INDUSTRIAL A N D ENGINEEfZINI: CHE.MISTRY
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Vol. 16, No. 6 -
Tools of the Chemical Engineer' 11--Weighing Devices By D. H. KiIleffer 19 EAsr 2 4 ~ aSr.,N B W YoRX, N. Y .
APID developnieiit of continuous processes in chemical work during rea:nt years has made the everimportant problem oi control of quantities of material t,lic key to successful results. Batch operation.: using t,he crudest of devices for determinirig quantities have often bem successful, but sooner or IiLtt'r human error or the failiirc rrf some part of w e n the simplest weighing or inensorem has resulted iu tlic complete onhalancing of the Continuous o1n:r:itions are not only more susto changes in the system from human or otlrer error, but :wEar more difiioolt to set going aguin ~ i thei proper ~ halarice has hem distorbcd. The oh1 p l m t s r d c and tlre old plant measuring rod arc equal to the handling of c ~ i i t,iniious flows of inatcrial only in tlie crudest FIowcvctr, t,liere has hecn developed during r onc might. almost ,say iiiorrthswhen the rapid adoption of some of the rlwicrs is considered---a wide t o weighing and measuring which brains. It is with these weigh article iteals. A mbseqoont iirticlr: mill discuss fluid iireasiirenient. W*:IGHIXG IVeight and volunw are closely and simply related by specific gravit,y mensuremcnts, and very ofterr tlre deterininat,ion of eiihcr will serve for both. In the case of liquids, i t is seldom necessary to rniploy scalcs except in calibrating a. volumetric measuring devicr. With solids, volume det,erminations liecome inereasingly inaccurate as particle six: increases and are seldosn sufiieiently reliable to he of ralne. Weighing devices must necessarily be employed if any considerable degree of acouracy is to he attained. Equal-armed balances become extremely awkward when large weights a.re to he handled and are seldom employed for wiglits beyond a few Idlograms. By employing balances of unequal arms only one large weight need he liandled, and the operation is peatly simplified with little loss of accuracy. The simnlest balance employing this principle is tlre steelyard. This coirh t s of a simple steel beam arrangcd to form a lever of unequal arms, the longer arm k i n g graduated for the placing of a poise weight. Its readings are ta.ken Erom the position of the poisc; weight on the beam. This scale has been i n use for generations and iiivolvcs the principle on mhich prat:tir:ally all vxighing devices are based. To make it more cunvcnierit the platform \vas added, so ~11.'pendd RS t u transmit. the weight upon it to the steelyard. 1,evers ase employed 8
Received February 1 8 , 11124.
Fro. I-Derrc~
*OR
for this purpose in various ways, dividing the weight 011 the platform to a convenient fraction for transmission t o the steelyard itself. ACCESS~RXES TO TIXE €'I.A~TFORMSCALE In the ~levelopment of weiglring devices the tendency lias been rather tomnrd rnakiiig tlie operation of the platfmn i easier and niorc accurate than . I'n~litihly the most iniportnnt a so fur devised Eor weiglriiig operntioiis 1ms l~ecii recording ihv weight after the sealc has 1m:ii bal:incid hy t,hii operator. This doviw is iiwa )lied only to scales for weighing hcavy nrticle~. It c( of a sat of figures cmbcrsnd on tlic under side of tho 1, poise weight diioh can only bl! sct in pi~itiorrfor tlicsc figiirm t.o print, and a fine adjiistriimt liiadc by moriiig a sinall h a - p r t of the larger poise rwight-on ~ v l i i ~ ~otlicr li figurcs are cmbo :tiid ~~liieli is activnbpd hy a rack and pinion. The opcriition is somediat as f0l111~vs:The lnrge poise wcigbt is inor~cdto that division on the bcanr i v ~ Below t that required for balancing. This brings thr first two or three figurcs representin5 the might in position to print,. For insta.ilce, if the weight is 36,540 lip., the poise weight i? moved to thi? point requircd for :3G,000 kg. and tire number 36 on t h e under side of the heam is in position to print. Now tho firie adjustment is moved to the position for accurate halanee, and this brings t.he last three figiires-in this case 5lO--on the under side of the smaller poise weight, or rod, into proper rclation with the first figures on the main beam, A card is theii inserted in a slot provided and the weight recorded on it by pressing a lever. The operator must balance the scale accurately to obtain an accurate result, hut once obtained the weight is perrrianently and legibly recorded. In this way the human error is nrinimized xvhere large numhers of weighings arc t,o be made and recorded. Illegihility of figures is completely obviated and tlre operation i s so simple that no special skill on thc part of the operator is required heyonti that for bringing bhe scale to balance. This device has found particularly wide application to track scales where t h e weighings must he permanently recorded.
