54 
BULLETIN 1036, U. S. DEPARTMENT OF AGRICULTURE. 
sometimes known as " relative viscosity" or " specific viscosity," 
but more correctly as "Engler number" or ''Saybolt number," as 
the case may be. Although there are a large number 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 different 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 obtained 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 
o o o 
present. Those which have a large 
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 
K 
*a? 
I 
ft 
I 
\ 
1 
K^ ./G 
r 
\ 
\ 
\ 
> 
\ J2 
\ 
;\ 
JX 
X# 
72/ 
y/°<£> 
?/?Tl 
>/?£ 
0£TSA 
>££S 
C 
20 <?0 ffO 30 /OO 
290 c?/<? &X> J£0 ,370 
/>£&/?££$ SZ2SOLC/7Z: 
Fig. 28. — The change in viscosity of coal-tar dis 
tillates with change in temperature. 
1. A high-boiling creosote. 
2. A carbolineum. 
by the use of the formula T 
where V is the absolute viscosity 
