure in the system rose owing to leakage, the float dropped and opened the valve to the pump, restoring the pressure a t the control point. Alternative means of supporting the rubber pad of the valve are shown in Figure 2.
ro PUMP -8-----,\
(
)--TOP
OF FLOAT
VIEW OF UNDERSIDE OF STOPPER SHOWING POSITION OF BAND
TO PUMP
-&=> STEEL STRIP SU5PE N510N
RUBBER PAD PIVOTS
-TO
SYSTEM
+woFFmr Figure 2.
MERCURY
Figure 1. Plambeck Model
Alternate Valve Seats
In order to facilitate the setting of the desired pressure, Pedlow (IO) suggested a central tube communicating with the inside of the float through a suitable stopcock. He also modified the valve arrangement, so the rubber pad was contained in a cup on top of the float. Bartholomew ( I O ) added a by-pass to facilitate rapid setting of the pressure, and this was essentially the form of the manostat first commercially produced by the Emil Greiner Co., a working draning of which is shown in Figure 3. Further modifications have resulted in the unitized instrument shown in Figure 4: which incorporates a single three-way stopcock to simplify operation (8). Many excellent suggestions ( I O ) have been made for further improvement of the manostat-for example, the use of a threeway stopcock as shown in Figure 5 in combination with a central
157
158
ANALYTICAL CHEMISTRY
tube. I n this manner air a t atmospheric pressure can be bled into the interior of the float independently of the rest of the system, so that the pressure in the system can be regulated a t will by manipulation of one stopcock. Cook ( 2 ) has suggested inserting a cork pad about 0.25 inch thick into the top of the inside of the diver, and adjusting the length of the diver so the cork comes in contact with the central tube before the diver strikes the bottom. The cork acts as a cushion t o prevent breakage if the manostat pressure is suddenly increased, and a partial seal prevents mercury from entering the central tube. However, because this seal is not positive, and eventually mercury does accumulate in the central tube, a small bulb of 5- to 7-ml. capacity a t the bottom (2)will act as a reservoir to catch the small amounts of mercury that may be spilled over.
TO c
f o PUMP
- JJ
ORIFICE
EIIIi Figure 4.
L~oPcocK CM 5 4
3
2 I
0
Figure 3.
Worlc.--g Drawing of Rlanostat
In another method of cushioning, shown in Figure 8, the rubber which serves as the orifice seal also acts as cushion and central tube seal. Although rubber gives an excellent seal with a minimum force, it is not very resistant chemically, and plastics have been substituted-for example, thin sheets of Teflon shown in Figure 7 in the all-glass apparatus for use n-ith chlorine gas. A reverse-acting Cartesian manostat has been devised by Spadaro et al. ( 1 2 ) in which the float motion permits air to bleed into the system, rather than throttle the vacuum supply. A very simple modification has been described by Ritzer et al. (11). A working drawing is shown in Figure 6 and data are given in the experimental section. The reciprocal sensitivity of the manostat can be approximated as a linear function of the absolute pressure (4)-i.e., the percentage error in the pressure control increases linearly Ivith the pressure. Sensitivity can be made independent of pressure by connecting a comparatively large volume with the interior of the float, so that changes in mercury level produce negligible changes in the pressure of the sealed gas ( 5 ) . The glass model has already been used above atmospheric pressures up to about 800 mm. of mercury gage. Here the orifice is vented to the atmosphere and the pressure source is permitted t o bleed gas into the system a t a rate smaller than the capacity of the orifice. The connections and the stopper must be wired in to hold the pressure. For increased pressures a metal unit is re-
Unitized Manostat
quired. An all-purpose metal manostat of high capacity and sensitivity for either vacuum or pressure control has recently been made commercially available ( 7 ) . In this unit, the central tube has been omitted to make the operation more fool proof, although less convenient in setting the pressure. A three-way valve a t the top facilitates use a t eithcr vacuum or pressure, and a micrometer adjustment on the orifice enables precise setting of the pressure. A smaller all-metal model for either vacuum or pressure (6) contains a central tube in combination with a rubber cushioning seal that maintains the vacuum setting even after air is admitted to the system.
ro
Figure 5.
Pedlow Jlociification
V O L U M E 23, NO. 1, J A N U A R Y 1 9 5 1
1
1.59 float chamber, while the orifice tube is connected to the low pressure source. In this method of operation, no excess gas is exhausted, because the float modulates the opening of the manostat orifice to maintain the set pressure differential controlling the flow of gas on both the upstream and downstream sides of the line orifice.