A ~ ~ T O M R T I C A LB LY ~innaso P
AUTOMATIC, OPERATION OF k4LES
L~TPO S c~i ~ Mi t s
One of the most recently perfected accessories for sinrplifyirig the operation of wighing is called the "weighman." It consists of z i iiiechaiiical arrangement for halaircing tli and at the same time tallying tire wcight on the platform. The main balancing beam of a platform scale of
June, 1924
ISDC7STRI.4L AnTD Eh7GirNRERING CHEMLWRY
ordinary type has placed parallel to it a revolving square shaft operated by an electric motor. On the shaft is a round sheath which is free to move axially alona it, but which must rrvolre with it, The round sheath is slightly longer than the graduated beam and the revolving shaft is a trifle more than twice as long. At a point corresponding to the center point of the graduations a disk is placed which has a constant positive hearing on the round sheath. This disk is normally perpendicular to the shaft when the scale is in balance, hut when imbalanced it is tilted in such a way as to cause the round sheath to move tciward a balanced position carrying with it the poise weight on the beam. With no load on the platform, the weight travels to the zero posit.ion and locks itself. When a load is run onto the platform, an electric contact must be made independently to start the mcchanism and unlock the poise weight. This is done to make sure that the workman does not weigh himself with the load, a.nd hence the switch for making the contact is placed some distance avay. As soon as this contact is made, the balancing weight moves to the equilibrium position wit,hout, intervention from the workman. I n so doing i t may actuate trains of gears to tally the weight in any convenient way. After tho weight is removed, the balance is automat.ically restored to zero and the mechanism is locked until released. Tho device is shown in Fig. 1. Its operation is mpid and fully as accura.te as that of the best of workmen. 1t.sgreatest use so far has been in weighing wheelImrrow loads of material being passed over it in unloading ca,rs or otherwise. The simplicidy of its parts is assurance of continuous operation with minimum trouble. Railroad cars are probably tlie most often weighed of any heavy weight,s,and many attempts h a w been made bo shorten the time requircd for this operation. One of the most successful of the various devices recornmended consists of an ntt.achment for the ordinary track scale vc~hercbyit is balanced by a spring at the end of the beam. At this point a comparatively small spring will carry the load, and this small spring can be enclosed in a t.hermostatic chambcr to prevent variations in it,s indications. The spring simply replaces the poise weight as a means of balancing, and the indication is ohtained from a dial revolved by rack and pinion movement to indicate its distortion. This dial has embossed on its circumference figures capable of printing the ivei&t of tlie car on a tape. The operation is great,ly shortened since the car itself, in rnoving oT'er the scale, operat,es the printing device in such a X Q ~as to obviate the iieccssity of stopping the train. As the car rolls onto the scale i t operates trips which put the scale into operat,ion to balance itself when d l eight wheels have passed onto it. The balance is secured almost instantaneously, and as the car moves off it trips the printing device to record its weight and sets the scale for the next car.
