APPENDIX A
PRACTICAL VISCOMETRY
The most essential part of the viscometer is shown in Fig. 29, p. 76.
To use the apparatus an appropriate amount of the liquid whose
viscosity is to be measured is pipetted into the right limb. The
liquid at the desired temperature is forced over into the left
limb until the right meniscus reaches the point N, it being noted
that there is sufficient liquid so that the surplus runs over into
the trap. The right limb is turned to air so as to prevent more
liquid from flowing into the trap. Having adjusted the working
volume, the left limb is connected with the pressure, and the time
required for the left meniscus to fall from B to D is noted. The
left limb is now turned to atmospheric pressure and the instru-
ment is ready for an immediate duplicate determination in the
opposite direction. In this second measurement the time is
noted which is required for the left meniscus to rise from D to B.
Knowing the pressure, p in grams per square centimeter, the
time, t, in seconds, the two constants of the instrument, C and C',
and the density of the liquid, p, the viscosity t\ at the given tem-
perature is given by the formula, (cf. p. 74).
T, = Opt - C'p/t (1)
DETERMINATION OF THE CONSTANTS OF THE INSTRUMENT
The second term of the right hand member of the above equa-
tion is the kinetic energy correction which should never exceed
5 per cent of the value of the first term. For this reason the
value of the constant C' and of the density p need be known with
an accuracy of 2 per cent only in order to allow viscosity deter-
minations to be made with an error of only one-tenth of 1 per
cent.
C' = 0.0446F/Z (2)
where V is the volume in milliliters of the bulb C between
the marks B and D, and I is the length of the capillary EF,
log. 0.0446 = 8.64895 - 10.
295