T O PUMP
The use of the Cartesian manostat t o control liquid flow by maintaining a constant gas pressure upon the liquid flowing through an orifice has been reported by Spillane and Goodwin (13).
GLASS BUSHING
-
TO SYSTEM
ORIFICE RUBBER SEAT GUIDE TIPS FLOAT
Figure 7 . \Innostat RIodified for Use with Chlorine Gas Figure 6.
Ritzer RIaiiostat
*
-L\
TO
f
Two other designs of the Cartesian manostat should be of ORIFICE special interest. In the first, the RUBBER BUNG manostat has been modified for CM. DOWNSTREAM use with chlorine gas, as shown in Figure 7. Adjustment is made by controlling the amount of liquid (75% c hl o r i n a t e d d i m e t h y 1pentane) buoying up the float. 0 A4pproximateadjustment is made Figure 9. Schematic Diagram of with most of the sealing liquid in Manostat the reservoir and practically none FLOAT in the float chamber. Then with THEORY GU/DE air above the liquid level in the reservoir, sealing liquid is allowed The relationship between sensitivity t o enter the float chamber until the and pressure in terms of the dimensions orifice is throttled. Fine adjustof the apparatus has already been dement is made by regulating the rived ( 4 ) ; however, this derivation negMERCURY amount of liquid in the float lected the area of the central tube. In a chamber. Arbogast ( I ) , who sugcorrection (S), the relationship including gested this type of manostat, recthe area of the central tube is given, alommends a 50/50 mixture of though no derivation is shown. ReChlorafin 42 and Chlorafin 52 ferring to Figure 9, the derivation of the Figure 8. Cartesian $lanostat as (Hercules Powder Co., WilmingFlow Regulator more rigid relationship is summarized ton, Del.) as the grease to be used below: on the joints and stopcocks. P = pressure in system expressed as height of sealing liquid The second design of the Cartesian manostat, as a flow regulap = pressure inside float in same units tor, is shown in Figure 8. The differential pressure created by the 1V = weight of float (corrected for surface tension) flow through a metering orifice is maintained constant, thereby a = area of orifice opening controlling the rate of flow. P , = pressure in orifice tube d = density of sealing liquid (mercury) In one method of operation, the upstream connection is made V = volume of gas trapped inside float to the float chamber and the downstream to the interior of the A I = area b e b e e n float and chamber float. The magnitude of the differential pressure is approxiA? = area between float and central tube mately adjusted by the amount of mercury in the chamber and A S = ring area of float finer adjustment is made by sliding the orifice tube up or domrn. A. = area of central tube The size of the orifice in the flow line should give approximately A = inside area of chamber = A. A I . + Az Aa the same differential ressure under the conditions of flow. The h, = depth of outside of float immersed in mercury rate is set a little higger than required when entering the manoh2 = depth of inside of float immersed in mercury stat and any excess flow is exhausted through the orifice tube of the manostat, thus maintaining a constant flow on the downConsider two equilibrium positions of the float-one in which stream side of the line. the pressure differential a t the orifice is acting on the float while it In the other method of operation, the upsteam side is conis on the verge of breaking away from the orifice, and the other in nected to the interior of the float and the downstream side to the which the pressure differential a t the orifice is no longer acting on
I:)
1
+
+
ANALYTICAL CHEMISTRY
160 the float while i t is on the verge of making contact with the orifice. The relative motion of the float between these two positions is considered negligible, and the difference in the ressure of the system between these two positions is considere! as an inverse measure of the sensitivity. The symb$s referring to the latter position are marked with a superscript, , and the difference in a quantity between these two positions is designated with a delta, A. Four simultaneous equations are then derived: Pressure equalities,
PRESSURE VR RIR TION RCCORDING
ro
EOURTION (/I/ WITH C,* 0.63
0 0
Bo\-le's law compression of gas in float,
I
1
I00
200
I
I
400
500
I
300
MM
P, PRESSURE,
p J 7 = p"T7"
p V " = pV" pAV --Vo.lp but AT'
i d
ACCORDING TO
Constant volume of mercury,
Figure 11.
I
600 HG-
I
700
1 0 800
Ritzer Manostat
Orifire diameter 1.3 mm. Pressure variation,
- A , A h = -41Ah,
L..