DIALIXDICATIKG SCALES The original dial indicating scales for rapid weighing8 were operated by springs, but since springs were subject to many errors and varied over rather wide limits in their indications over long periods of constant use, they have never found much favor in industrial operations. To ohviate this difficulty and.at the same time to furnish industry a scale that would he rapid in its action, a type of scale has been developed which depends on the displacement of a pendulum from perpendicular for balancing the weight placed upon it. These scales have been built in practically all sizes, from the accurate, small weighing devices of the apothecary to scales for weighing automobile trucks and their loads. In practice i t is usual to balance the weight with two pendulums moving in opposite directions, to avoid
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the error that would be introduced by any change from perfect level of the scale. Two general types of pendulum scales are in use-one whose pendulums are permitted to swing ch weight is through only a small arc and in ~ ~ f r i additional balanced by adding poise weights to the beam in a definite position, and another in which the pendulums are arranged to swing through an arc of nearly 90 degrees to balance the entire weight. In the latter type it is necessary to transmit the motion of the pendulums to the dial through steel tapes and cams in order to make it possible to have an evenly graduated dial. The reason for this is that the movement of t.he pendulums from a vertical position balances a wcight which varies according to the sine of the angle through which they move, and this would make necessary a dial so graduated unless provision were made t,o change this motion to a reylarly increasing onc. In the first type the motion of the pendulums is confined to a very small arc so that the difference in tho graduntiim. is inconsiderahl(~.
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BOK rris A I I T O \ ~ ~ ~ W T I C I ~ I N0C 8 nunr Qr;nrrrrics 01MA~RIIAL
2- .HOPPI%Scrim NiTE
L)rscoxmmoi;s WEIGHI. Where it is necessary to weigh quantities of material so great that a scale for carrying out the operation would be unwieldy or where many quantities of t,he same siw are to be weighed, it is possible to employ scales designed to divide a bulk of material into convenient lots and weigh these automatically. Perhaps the best illustration of this is to be found in the case of scales for grain elemtors. Carloads and shiploads of grain must be weighed accurately both into and out of storage, and to accomplish this with minimum effort in minimum time a discontinuous automatic scale has been widely used. Typical equipment consists of a hopper on one end of a balancing lever and a weight on the other sufficient to balance accurately the desired load. The hopper is free to move vertically for a short distance and when empty rises sufficiently to open an inlet port in a chute. As the load grows, the approach to a balance reduces the size of the inlet opening so that more accurate weighing is possihle. When the predetermined weight has beon let into the hopper, its further downward movement closes tlie inlet completely and opens the bottom. of the hopper for discharge. When discharge has been completed, the bottom of the hopper is closed automatically and the inlet is again opened by the same mechanism which closes the bottom, it being necessary for the hopper to he entirely empty before the bottom closes and for the outlet to he closed
Vol. 16, No. 6
INDUSTRIAL AND ENMNEERINC CHE.MISTRY
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before the inlet can he opened. A counter is provided t o tally the number of times the operation is completed, and thus the amount of material weighed. It is possible to vary the quantity of niaterial delivered at an operation within rather wide limits by varying tlte weight to he balanced. Anothcr modificat.ion of the same principle feeds the hopper from a belt conveyor which is operated by a n clecdric motor and which is stopped by the movement of the weighing hopper, thus stopping the feed of material until the load is discharged.
I
secrecy is desirable, and prevent mors in reading the position of the pointer by inexperienced employees. A scale has been developed for indicating numbers of articles on a dial by comparing the might of a definit,enumber with that of a quantity. In this case the balancing weight is one or more of the articles to be counted, and by balancing this weight with similar articles in the scale pan B definite number may be easily and quickly determined. S w h scales are made givingratios of 25 to 1,40 to 1, 50 t,o 1, and 100 to 1, hy means of which practically any xiredetermined number may be counted. .4notlier modificakxi, applied both to dial and steelyard scales, counts an indefinite number by comparing its weight to that of a known number.
