Improved McLeod Gage and Manometer HAROLD SIMMONS BOOTH,Western Reserve University, Cleveland, Ohio
I
N SPITE of the numerous improvements made in the McLeod gage since the original devised by McLeod (a), it still suffers from certain defects. I n order to overcome some of these, there has been devised in this laboratory the modification shown in Figures 3 and 4. A better appreciation of this device may be obtained, perhaps, if the original McLeod gage, as shown in Figure 1, be considered first. I n this device the apparatus whose pressure is to be measured is connected to the body of the McLeod gage through the capillary between B and F. When equilibrium has been attained between the McLeod gage and the apparatus, the mercury reservoir, R, is slowly raised, imprisoning above the cut-off, C, the gas of volume VI and compressing it into the calibrated capillary, OT. Simultaneously, the mercury will rise in the capillary BF. This instrument may then be used in either of two ways: First, the mercury may be allowed to rise in capillary OT exactly to 0, and the difference in height, H, between 0 and the top
useful for low pressures, the mercury is adjusted so that the height of the column in the capillary BF (Figures 1 and 2) or OF (Figures 3 and 4) coincides exactly with the height of T, the mercury in the capillary OT being a t some height, L. If we let H represent the difference in height in mm. between L and T,and if v represents the volume of the capillary OT per mm. of height, then the equation becomes
P m H X -Hv
However, in neither of these formulas is the fact recognized that pressure H should be corrected for pressure P being measured. Formula 1 then becomes P = (H
+ P ,) ;
whence P = H
V2
- Va
(3)
HZv whence P = ___ Vi - HV
(4)
~
Vi
and Formula 2 becomes P = (H
4'
(2)
VI
+ P ) Hv T,
I n measuring low pressures this correction may be omitted. However, in the use of the vacuometer to measure pressures of the order of several millimeters, it should be included. The type of vacuometer shown in Figure 1 is usually used for measuring relatively low vacuums, whereas that in Figure 2 is more commonly used for measuring high vacuums. I n Figure 2 the diffusion of the gas between the vacuometer and the apparatus has been greatly facilitated by using a tube of large bore and making the capillary a by-pass. However, this type of apparatus can only be used according to the second method. If one desires, therefore, to measure a considerable range of low presvure, it is necessary to have two vacuometers, one such 8s Figure 1 and the other such as Figure 2. This has been obviated in our design as shown in Figure 3. A tube of large bore, B , connects the vacuometer and the apparatus whose pressure is to be measured. At a distance of about 85 cm. below the constriction, C, the side tube, EOF, is attached. The upper part of the tube is made of capillary tubing of the same piece as was used in the body of the vacuometer. Thus there is no capillary error due to differences in diameter. Above the capillary tube at F there is a short space of 7-mm. tubing surmounted by another capillary tube, which is bent downward and again upward a t the overflow well, W . The distance from the top of the capillary tube to W should be about 760 mm. The total length of the capillary from 0 to F should be about 1 meter. CALIBRATION AND PREPARATION OF GAGE FIGURE2
FIGURE1
of the column of mercury in the capillary side tube, BF, noted. Then it is obvious that, assuming the perfect gas laws, the true pressure, P , in the system connected to the gage is P = H X -vz Vl
(1)
where Vz is the volume of the gas in the capillary OT. I n the other method of operation, which is particularly
The volume v of the capillary tube in cubic millimeters per mm. of length should be determined before attaching the capillary to the body of the tube. The capillary should be tested first for uniformity by placing in a suitable length of the capillary tubing a drop of mercury and measuring the length of this drop in different positions in the capillary. In a capillary of uniform bore the length of the drop of mercury will be,constant. To calibrate it the capillary tube is almost filled with pure mercury. The length of this mercury column is measured, the temperature noted, and the mercury run out and weighed. The capillary tube is then sealed on to the gage body and sealed off a t the top, T,with
380
October 15, 1932
INDUSTRIAL AND ENGIKEERING CHEMISTRY
as blunt an end inside as possible. The gage is then clamped in inverted position and filled with pure mercury up to the constriction, C, the temperature noted, and the mercury removed and weighed. If the temperature a t which both
381
only on hydrogen or nitrogen, or a similar inert noncondensable gas. Oxygen a t these low pressures tends to smut the mercury, probably because of the formation of ozone by the activation of the oxygen by the electrical discharge produced by the moving mercury. This smut deposits as B film in the capillary, obviously introducing errors. From pressures between 17 and 0.1 mm. (gage A in Table I), the procedure is as follows: The reservoir is raised slowly until mercury in the body of the tube has risen to the etched mark, 0, in the capillary, OT. The difference in height, H, between 0 and the column of mercury in OF is noted. Whence