A P m a X 100, %
flow rate, S.T.P., litere/min. at 15'
F.
Combining and simplifying, where Equilibrium of forces on float,
(P + hl)Aad
+ p ( A o + An)d - P ( 4 o f d:,+ -4, - ~
m
,NEEDLE
) d Paad - W = 0
VALVE
CA
Equation 6 is a measure of the sensitivity expressed in terms of the per cent deviation in pressure as a linear function of the pressure. I t is applicable to liquids other than mercury, but i n that case the pressure must be expressed as heights of the liquid. For a reverse-acting orifice (f,")-one which permits gas to bleed in as the float rises-the same equation applies, except that the result i q negative, because P. is greater than Po. The capacity of the manostat is a measure of the gas which passes through the orifice when it is open and ( 4 ) it may tw derived from Fliegnrr's equation (9), which may he mitten as: 1L'
where us
=
=
0.533
C,aP
-__
T
flow of u r , pounds per second
rb = orifice coefficient n
P 7'
= = =
area of orifice, square inrhes upstream pressure, pounds per square inch absolute temperature, ' R.
It is based upon a critical pressure ratio of approximately 0 6 (ratio of upstream to downstream pressure) and is derived froni the follon-ing equation for anv gas:
N
Figure 10.
.irrangement of ipparatiis
Equations 1 to 4 can be solved t)v first eliminating hz from Equations 1 and 2, thrn combining the result with Equa on 4 to elimiriatc A h . Combination of Equations 3 and 4 to I iminate Ahi thrn leaves two equations in 4 p and 4 P , which ma) ,e combined to eliminate Ap. Because 6P is to be relative small, quadratic terms are eliminated with negligible error, ani further simplifications can he made with negligible error, resulting in the folloir.ing equation:
here g K
gravity constant ratio of specific heats of gas (constant pressure to constant volume) .If = molecular weight of gas R = gas constant Converting to c.g.s. units and volumetric flow, for any gas:
where
= =
= gas flow in S.T.P., liters per minute a t 75' F. D = diameter of orifice opening, mm. P o = pressure in system, mm. of mercury I'
Equation 11 is applicable when P. is less than 0.5P0, which is generally true for vacuum operation. In the previous paper ( 4 )
V O L U M E 23, N O . I, J A N U A R Y 1 9 5 1
161
an orifice coefficient of 0.6 was assumed, because this is in general the lowest value that may be obtained; and the equation then reduces t o that given in the previous paper. However, a coefficient of over 0.8 was obtained experimentally; hence larger capacities can be expected than according t o the equation given in the previous paper. Equation 1 1 may also be applied to pressure operation, except that P, must be less than 0.5P". When P. is greater than 0.5Po, the flow may be approximated by 21
stat was placed between the flask and the vacuum pump in the recommended manner and all measurements were made under vacuum. The variation in pressure was taken as the difference between the maximum and minimum values observed during the cycle. .4pparently, the manostat operates by either shutting and opening the orifice in cyclical fashion, or simply by throttling the orifice opening. Because the cyclical action was desired for reading the pressure fluctuations, experimentation was conducted to determine how this operation could be obtained. It was found that by carefully throttling the vacuum line to the pump with a stopcock, so that the rate of evacuation was only slightly larger than the rate of leakage which was adjusted to from one half to three fourths of the capacity, the cyclical action was obtained. This cyclical action is also desirable because it prevents sticking of the float, which may result in anomalous behavior. The capacity was taken as the maximum leakage a t which a set pressure would remain constant. This was observed by adjusting the flow until the pressure just started and continued to rise. Experiments were conducted over a range of pressure between 50 and 760 mm. of mercury. Lower pressures were not used because the quantities to be measured were too small to be accurately measured.
DyPo - Pa)
= 0.12:3c,
fif
For R reverse-acting orifice, Equations 11 and 12 may be modified a$ follows: For P o I(>SP than 0.5P,,
1;or P o greater than 0.5Pa, 0
=
0.123C,.
LP (P.