Commvons WEIGKIKG DEVICES Two types of devices are in use for determining the weight of material delivered over a belt conveyor continuously. One of these continuously determines and records the weight passing, taking into account variations in load and belt speed, while the other controls the weight of material on the belt, at all times assuring const,ant flow, and may easily be made recording by the use of a n indicator for showing belt travel Typical of the first type is a device consisting of a counter whose record is dwendent on the weidit on the belt and a t the same time on tlk amount of belt travel. The two motions are transmitted to a disk (1 in Fig. 5 ) . The weight is s u p ported through levers by an iron Boat in a chamber of mercury whose travel is proportions1 t o t h e I R
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Pilo. ~ - I N S I A L L A ~ O N FOR CONTINUOUSLYWVEIUNINC M A T ~ R X A G O N BELTCDNYPYDR
Such automatic aeighing machines are widely used, both for neighing quantities too large to be weighed in a single load and, on the other extreme, for weighing material into packages-bagging grain or flour and even filling packages of a kilogram or ICESin content. Ny varying the size of the machine and the weight of the draft, practically any task of ~veighingt.hat requires many duplications of the same weight may he easily accomplished. The speed a t which the machine operates is lnrgdy dcpendeirt on the hime required for charge and discharge, but the cycle nccd seldom exceed s few seconds in duration.
CHECKIKG WEIGHTS While dealing ~viththe subject of predetermined weight rveighings, a device based on the dial principle already discussed cannot be overlooked. It consists of an ordinary dial scale, platform or othervise, with the indicating scale abbreviated to only a trifle more ihan the limits of tolerance of the weights of the articles to be weighed. The scale is set for the definite weight desired by means of an enclosed beam with a movable poise wcight and then locked. This prevents tampering with weight.s by the operator and gives an accurate indication of the shortage or ayemge of quantities tested.
*ic 4 - E a m ~ ~ n mPOR Co~morirhicQu~mnr on M i i T m i d ~CONTINUOVSLY Fan TO A BBLTCONVBYOI
Still another development of the dial indicating scale consists of movable tally markers placed on the dial to indicate definite quantities of materials required for batch operation. These may he so arranged that the operator need not know what weight he is delivering, in eases where
"IYOR
mittei to a small belt ( 2 in Fin. 5 ) which is in Dositive contact at all times with
I S D C S T R I A L A N D ENQIAfEERING C H E M I X T B ~
June, 1924
what its rate of travel. It is particularly useful for semifluid or quasifluid substances-i. e., granular materials and pastes. It is necessary that a quantity of material sufficient t o keep the belt loaded properly be continuously available a t all times, and the material fed should be reasonably uniform as to lumps, etc. The advantage of this device is in the uniform rate of supply of material that can be preserved within comparatively narrow limits, but its accuracy as a weighing device does not put it in the class with the others discussed except under the most favorable circumstances. It is also possible with this device to feed liquid a t a rate proportional to that a t which solid is fed, or vice versa. The manufacturers claim an accuracy for this device^ within a fraction of 1 per cent of actual when working under favorable conditions. Still another device for arriving a t the approximate weight of solid granular material passing through a chute consists of a short screw arranged in the center of the chute to be revolvetl by the passage of material by it. This arrangement is satisfactory where a fairly uniform slow rate of flow
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is maintained in a grallular material which is not subject
to caking or sticking, such as grain. It has a counter attached which records the linear passage of material past a given point in the chute, and thus, to reach an approximation of the weight ‘of material passing, the chute milst be a t all times equally full of material. The counter can be arranged to indicate either volume or weight of material passing.
CONCLUSION This discussion of weighing devices is known to be incomplete; yet it is hoped that it will Serve a useful Purpose in calling attention to some of the developments in this field. It cannot be concluded without mention of the fact that, whatever refinements may be introduced, none has yet appeared which can equal for accuracy the operation of a standard steelyard or platform scale in the hands of a skilled operator. Any of the improvements mentioned above may save time or labor or both, but it must be understood that they do so a t the sacrifice of a certain amount of accuracy.
