I.
Introduction
The Ostn.altl viscoiiieter. shown in Fig. I , is the standa~rcliiistmruiiieiitof the rubber experimental stations in the Dutch East Intfies for tleteriiiining the consist,ency iusually called “viscosit,y”) of henzeiic solutions of crude rubl)er.’ ordinarily iisetl t h e flow takes place solely under hydrostatic had. In connection n-ith in i.rlstigational work of the (‘rude 1ypeof inst’riinient in ivhich it is poi;sihle t,o iiiake iiieasurernerits at various average rates of flow. The Ost,waltl viscometer. as ordinarily used! is not suitable for this piirpose unless provided \vit,h inore tlinn one lower bulb, so that it niay he fillet1 t o different height,s as in the iiiotlified forin of instrument used tiy tic J011g3. I t is however possiljle as will he shown, t o use the usual form of 0stm.ald viscometer with an external soiircc’ of pressure, as is done with tllc Birigliani viscoiiieter.i At thc Bureau of Stantlarcls the Binghain viscoiiicter is t i :try deterxinntion of viscosity. I-Ion-ever. since thc Ostn-ald viscometer is in general u s e throughout thc ruhber industry, and its simpler coiistruction facilitates cleaning, it seenied desirable t o investigate t8hesuit’abilitj- of this latter instruinerit as a consistoiiiet,er, i. e., a.n instrmnent u-hich limy lie used to distinguish IietTvcen 2 viscoiis amtl a plastic material. arid to iiieasiirc the consistency of either. The two instruiiimts of Fig. I . used in this investigation, TI-ereo h t a i n d through the kindness of Dr. E, van \-alkeiil)urg;?.’as t)ypical of t hose iiscd in testing 1wiizt.iie solutions of crude rubher. 2.
Methods of determining Mean Effective Heads
\17ith the F3ingh:iiii viscometer. in which the effect of the hylrostatic lieatl very sinall. the times of flow in hoth directions are iiscti in tleterinining the inean effective, usually called the average pressurc. \\-itli thc Ostn alti viscometer wliei c the hytlrostatic head 1s conqiderahlc, the asqumptioiis in\ olvrd
I
during tht. run. -7. Washburn ancl IYillianis? took for the average pressure. the pressnrrl at inean time. This method, according to Applebey, is approximately correct. Methods I and 2 were rejected 3s inapplicable to Ostwald instruments with short times of flow.
\ L t h cylindrical or spherical bulbs. 3. under certain condition.., the average head h may be calculated by equation.. or inorc conveniently by Fig. -7. By average head is meant that head which, if inaintmainetl constant, would cause the time of flow to he the m i l e a< with the variahlc liead actually eyisting. The lower c u r v ~applies to an Ostwald viscometer with two equal cylinclrical l)ulbs, and also to an instrument of the pipette type with one cylindrical bulb. The uppcr curve may be calculatccl froin formula of Barr3 for a pipette n ith spherical tiulh, antl it apl:lic:, equally well to an 0-twaltl ~ i ~ c m i i e t e i with t m equal spherical bulbs, provided the menisci a t the start of run are at the top of the upper bulb and at the bottom of the lower. Fig. 2 shons that there is less error in assuming that the average head i.i q u a l to t h e aritlinietical iiiean with spherical than n it11 cylindrical bulbs. If hl is the initial and h2 is the final heatl. Barr’s equation may be written in the more convenient forni, hi - hz
1
a
11. P. dpplehey: J. Chem. doc. 97, 2000 (1910). C,’lw~u S o c.. 35, 737 [ 1913 1 E. I\-.\\-ashburn and G . T.Williztms: J. G . Barr, J. Sor. Chem. Ind., 43 29T j192+).
which may be compared vith the corresponding formula for cylindrical bulbs, h
=--
hi - h? hi lop,
6
(2)
The attempt was iiiatle to apply equation ( 2 ) t o the inrtruriierits of Fig. I . but it n-as found that at Ion. pressuies iiiore concordant resu1t)s could he 01)tainecl the qiniplcr equation
P =
where
PI11
+ h, P
(3)
p = density of liquid in grams per cc 1 3 , ~= external pressure, calculated from iiianonleter reading, in grams per ctn? h, = average hydrostatic head in c m
Equation (3) is known to be only approximate when pl,, is small. According to Bingham, equation ( 2 ) should he used when pn, is less than 30p multiplied by the height of bulb.