- PO)
dZ
In Equations 13 and 14, D represents the diameter of a circle having the same idea as the free area of the orifice. inasmuch as part of the orifice opening is obstructed by the pin required to lift the orifice seal. EXPERIMENTAL
The experimental unit was set up as shovm in Figure 10 to measure both the sensitivity and capacity of the Cartesian manostat. Two models were studied-the Ritzer (11)(Figure 6) model i n which the central tube is omitted, and the latest Greiner Inodel (Figure 4) which contains a central tube. In the latter niodel sizes of two different orifices were tried. The setup consisted of a 4-liter flask serving as the system. To the flask an accurately calibrated flowmeter was connected through a needle valve, so that the rate of leakage into the flask from the atmosphere could be controlled and measured. Also connected to the flask was an accurate absolute reading mercury nianometcv. The pressure fluctuations were observed through a (,sthetometer containing a telescope having a 30X magnification. Fluctuation of 0.05 mm. could ensily be observed. The mano-
05
t
1
p
I
PRESSURE VARIATION IICCWDING TO
1' 3 5 2
-4
ACCORDING TO EWATtON ( I I ) WITH C ,. 0.84
0
IO0
200
300
400
P ' , PRESSURE,
500 600 M H HG-
-2
700
800
-
l
0.4
a b
0 I
VI)
-
0.85
40.5
J 0
IO0
200
300 400 P," PRESSURE,
Figure 13.
500
700
600
Greiner Manostat
+
Orifice diameter. 1.0 m m .
w
Pressure variation. P - P a x 100, D, flow rate, S.T.P.. liters'min. at 15' F.
800
Mht HG.-
Greiner Manostat
Orifice diameter, 0.6 mm. Pressure variation,
A- pP _ _ a x
100, %
rate, S.T.P., litersfmin. at 75O F.
The results of the experiments are shown in Figures 11, 12, and 13. Each figure contains tn-o curves, one for per cent deviation in pressure, and the other for capacity in S.T.P., liters per minute at 75' F., both plotted against absolute pressure. The data on the Ritzer model are shon-n ill Figure 11. For the flow measuremerits, the btist straight line was drawn through the experimenhl points and C, was calculated from the slope of the line and Equation 11. -4value of 0.83 was olitained using a 1.3-mm. orifice. For the sensitivity measurements, the theoretical curve for per cent pressure fluctuation was calculated from Equation 6 anti compared with the experimental points. For the Ritzer model, Equation 6 is simplified, because ;lo = 0. The theory appears adequate, in spite of the fact that all the points are a little below the line; the slope of the theoretical line, however, appears to be in very accurate agreement with the data. The data on the Greiner model are shown in Figures 12 and 13 for a 1.0-mni. and 0.6-mm. orifice, respectively. For flow measurements values of 0.84 and 0.85, respectively, were obtained for C,. I t is therefore recommended that for operation of the manostat in the pressure ranges presented and with orifices of the sizes employed, an average value of C, = 0.84 be employed in Equations I
Figure 12.
5
P
PRESSURE VARIATION ACCORDING TO EQUATION (6)
u, flow
lo-
'
ANALYTICAL CHEMISTRY
162 11 t o 14. Excellent agreement in sensitivity measurements was obtained between Equation 6 and experimental data for both orifices. ACKNOWLEDGMENT
The assistance of Jesse P. Nehrulst and Robert E. Bader of the Emil Greiner Co. in performing some of the laboratory experiments, the help of Charles F. Saladino in preparing the figures, and the permission of the Emil Greiner Co. t o publish this information are gratefully acknowledged. LITERATURE CITED
(1) Arbogast, J. F., personal communication, Aug. 26, 1949. (2) Cook, N. C., personal communication, NOT, 8. 1947. (3) Gilmont, R.,ASAL. C H E X . , 20,89 (1948).
Gilmont, R., ISD.ENG.CHEM.,ANAL. ED., 18, 633 (1946). Goodwin, R. D., J . Chem. Education, 24, 511 (1947). Greiner Go., Emil, Xew York, N. Y., Bull. C.M. 97 (1949). Ibid., I.C.M. 96 (1947). Greiner Co., Emil, New York, K.Y., Catalog G 15070 (1950). Perry, "Chemical Engineers' Handbook," 2nd ed., p. 847, S e n York, RIcGraw-Hill Book Co., 1941. Plambeck, L., Jr., Pedlow, G. W.,Jr., and Bartholomew, W ,H., personal communication, April 7, 1947. Ritzer, J., et al., T h e Brinewell, 3, S o . 14, 1 (1946). Spadaro, J. J., et al., IND.EXG.CHEV., A N ~ L ED., . 18, 214 (1946). Spillane, L. J., and Goodwin, R. D., J . Chein. Education, 25, 78 (1946). RECEIVED October 18, 1949. Presented before the Division of Physicai and Inorganic Chemistry a t the 116th Lleetinp of the A\IERTCN CHEIIICAL S o c ~ r :Y.,~.itlontir City, S . .I.
