A Simplified Precision Oil Manometer T. C. CHADWICK
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AXD
S. PALKIN, Bureau of Chemistry and Soils, U. S. Department of Agriculture, Washington, D. C.
S A PREVIOUS paper (1) the advantages of oil manome-
ters for precision measurement of pressure, particularly in the range of 40 mm. or less, were pointed out. An oil gage was there described which was, in effect, a double U-tube type manometer, in which both oil and mercury are employed, the mercury serving, however, as a mobile “backing” medium for the oil column and not for pressure measurement. The oil manometer described in the present paper is much simpler in construction, and is, in effect, a single U-tube requiring no second backing liquid such as mercury to render possible movement of the oil column from the closed end. ii grease-sealed, well-ground stopcock constitutes the closure mechanism, comparable to the closed end of an ordinary Utube. The dislodging of the oil is accomplished by a slight rotating movement of the plug of this stopcock. When
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u
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opened, the stopcock permit,s communication with the pump ing system. 9 large expansion chamber just below this stopcock, as in the case of the earlier mercury-oil gage ( I ) , serves to reduce to a negligible value the effect of any residual trace of gas in the closed end. The lower chamber serves for degassing and “conditioning” of the manoinetric liquid, as explained below.
Preparation of Manometer for Use A quantity of purified oil, ester, or other nonvolatile liquid, sufficient to fill reservoirs f and g and still leave several cubic centimeters of liquid in bulb h, is introduced through u before the fine capillary and rubber tube, b, is attached. Tube b is slipped on u, stopcock P is locked, stopcocks d and e are opened, and a good vacuum pump is connected at c and run continuously during the conditioning of the manometer. After rapid degassing has ceased at room temperature, the liquid in reservoir h is gently heated until the volatile material has been driven off. In order to remove any volatile material which may have collected in k and f, the hot oil is manipulated a couple of times between reservoirs h and f. After the oil has cooled somewhat, it is forced from reservoir h to a level slightly above stopcock d by closing stopcock e, and allowing air to leak very slowly into reservoir h through stopcock P. When the liquid passes through stopcock d, the plug is crossed in d and then in P. If the manometer has been thoroughly conditioned, the leyel of the oil will not fall when stopcock e is opened even at low pressure. A rotation of the plug of stopcock d through a few degrees will be necessary to dislodge the oil. The pressure is read by measuring the difference in levels of the oil in k and M . Tube k and auxiliary tube M have the same inside diameter, so that errors due to capillarity will be eliminated. Tube M is constricted at r to temper undue or sudden movement of the oil through the tube. At the end of each day of use, and while the manometer is still evacuated, stopcock e should be closed and air should be allowed to leak slowly into bulb h, forcing the level of the oil through reservoir f. Stopcock d should then be carefully opened, permitting a small amount of additional oil to pass through, which will carry with it any trace of gas that may have collected. Xone of the liquid already above the stopcock should be allowed to run into reservoir f , except when the oil is to be reheated for reconditioning. To preserve high accuracy of the gage, it should be reconditioned when the oil can be dislodged without rotating the plug of stopcock d. When not in use, the gage should be left with stopcocks d , e , and P crossed. I n Table I are shown comparative readings made on five manometers: two separate simplified precision oil manome-
TABLEI. COMPARATIVE RE.4DINGS (26” C.)
65 CM. 9
Simplified Oil Gages Light mineral oil
A ~ i e one-A z
I-;--
D M.
M.
Mm.
Mm.
OaZ
Hgb
11 14 33 52 69 88 98 111 128 142 162 202 257 350 470 598 39
0.71 0 90 2.12 3.34 4.42 5.64 6.28 7.11 8 2 9.1 10.4 12.9 16.5 22.4 30.1 38.3 2.50
Mm. 011 11 14 34 53 70 90 100 113 130 145 165 205 260 356 478 608 40
Mm.
