NOVEMBER 15, 1938
ANALYTICAL EDITIOX
simple forms to be explained, is found. This difficulty is encountered only in the research that establishes the constant.
Illustrations Figure 1 is a n illustration of etch figures formed on a single crystal of silver dichromate by refluxing solvent flowing over a crystal plate lodged above the “water line.” Parallelism of corresponding sides of identical angles on different forms is evident. Figure 2 shows crystals of silver dichromate with the sides of identical angles parallel, as far as possible. Dark colors are crystals; light colors denote etch figures selected from Figure 1. On the octagons and heptagon the angle designated as A is a t the top to the right and the others follow in
665
counterclockwise order. These figures are the principal ones from which angular data were taken. Figure 3 is an octagon of cinchophen hydrochloride hydrate. The last values of Table I constitute the constant to be recorded for t’he asymmetric octagon of silver dichromate. .A quadrilateral of silver dichromate had the following angles: 88.9”, 91.7’, 88.2’, and 91.4’; sum, 360.2’.
Literature Cited (1) Reichert, E. T., and Brown, A. P., Pub. 116, p. 145, Carnegie Institution of Washington (1909). (2) Shead, -4.C., IND.EKQ.CHEM., Anal. Ed., 9, 496 (1937). RECEIVED Sovember 1, 1937. Presented before the Microchemical Section at the 9 h h Meeting of the American Chemical Society, Dallas, Texas, April 18 to 22, 1938. This is the second paper in a series; for the first, see reference ( 2 ) .
An Accurate Micromanometer C. C. FIXDING
AND
T
F. H. RHODES, Cornell University, Ithaca, X. Y.
HE differential micromanometer described below was
designed and built to measure small pressure differentials with an accuracy at least equal to, if not greater than, the usual two-liquid manometers. Pressures larger than 2.5 em. (1 inch) of mater cannot be measured with this instrument, but this range iq large enough for many of the differential pressures encountered in the flow of gases. It has been used satisfactorily for Pitot tube and pressure drop measurements in air ducts. With the ordinary two-liquid micrornanometers, the difficulty of locating the meniscus and parallax introduces unknown errors t h a t may be as large as several hundredths of a millimeter. Since the level of the water in this manometer is measured by the completion of an electric circuit, and not by the visual location of a meniscus, these errors are not present. The absolute accuracy of the instrument depends chiefly on the accuracy of a micrometer screw which can easily be read t o *0.003 mni. Because a slight amount of vibration is usually present , unless extreme precautions are observed in mounting the instrument, this accuracy cannot be attained in practice. This manometer has been in use for approximately one year, operated chiefly by students nho had not even seen the instrument until just before taking readings. With this type of inexperienced operator, duplicate readings of a constant pressure usually do not deviate more than *0.01 nim.
from the average. In addition, a constant error due to the design of the instrument is present. One end of the E-tube which forms the manometer is fixed in a pivot, TT-hilethe other is free to rotate in a vertical plane. Thus the opposite end, where the measurements of levels are made, moves in an arc with the pivot a t the center. The platinum nire which makes contact with the surface of the liquid in the manometer (Figure 1) remains at a fixed distance from the pivot, but the point of contact of the end of the micrometer screw and the plate that rests on it is free to move outward froin the pivot as the angle of t h e manometer arm x i t h the horizontal is increased. This outward movement must be permitted, as the arm carrying the contact plate i s moving in a circle tT-hile the micrometer screw moves in a vertical line. A simple analysis of this construction reveals that the distance measured by the screw is not exactly the same as the vertical distance traveled by the end of the platinum \Tire in any deviation from the zero, or horizontal position. The difference between these two distances is the constant error of the instrument and is dependent on the angle of deviation from the zero position and the distance between the pirot and the point of the electrode in the opposite arm. The following derived relationship permits calculation of the magnitude of this deviation: Ad = l(tan
21”
-MICROMETER
U
FIGVRE1. DIAGRAM
OF
~IANOMETER CONSTRVCTION
CY
- sin
a)
nhere I d is the error, 1 is the distance from the pivot to the platinum wire, and a is the angle through which the instrument must be rotated in order to measure the difference in level caused by tn o different pressures. Although the error is directly dependent on the length of the manometer, I , the difference between the tangent and the sine decreases much more rapidly than the increase in 1. When 1 is infinite this dif-
666
ISDUSTRIAL -4ND ENGISEERIKG CHEMISTRY
VOL. 10, NO. 11
7 5 WATT LAMP
4 - VOLT - c
TO COhTACTS
FIGCRE2 . THYRATRON RELAYCIRCCIT ference is, of course, zero. Table I s h o w the magnitude of the error when I is 25 and 52.5 cin. (10 and 21 inches), The 52.5-cm. (21-inch) length was used in the two manometers t h a t have been constructed. If t h e accuracy with which readings are to be made is limited t o 10.01 mm., the length, 1, must be 90 cm. (36 inches) or more. On the other hand, shorter lengths can be used, provided a proper correction is made. I n the inst,runients t h a t have been used in this laboratory, a length of approximately 52.5 cm. (21 inches) has been used and corrections have been applied for pressures between 2 and 2.5 cin. (0.8 and 1.0 inch) of water.