Paper Chromatography of Amino Acids E f e c t of p H of Sample 11.1.
ALTON J. LANDUA, ROBERT FUERST, AND JORGE AWAPARA D . Anderson Hospital for Cancer Research, Uniuersity of Texas, Houston, Tex.
It was found that many amino acid solutions could be made to yield good chromatograms by adjustment of the pH of the sample before placing the sample on paper. This was especially true for acid protein hydrolyzates from which most of the acid had been removed by repeated evaporation. The pH of the sample was found to affect the spread of a spot and also its position on the final chromatogram. Similar results were obtained for individual amino acids and for mixtures. The data may be used to determine what pH of the sample will result in the most compact spots and most advantageous positions on a final chromatogram for a desired separation, or may serve as a basis for further interpretation and theoretical speculation on the mechanisms of these separations.
P
APER chromatography has been widely used for the separation of amino acids ( 3 ) . One of the most important factors affecting the quality of a chromatogram is the pH of the solution t o be analyzed. pH effects were recognized by Consden, Gordon, and Martin ( 4 ) , investigated briefly by Bull, Hahn, and Baptist (2), mentioned by Miettinen and Virtanen ( 5 ) ,and considered in some detail for lysine by Aronoff ( 1 ) . Hydrochloric acid protein hydrolyzates, repeatedly evaporated t o dryness, fail t o give good chromatograms by two-dimensional chromatography, when phenol is used first and 2,4-lutidine second as solvent, unless the pH of the sample is further adjusted before application t o the paper. Adjustment of samples from other sources also results in good one- and two-dimensional chromatograms, using other solvents and combinations of solvents. The present investigation was undertaken to determine the effect of sample pH alone on the final chromatogram, and also whether such data could be used t o advantage in resolutions of amino acids and some related substances.
Serial dilutions of 0.2 N hydrochloric acid and 0.2 N sodium hydroxide from 2 X 10-1 N to 2 X 10-6 N were made up and equal volumes of each dilution and the amino acid solution were mixed. One sample consisted of equal volumes of glass-distilled water and the amino acid solution. The pH's of the resulting solutions were measured with a Beckman pH meter, Model G. Then 0.02 nil. of each sample, equivalent to 2 micrograms of amino nitrogen, was placed on Whatman No. 4 filter paper in such a way that the solvent ascended (6) in the short or slow direction; one sheet was used for each compound in each chromatographic solvent. The range of the distance traveled by solvents for different chromatograms was 26.3 * 3.8 em. in phenol, 26.0 * 3.5 cm. in lutidine, and 27.6 * 3.7 cm. in butanol. The solvent boundary was marked with a paper punch. Chromatograms were dried in an oven a t 100" C. for 15 to 20 minutes, sprayed with 0.057, ninhydrin in water-saturated 1butanol, redried for 15 to 20 minutes at 100" C., and then placed on an incandescent illuminator. Here the top of the spot, the bottom of the spot, and the center of most intense color were marked. These distances from the starting point were then measured and Rr was calculated by:
R/ = EXPERIMENTAL PROCEDURE
Solutions of amino acids and related substances were made up to contain 200 micrograms of primary amino nitrogen per milliliter of glass-distilled water, the same concentration of secondary amino nitrogen for proline and hydroxyproline, and the same concentration of primary a-amino nitrogen for substances also containing other types of nitrogen in their molecules. The chromatographic solvents included phenol, 2,4-lutidine, and 1-butanol. All were C.P. grade and were saturated with glass-distilled water a t the temperature of the experiment, the range of which was 21.2" * 2.3 C. Lutidine and butanol were used without purification, but the phenol was distilled from zinc dust, that portion boiling from 177 to 181' C. being used.
distance to point in question distance of travel of solvent boundary
These values were plotted against the pH of the original applied solution. Several solutions containing from four to eight compounds, each having 2 micrograms of amino nitrogen, were then run by one-dimensional chromatography and by two-dimensional chromatography, using different solvents, combinations of solvents, and various pH's. DISCUSSION
It was decided that a graphical presentation of the results would illustrate the effect of pH of the applied solution more