Hgb
0.70 0.89 2.15 3.35 4.43 5.70 6.33 7.15 8.2 9.2 10.4 13.0 16.5 22.5 30.2 38.4 2.53
AT R O O M
Previous Mercury-Oil Gage‘c ADiezone-B Mm. Mm. 071 11 14.5 33 52 69 88 98 111 128 142 162 202 256 351 470 598 39
Hob 0.71 0.93 2.12 3.34 4.42 5.64 6.28 7.11 8.2 9.1 10 4 12.9 16.4 22.5 30.1 38.3 2.50
TEMPERATURE ZimUmerli hlanomeGaee ter Mm. Mm Ho Hg 0.7 0.9 2.1 3.4 4.4 5.7 6.3 7.1 8.2 9.1 10.4 13.0 16.5 22.5 30.2 38.4 2.5
0.7 1.0 2.0 3.2 4.6 5.7 6.0 7.4 8.5 9.3 10.3 13.2 16.1 22.7 30.4 38.5 2.3
0 The mercury-oil gage was provided with an auxiliary tube similar to that used in the oil gages. b Calculated.
r
399
400
INDUSTRIAL AND ENGINEERING CHEMISTRY
ters, one containing Apiezone-A,' and the second light mineral oil (white paraffin oil, U. S. P., Eimer and Amend Co., New York, N. Y.); the mercury-oil manometer previously described ( I ) , using Apiezone-B;l a Zimmerli vacuum gage (3); and a webmade, simple U-type mercury manometer. I n order to ensure constant pressure during each set of observations, a pressure-control unit (2) was used in the system with the communicating tube placed a t a point equidistant from all the gages. Readings on the Zimmerli gage had to be made with considerable care, and were time-consuming because of the need to ensure accurate adjustment of the levels for each observation. However, with the aid of automatic pressure control, constant pressure was maintained a t each point of observation, and with some practice it was possible to obtain readings with an error not exceeding 0.1 mm. of mercury, as shown by comparative readings. 1
.&piezone-A and Apiezone-B (J. Biddie and Co., Philadelphia, Pa.).
VOL. 10, NO. 7
Readings of the simple mercury C-manometer were accurate to about 0.3 mm. Calculated values in Table I were obtained by dividing the oil readings by the ratio of density of mercury a t the room temperature to that of oil a t the same temperature. TABLE11. RATIOOF DENSITY OF MERCURY TO OILS Oil Used hpiezone-B Apiezone-.I Mineral oil
Equivalence of 1 mm. H g in Terms of Oil .It 20' C. A t 25' C. At 30' C. I t 35" C. 15.54 15.58 15.63 16.67 15.55 15.73
15.59 15.78
15.64 15.83
l5,68 15.87
I n Table I1 are given data on the equivalence of mercury to oil readings a t the temperatures indicated.
Literature Cited (1) Palkin, S., ISD. EXG. CHEM., Anal. Ed.,7,434 (1935). (2) Ibid., 7,436 (1935). (3) Scientific Glass A p p a r a t u s Co., Item 19,115,p. 108,1936 catalog. RECEIVED March 2 3 , 1938.
Improved Trap for JMoisture Determination by Distillation J
EARLE E. LANGELAND AND RICHARD W. PRATT Union Paste Company, Medford, Xlass.
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N D E T E R h I I N I S G moisture in materials by distillation with an immiscible liquid, certain advantages are to be gained by the use of a liquid heavier than water. In the case, for example, of tetrachloroethylene (1) the high specific gravity of the solvent permits most of the materials to be dried to float a t the top of this liquid, preventing localized overheating and charring which frequently take place when lighter distilling liquids are used, and (2) there is complete freedom from fire hazard. Because the water which is to be measured floats on top of the distilling medium in the trap, when liquids heavier than water are used for this purpose, i t is not possible to effect the necessary return of solvent to the boiling flask by means of overflow, as in the Dean-Stark trap, and a different principle must be employed. il trap to effect this result has been described by Bailey ( 1 ) in which a tube connects the boiling flask with a stopcock sealed into the bottom of the graduated trap. I n operation, when steady conditions have been reached, the stopcock is opened just enough to permit the distilling medium to return to the boiling flask a t the same rate it is received into the trap from the condenser. I n using the Bailey trap one primary difficulty confronted the authors. Because of unsteady and widely fluctuating thermal environment, which was not susceptible of easy control, constant manipulation of the stopcock was required during a determination to prerent complete drainage of the trap or the overflow of the water a t the top. This difficulty led them to produce the trap pictured in Figure 1, in which all trouble from unsteady thermal conditions was eliminated. Once the water has been received in the trap it is impossible for it to return to the boiling flask under any reasonable conditions of heating. When the apparatus has been set up and the heat applied to the flask, no further attention is required until the completion of the distillation.
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15MM OD
FIGURE1. DIAGRAM OF RECEIVING TRAP
