Automatic Effusiorneter for Determination of Specific Gravity of Gases GEORGE V. FESKOV Standard Oil Company of California, Richmond, Calif.
A n automatic effusiometer has been developed which is considered more accurate than the regular manually operated type. It eliminates personal error in handling a stop watch, and gives results which are very reproducible and accurate.
T
HE specific gravity of gases is usually expressed as the
If the specific gravity of air is taken as unity, then
ratio of the weight of a given volume of the gas to the weight of a n equal volume of air, both measured under the same conditions of temperature and pressure. However, the method of direct weighing is not well adapted to routine testing as practiced in the various branches of the petroleum industry; so when a large number of determinations is required , comparison methods are almost universally employed. The comparison methods fall into two classes: (1) effusion methods, and ( 2 ) indirect weighing methods. since the author has had extensive use for the effusion method, work has been done to improve its accuracy, particularly with respect to timing, The effusion method is based upon the fact that the times required for the flow of equal volumes of two gases through the orifice are approximately proportional to the square root of their specific gravities. The specific gravity thus can be calculated from the following equation: Sp. gr. of gas Sp. gr. of air
Sp. gr. of gas
t 2 gas t2 air
where t is the time in seconds required for the flow of a given volume of the gas Or air* Edwards (2) has made a n extensive investigation of the accuracy of the effusion method and found that although no results of high accuracy can be expected from apparatus of the effusion type, yet it should serve well for approximate results, or for control work where relative values only are needed. He also pointed out that by the observance of certain precautions in the COnStrUCtiOn and use Of the apparatus, it is possible to secure results accurate to about 2 per cent. Greater accuracy and reliability can be obtained by standardizing the apparatus. The method, because of the relative simplicity of the apparatus originally designed by Bunsen ( 1 ) and later improved by Schilling (S), is widely used.
- t 2 gas t2
=
Regular Effusiometer The apparatus consists of a glass cylinder 3.5 inches in diameter by 12 inches dee with a metal lop plate. An armored glass tube extends throu the metal cover from the top and almost to the bottom of t f e cylinder, which is filled with water. Attached t o the glass tube is a three-way brass stopcock which contains a glass tip. The tip itself contains a small platinum disk with a standardized orifice.
air
R
This apparatus finds wide application in the routine testing of the specific gravity of gases, particularly in refineries and plants in locations which are too small for practical use of larger assemblies of apparatus. It is generally recognized that when water is used as a confining fluid, some of the difficulties encountered in obtaining accurate results may be attributed to the condensation of moisture on the edges of the orifice. The solubility of the gases in water also has some effect. Mercury, on the other hand, does not possess these disadvantages, since its vapor pressure is negligible, and as far as solubility is concerned, the only trouble encountered is due to hydrogen sulfide However, even with mercury as a confining fluid, it is still difficult to obtain reproducible results, because of the error in timing. A considerable error in this regard may be introduced if the eye is not kept on the same level with the graduation a t the time of starting or stopping the stop watch. I n order to eliminate a n error from this source, an effusiometer containing a n automatic timing system has been built and is herewith described.
Automatic Effusiometer The apparatus, as constructed, is essentially a modification of the conventional design using mercury as a displacing medium. The general arrangement of the apparatus is shown in Figure 1.
FIGURE1 653
INDUSTRIAL AND ENGINEERING CHEMISTRY
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VOL. 11, NO. 12
the gas chamber but inside a mercury reservoir which is open to the air. If the electrical contacts were placed within the gas chamber there would be a potential danger of explosion with some gases under test. Explosive mixtures can be formed in case of incomplete flushing, or the sample itself may be ignited by an electric spark produced at the moment contact is made. By placing the contacts in a mercury reservoir, the apparatus is safe to use even when the gas being tested is within the limits of explosibility. The obvious advantage of the automatic timing system is the fact that it eliminates error in timing due to the human factor. and at the same t i g e makes the determinations less tedious to perform.