where V Zrepresents the volume of the gas compressed in the capillary above 0. Since there is no pressure above the side capillary, OF, no correction for this is necessary as was the case in the original design shown in Figure 1 (see Equation 3). TABLEI. DIMENSIOXS AND CONSTANTS OF Two TYPICAL NEWT Y P GAQES ~ A 1.75 260 16 90 1.030 16,260 1/56.98 1/14,815
Inner diam. of capillary O T mm. Lengtb ,of capillary OT,mm'. Inner diam. of body of gage mm. Ap rox. length of body of g&e, mm. VoPume u of capillary, cu. mm. per mm. Volume Vi of whole gage, eu. mm. Gage constant Vz/ VI Equation 1) Gage constant v/Bi (Aquation 2)
B 1.07 320 28 150 0.9052 77 720 l/i68.3 1/85,856
OPERATING G A Q E ACCORDINQ TO EQUATION 1
2
If P = 1 mm.. 3 mm., 10 mm., 17 mm.,
56.98 mm. 268 3 mm. 170.94 mm. 804.9 mm. 560.8mm. Beyond range 968.7 mm. Beyond range
3 3
3
--
OPERATINQ GAGE ACCORDING TO EQUATION 1
f
FIGURE4
H H H H
w FIGURE3
the capillary and whole gage are filled are the same, it is unnecessary to calculate the true volumes, and the relative weights may be used instead to get the constant v/Vl for the gage. In operation the vacuometer is first carefully cleaned and thoroughly dried by passing dry air through it for a number of days, or better, by repeated rinsings with dry air followed by evacuations. Then the reservoir, R, is filled with pure mercury and kept at such a low level that when a vacuum is applied to the gage the mercury will slowly rise in the lower tube. When a satisfactory vacuum has been obtained, the reservoir, R, is raised until the mercury overflows into W. The reservoir is then lowered a t once as low as possible, the column of mercury breaks at F , and opportunity is given for adsorbed gases to escape from the walls of OF. After a few minutes, R is again raised, thus driving over any minute amounts of gas which may have escaped from the walls of OF. The instrument should then be allowed to stand with reservoir R as low as possible, and a t daily intervals R should be raised to drive over any gas while the space above W is evacuated to expand the bubble and prevent its lying down in the capillary. When no evidence is seen of a bubble of gas being carried over into W , it may be assumed that the space in OF is completely evacuated, and the gage is ready for use. This instrument may be used over a wide range of pressures. One made in this laboratory is sensitive over the range from 760 to 0.00001 mm., although according to Gaede (1) accurate measurements below 0.0001 mm. can be made
If P = 1.00 mm., H 0 01 mm H 0:0001 m;n H O.OOOOI m;., H
122.0 mm. 293.0 mm. 12.2 mm. 29.3 mm. 1.22 mm. 2.93mm 0.39 mm. 0.83mm.
3
P
If low pressures are to be measured, the mercury is allowed to rise in OF to the same level, T,as the top of the inside of the capillary on the body of the gage. The difference in level, H , between T and the mercury column of the capillary on the body of the gage is noted. Then the true pressure
P
=s
H'X
V -
VI
where V I equals the total volume in cubic millimeters of the gage above the constriction C, and v equals the volume in cubic millimeters per mm. of length of the capillary on the body of the gage. Again there is no correction for. the pressure in the external system (see Equation 4). To measure higher pressures-that is, to use the gage as a manometer-the design shown in Figure 4 equipped with capillary side tube S is required. The reservoir is adjusted so that the mercury level in S is at 0 and the difference in levels between the mercury in OF and in S is the pressure of the outside system being measured. I n building a gage of this new type it is important to have the proper ratio between the total volume of the gage, VI, and the total volume of the capilIary. To avoid sticking of the mercury in the capillary, its inner diameter should be greater than 0.5 mm. Although for very low pressures the body of the gage may be even as large as 500 cc., for most uses in the laboratory it is best to have this volume around 50 cc. For the convenience of those wishing to build one of these new gages, there is described in Table I the complete dimensions and pressure data of two gages of this type of different range actually in use in this laboratory. To minimize smutting of the mercury, these gages may be
ANALYTICAL
382
operated by vacuum-pressure control arrangements, such as that devised by Gaede ( 1 ) or the General Electric Company modification (2), provided the reservoir is connected to an air-pressure line to force the mercury over the top when evacuating capillary OF.
EDITION
Vol. 4, No. 4
LITERATURE CITED (1) Gaede, Ann. Physik, [4] 41,313 (1913). (2) K a m "High Vacua," P. 128, Longmans, 1927. Mag.* 47*llo (3) McLeodi
RECEIVED Aprll 6,1932.
