54 



BtlLLETIlSr 1036, U. S. DEPARTMElsrT OF AGRICULTURE. 



sometimes known as "relative viscosity" or ''specific viscosity," 

 but more correctly as "Engler nmnber" or ''Saybolt number," as 

 the case may be. Although there are a large nimiber of viscosi- 

 meters on the market, no two makes will give exactly the same result 

 because of the fact that these instruments work under a different head 

 of liquid and have slightly difi'erent sizes of orifice. Results ob- 

 tained by one make of instrument are, therefore, not directly com- 

 parable numerically with those obtained by another make. For that 

 reason it is necessary to give the name of the instrument when 

 viscosity readings are published. The instrument which seems to be 



in most general use in this country 

 for measuring the viscosity of oils is 

 that known as the Engler viscosi- 

 meter. The efflux time of water at 

 20° C, the standard temperature, is 

 generally between 50 to 51 seconds. 

 The Engler viscosimeter used at the 

 Forest Products Laboratory has an 

 efflux time of 50.8 seconds with water 

 at 20° C. Tables have {15), how- 

 ever, been worked out by which re- 

 sults obtamed in the Saybolt, Engler, 

 and Redwood viscosimeters may now 

 be converted to absolute viscosities, 

 and hence from the readings of one 

 instrument to those of another. 



The viscosity of coal-tar creosote 

 varies somewhat with the percent- 

 age of the higher-boiling constituents 

 present. Those which have a largo 

 proportion of the higher-boiling oils 

 are, of course, more viscous than 

 those having a small percentage of 

 these oils. The extremes are represented by low-boiling creosotes on 

 the one hand and by carbolineums on the other. The viscosities of 

 creosote as well as of all other oils vary also with the temperature. 

 In general, the higher the temperature the less the viscosity. This 

 is not, however, a straight-line relation. Figure 28 shows the change 

 in absolute viscosity with the change in temperature for an average 

 coal-tar creosote and for a carbolineum. 



The change in viscosity of creosote or carbolineums may be cal- 

 culated from two or three determinations at different temperatures 



by the use of the formula V= rpj^i where V is the absolute viscosity 



























































































































































































V 





















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\ 



\i 





1 



\ 















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\ 







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\ 





^ 



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N 



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s 



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^ 



L 







7Z7 



f/=£-A 



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290 .S/O ,3JO J£0 ^70 



Fig. 28. — The change in viscosity of coal-tar dis 

 tillates with change in temperature. 



1. A high-boiling creosote. 



2. A carboUneum. 