Construction A length of 13-mm. glass tubing is bent to form a flat U-tube 52.5 cm. (21 inches) across and 10 cm. (4 inches) high. Each arm of the U-tube is provided with a short length of 7-mm. tubing for rubber tubing connections. The glass tubing is held in a frame mounted on a flat’ plate, which is, in turn, supported by three leveling screws. One arm of the U-tube is held in a swivel joint by means of a clamp around the tubing. The other arm carries a small flat plate clamped to the upright section. In operation this plate rests on the top of the micrometer screw. About 15 cm. (6 inches) from this end of the instrument, two vertical guides are provided to prevent horizontal movement. The position-i. e., the height-of the movable arm is controlled and indicated by a micrometer screw graduated in millimeters to 0.01 mm. A platinum wire is sealed into the cross arm; a second platinum wire is sealed through the center of the top of the movable arm and extends downward about 5 cm. (2 inches). The end of this platinum \Tire is sharpened to a very fine point,, so that a point contact can be made with the center of the meniscus. These two electrodes are connected to a sensitive Thyratron relay which operates a small incandescent bulb and is so sensitive that the conductivity of tap water, or distilled water containing a few parts per million of salt, is great enough to operate it. Thus it is possible t o use water with a density of one and obtain readings directly in millimeters of water. The construction of the manometer is shown in detail in Figure 1. The two platinum contact electrodes are connected to the contacts of a Thyratron relay circuit shown in Figure 2. This circuit is similar to the ordinary Thyratron relay except that a low-power tube is used to activate an indicating lamp. The relay circuit is included for the convenience of those who do not have it readily available. Operation I n operation, a zero point is first determined by noting the reading on the micrometer screw when it is slowly lowered until the pointed platinum wire just touches the surface of the meniscus, causing the lamp on the relay to light. This zero point is found with both ends of the manometer open to the air. The manometer is t,hen connected across the desired pressure differential with the high-pressure side connected t o the movable arm. The micrometer screw is carefully lowered until the light flashes on, and the new reading is taken. The difference betLveen the zero and final readings gives the pressure directly in millimeters of water. All readings must be made in the same way, with the
micrometer screw advancing in the same direction, 60 that contact is always made between the pointed platinum nine and the surface of the Rater. There is a difference of a few hundredths of a millimeter between the “make” and the “break” points. It has been suggested that this difference might be used to mea.ure the surface tenqion of the liquid if a larger platinum wire neie w e d , but no experimental n ork of this nature has been done. TABLE I. ERROR Presqure
Inches of IT ater
~-
Error
7
1 = 10 Inches Inch Jfm.
1 = 21 inches
Inch
.lint.
The manometer is very simple t o use and no eyestrain i, involved in reading it. The operator does not watch either the manometer or the micrometer screw, and merely needs to place the relay in some position in his line of vision. After t h r correct level is located very roughly, the screw is backed off until contact is broken and is again advanced very slonly until contact is just made. In this manner duplicate readings of the same pressure may be obtained in 1 or 2 minutes, depending on the operator’s familiarity with the instrument. The micrometer screw is read as simply as a n ordinary micrometer caliper and can be obtained from any concern manufacturing micrometer calipers. Several other applications of this principle to the measurement of liquid levels are possible. T h e only requirement that must be observed is t h a t the liquid have a moderate conductivity. The sensitivity of the relay can be altered over a wide range by slight changes in the circuit and by the use of other tubes. With the present equipment the relay will operate using methyl alcohol containing 1 drop of hydrochloric acid in 200 cc. hIany other liquids can undoubtedly be used if a very small amount of an electrolyte is added. The change in level of a liquid in an accurately machined cylinder caused by the addition of solids of odd shapes could be determine(1 with extreme accuracy for volume or density determinations of granular solids. In addition, i t should be possible to measure the thickness of fluid films fairly accurately, provided they are not below the range of accuracy of the micrometer screw.
Acknowledgment T h e authors wish t o acknowledge the assistance of W. C. Ballard in the construction of the electronic relay and of H. S. Bush in the design and construction of t h e manometers. RECEIVED August 3 , 1938.