Determination of Nitrate Nitrogen’ A Colorimetric Method By F. M. Scales and Arthur P. Harrison DIVISION OF SOILBACTERIOLOGY, BURBAUOB
F T H E more common colorimetric methods for the determination of nitrate, Sprengel’s2phenoldisulfonic acid method, modified by Chamot and his collaborators,3 is probably the most popular. The brucine4 reaction and also that brought about by the use of diphenylbenzidine6 may be employed to lesser extent. None are free from objection, however, and in an effort to obtain an accurate and rapid method for the determination of small amounts of nitrate in biological solutions the method here presented was developed. It is based on a report of Denig&s.6
0
APPARATUS A dark box is employed, made in the form of a solid block of wood 38 cm. square, 15 cm. thick, with 100 holes 2.5 cm. in diameter and 11.5 cm. deep, to accommodate ten rows o€ tubes. It has a hinged cover and is finished in black. The tubes should be of uniform size and color. Very satisfactory dimensions are 13 x 1.5 cm. A buret of 100-cc. capacity with a cock and tip of 4-mm. bore is used for the rapid delivery of concentrated sulfuric acid.
REAGENTS 1-Concentrated sulfuric acid. 2-Standard nitrate solutions, prepared by diluting a solution of pure nitrate t o the desired strength. From a solution of 1 mg. of nitrate nitrogen per liter-e. g., 6.07 grams sodium nitrate to 1000 cc. distilled water-dilutions corresponding to 0.01 mg. or more per liter can be conveniently prepared. 3-Reduced strychnine. It is better to purchase the strychnine sulfate for this reagent in 5-gram bottles and to keep i t well protected from contact with the air. Equal volumes of a 0.5 per cent solution of strychnine sulfate in concentrated hydrochloric acid and a 0.1 per cent solution of mercuric chloride in distilled water are mixed. Twenty-five cubic centimeters of this mixture are poured cautiously over 1 gram of magnesium
* Received December 13, 1923. * Pogg. Ann,, 121, 188 (1863).
* Chamot, Pratt, Redfield, J. Am. Chem. Soc., 33, 381 (1911). 4
5 6
Lunge and Lwoff, Z . angcw. Chem., 12 (1894). Letts and Rea, J . i7hem. SOG.(London), 106,1157T (1914). Bull. soc. chim., [4]9, 544 (1911).
PLANT INDUSTBY, WASHINGTON,
D.
c.
powder in a 300-cc. Erlenmeyer flask. The reaction is almost violent. Three or more flasks are prepared in this way, then combined, and when cool the liquid is filtered or decanted and is ready for use. It is better to make several reductions in this way and combine them than to make up the reagent in larger quantity, as it is safer and at the same time neutralizes any variations in the reduction. The reduced strychnine should be used within a few hours.
Great care must be exercised in the preparation of the reagent in order that it may be the same each time. I n reducing the strychnine sulfate with zinc, it was found that an excess of metal practically always gave a reagent that produced color in distilled water. Zinc usually contains small amounts of lead as an impurity, which makes it an unsatisfactory reducing agent, as lead chloride in comparatively minute quantity adds considerably to the color-producing quality of the reagent. Magnesium was selected because it is comparatively pure, gives a vigorous reduction, and if used as recommended will produce a reagent that will give no color to tubes of distilled water. The color-producing power of the reagent is enhanced by the addition of a small quantity of zinc, mercury, or lead chloride before the strychnine is reduced. The best concentrations for the chlorides in the reagent are 0.5 per cent for zinc, 0.05 per cent for mercury, or 0,001 per cent for lead. As an equal volume of the chloride solution is added to the strychnine solution, the former would obviously be prepared twice as strong as the percentage given for its presence in the reagent. In the present work mercuric chloride was used, because it is more soluble than the lead and not deliquescent like the zinc salt. It is possible that the effectiveness of the zinc and mercury chlorides may be due, in part a t least, to the presence of lead as a minute impurity. The results of tests with various chlorides are given in Table I. The reagent prepared without the use of any chloride before reduction is semitive to 1 part nitrogen in 5 million. Lead, zinc, or mercury chloride in proper amount increases the-