A
ORIF,CE
110 v
FIGURE 2. TIMING CIRCUIT
The gas chamber surrounded with a water jacket is sealed to a two-way stopcock. One way leads to the orifice and another serves to receive a gas sample from the sample tube. A drying tube containing calcium chloride is used to remove moisture from the gas sample, and is located at the back of the equipment, and so is invisible in Figure 1. The mercury reservoir is made of either two pear-shaped bulbs or a straight cylindrical form. When a sample is introduced into the gas chamber it displaces mercury from the latter into the mercury reservoir, which has three electrical contacts sealed through its wall. An electric stop clock is connected to the contacts as described below; as long as the mercury covers all three contacts the stop clock doe8 not run. The stopcock is opened to the orifice, the gas begins to escape, and the column of mercury in the reservoir begins to descend. At the instant the mercury passes the upper contact, the stop clock is started and continues to run until the middle contact is passed. This stops the clock. Reading of the clock then gives time of efflux in hundredths of a second if desired.
Results As a part of this investigation a series of tests was made on several samples of gas in addition to air which served as a standard. The time of efflux was found to be easily reproducible. This reproducibility in timing allows for good checking even when determining specific gravity to the third significant figure. Results obtained are as follows, air being used as standard:
Timing System
Each, determination consisted of several runs on the test gas, preceded and followed by several runs on air. T h e number of runs in each determination was usually from three to five. Results by the balloon method represent single determinations. It is not to be expected that the balloon method values will check the effusiometer ones, because of the different principles involved in the two tests. The time of efflux is reproducible within 0.2 second and usually within 0.1 second.
I n designing the automatic timing system it was desirable to avoid use of a relay. It was evident that this would increase the cost of the apparatus and a t the same time would r e q i r e more servicing and replacement. The problem was solved by utilizing the principle of the series-parallel circuit. The circuit as constructed consists of two parallel resistances Rz and Rs, in series with a line resistance, R,, as shown in Figure 2. I n the parallel circuit of two resistances Rz and R3, the currents are inversely as the resistances:
Gas
Effusiometer
Balloon Method
Deviation
Commercial H, Manufaotured Ha Hn CHI CHI CzHs Nz CHI
0.0850 0.1290 0.3467 0.678 0.842
0.0824 0.1243 0.3444 0.660 0.843
+3.2 +3.8 $0.7 f2.7 -0.1
%
+ ++
Examples of Actual Runs 1. 55.85 55.88 55.88 seconds
2. 3. 4.
The current in each branch may be represented as follows: I2 =
E
zz
46.94: 46.84: 46.86, 46.89, 46.91 seconds 56.09, 56.09, 56.12, 56.19, 56.02, 56.12 seconds 19.76, 19.76, 19.80, 19.73, 19.70 seeonds
Table I shows how a variation of 0.1 to 0.2 second affects value of the specific gravity of different gases (calculated). TABLE I. EFFECTOF VARIATION IN TIME Time of Efflux for Air
Ra is resistance of the stop clock and has therefore fixed value. With Rz less than R3more current flows through Rz. By selecting a proper value for Rz it is possible to reduce I8 to such an extent that the stop clock will not run when all contacts, A , B, and C, are submerged in mercury. Then, as the contact, made through the mercury, is broken at A , all the current becomes available for Ra-i. e., to start the clock. Breaking of the circuit at B then stops the clock. Wire-wound resistors are used, for they are completely coated with a vitreous porcelain enamel which protects the windings against moisture and mechanical injury. The stop clock used is obtained from the Standard Electric Time Company, Springfield, Mass. It has a magnetic starter.
I
15 Sec.
See. 54.5 54.6 54.7
Time of Efflux for Gas 45 Seo. 60 Sec.
30 Sec.
75 Sec.
Specific Gravity 0.0758 0.0755 0.0763
0.303 0.302 0.301
0.682 0.679 0.677
1.211 1.207 1.203
1.893 1.886 1.881
This automatic effusiometer is recommended for routine determinations of the specific gravity of gases, except those containing hydrogen sulfide.
~
The timing system is different from one described by Edwards (2) in two respects: (I) Instead of a chronograph, an electrical stop clock is used, thus eliminating use of graph paper; and (2) the platinum contacts are not situated within
Literature Cited (1) Bunsen, R. W., “Gasometrische Methoden”, p. 128, F. Vieweg und Sohn, 1857 (English translation by Henry E. Roscoe, London, Walton & Maberly, 1857). (2) Edwards, J. D., Bur. Standards, Tech. Papers 89 and 94 (1917). (3) Schilling, E., “Handbuch der Steinkohlengas-Beleuohtung”,3rd ed., p. 100, Miinchen, R. Oldenbourg, 1892.