Determination of Silicon in Steels Robert M. FOWLER, Bureau of Standards, Washington, D. C. LTHOUGH it is a wellTable 11 shows the r e s u l t s The silicon content of three typical steels has known fact, that in the Obtained by thirteen been determined by solution, dehydration, and determination of silica in on a standard steel. C o l u m n recovery of the dissolved silicon by volatilization silicates a c c u r a t e results can shows values they reported of the iron with hydrochloric acid gas. The be obtained only when several on first a n a l y z i n g the steel, percentage of silicon obtained by a single ded e h y d r a t i o n s a r e m a d e , it and column 5 the results they has l o n g b e e n t h e p r a c t i c e obtained when the steel was rehydration with jive common methods for the of steel chemists t o c o n s i d e r t u r n e d to them for determination of silicon is compared with the the amount of silica left in iron with a request that they make values for total silicon. An umpire method for s o l u t i o n s after a single dehytwo d e h y d r a t i o n s for silicon. silicon in very low-silicon irons is also described. dration as b e i n g negligible. T h e s e d a t a i n d i c a t e that if However, in the last few years the silicon content of a steel is there have been several attempts to show that for accurate as high as 0.4 per cent, a second dehydration is necessary work more than one dehydration is necessary, especially if for accurate work. the silicon content is high. For example, Pinsel (6)claims 11. SILICON IN STANDARD STEELREPORTED BY THIRTEEN that when cast irons are analyzed by the nitro-sulfuric acid TABLE ANALYSTS method the results are low, and that for umpire work several -FIRST ANALYSIS-SECOND ANALYSISevaporations cannot be avoided. No. of ,d& Values No. of .deValues ANALYST hydrations reported hydrations reported Stadeler (7), while studying the relative advantages of % ol, ," the various methods for silicon, had three steels of different 1 0.441a ... ... 2 silicon contents analyzed by twelve laboratories, employing 0.434'" ... 3 0 . 38Sa 2 0:iis. five different methods. All of these laboratories found sili4 0.412a 2 0 . 440a 5 0.409b 2 0.4410 con in the filtrate after a single dehydration. The twelve 6 0.408b 2 0.436'" 7 laboratories varied widely in the percentages of silicon re0.420b 2 0.446" S 0.407d 2 0.441" covered in a second dehydration, averaging 0.01, 0.02, and 9 0.422 2 0.4394 10 0.417 0.440 ... 0.14 per cent on an 0.08, a 0.3, and a 4 per cent silicon steel, 11 0.427C 2 0.451C 12 0.422d respectively. However, the results for the first evaporation 1 0.4346 13 0.4356 ... ... differed so greatly as to throw doubt on any conclusion as Av. 0.419 Av. 0.441 to the amount of silicon recoverable in a second dehydraa Sulfuric acid method. b Nitro-sulfuric acid method. tion. Wolf and Heilingotter (10) analyzed a sample of highNitro-hydrochloric acid method. silicon steel by five methods, making four dehydrations by d Hydrochloric.acid method. e Perchloric acld method. each method. For their standardization procedure they chose a nitro-sulfuric acid method, which yielded an average DETERMINATION OF MOST PROBABLE VALUE^ FOR of 4.004 per cent of silicon in the first dehydration, 0.068 in SILICON The the second, 0.039 in the third, and 0.019 in the fourth. I n view of the uncertainty concerning the amount of silitotal, 4.130 per cent, was considered to be the silicon content of the steel. Four dehydrations by each of the other con remaining in solution after dehydration by the usual methods gave values ranging from 4.078 to 4.116 per cent, procedures, it was decided to attempt to determine the true results which were from 0.014 to 0.062 per cent lower than silicon content of three typical steels and then to compare these values with those obtained when the usual procedures those obtained by the nitro-sulfuric acid method. I n the analysis of Bureau of Standards standard samples were applied. Steels representing the ranges of silicon usually of steel it has been the practice to make two dehydrations encountered in steel laboratories were selected for this work. by the sulfuric acid method. The results obtained on a The samples were in the form of fine chips as prepared for Bureau of Standards standard samples (4). For convenience number of these standards are given in Table I. they will be referred to as steels A, B, and C throughout this OBTAINEDBY SULFURIC ACIDMETHOD paper. Their silicon contents were approximately 4.7, 0.4, TABLEI. SILICON and 0.1 per cent. Si Si OBTAINBD OBTAINED The other constituents were as follows: wl'. OF IN 1ST DE- I N 2ND DE- TOTAL
A
TYPEOF STEEL
0.4 Carbon B.0.H. 0.1 Carbon' Bessemer 0.4 Carbon' Bessemer 0.6 Carbon: A. 0.H. Medium manganese Chrome-nickel 0 6 Carbon A 0 H. 110 Carbon' A: 0:H. Aoid electric
SAMPLE$ HYDRATION arums % 10 0.017 10 0.020 10 0.062 10 0.105 10 0.192 10 0.213 5 0.276 10 0.381 10 0.434
HYDRATION
% 0.001 0.001 0.002
0.002 0.003 0.007 0.004 0.007 0.007
SILICON % 0.018 0.021 0.064 0.107 0.195 0.220 0.280 0.388 0.441
STEEL TYPE
C
%
Mn %
P
S
Ni
Cr
V
%
%
%
%
%
0.064 0.10 0.010 0.024 A Unknown B Acidelectric 0.258 0.748 0.036 0.048 O:ii2 0:ih 0:1%7 0.674 0.630 0.062 0.030 0.161 0.166 0.008 C A.O.H.
In the analysis of silicate materials, such as rocks, it is usually assumed that the silica which escapes the dehydrating