WIS-LOW H. HER-CHEL
I220
%ST,K .
BLLhLCT
In t'he instruiiirmts of Fig. I . thv h l h s are uiicqual and the ineniscus is near t.he center of the lower h l h at the start of it riin. Thus since neither equation ( I ) nor ( 2 ) is strictly applicable, it nppcx,rctl preferable to 1 1 s ~ t,he siiiipler though nppr(?siiiiatc t)quatioii i ,i 1. 4. For any given instlruiiientl. Ii,, will l w constmt. at> sufficiently high pressures. no matter what the shape of the. bulbs. It can not in general hc measured directly 011 thc instruincnt , hut niust be considered as an instrumental length which iiiust. lie tleterinincd hy flow tests. -\fter h,, has been determined, equation i:,1 may be u m l to tletcririint p in the calculation of viscosity. 3.
Calibration of Ostwald Viscometers
)iiwtcrs whcrr tlic (lriving force is cntircly due t o hydrost8atichead, the relation hrt n.ccw viscwi;ity anti tiiiie of flow is given hy the. equat ion
viscosity in poisrs density in grams per c r t = time of flow, in scconds d and 13 = instruiiieiitnl constants Ivliirli limy tw oht:iiiictl l>y Higgiris' graphical niethorl.' Equat'ioii (4)is applicablc t o tmheOst8\Taltlviscoiiictcr as ortlinaril>-meti (n.it(hout external pressure). Jl-hen thew is a n external pressure p,]: in acldit,ioii to t'lic hydrost'atic pressure h, p where ,u
=
P =
as with the I3ingha,in visromrter. providecl pil,is not too small in coiiipitrisoii with h, p . Since the time of flow mal- tie very short, for a calibrating liquid like lienzene or water, it, is desirable to use as low a pressare as possible in order t'o reduce t,he t,iiiring error. On t#heot,her hand. when very viscous liquids are under test it is an advant8agetjo use a high pressure in order t o reduce the time of flow-. Thus it is desirable t80calibrate instrunentjs by both equations (1) and i;) since a wide range of pressures is of advantage with viscous as well as with plastic materials. ,llthouph these equations do riot apply for pla.stic iiiaterials, as nll1 be seen later, the const,ant, (' is convenient for use in calculating the diameter of the capillary. The neglect of a kinet,ic energy correction because there is submerged discharge does not' appear to be justified, and the practice of Ringham in applying it has accordingly been followed. 1IT.
L:. Higgins: J . $or, Chem. Ind.. 3 2 , j68 ( 1 9 1 3 ) .
THE O5TW.iLD V I S C O M E T E R At, -1 C O S h I S T O J I E T E R
1221
Since the kinetic energy correction is small in comparison with the previoiis term in equation (4), for the Bingham viscoineter it is custoinary to calculate the value of R from the equation
whtw Q
=
1
=
volume of upper bulb, in cc. length of capillary in cni ni = coefficient for which. on the lmsis of available data, the value of I . I 2 is uFually assunied. It is believed that. for the Ostswaltl vibcoineter, the values of E obtained by Higgns' method, are to he preferred to those obtained by equation (6). The absolute viscosities of the calibrating liquids were obtained by the Bingham viscometer. In using these liquids to calibrate the Ostwald viscometer, a series of about 6 runs was made with each liquid with varying values of pill and a value of h, was found by trial which would make C as nearly constant as possible. The average value of h , obtained with all liquids mas used in the final calculation of the average value of ( ' its given in Table I.
TABLE I Dimensions and Instrumental Constants of Ostnald 1-iscometers
-
1
Length of capillarj , cni ('apacity of upper bulb, cc Time of flow for benzene, 3 o o C , sec. without external pressure Time of flow for water, .jo0(', sec., xitholit external pressure Constant. X, in equation ( 4 ) T-alue of B: By equation 16), for coinparison only By Higgins' method, arid used in calculating C Sumber of calibrating liquids ;\laximum mriation in C' with any one liquid,
I
