Cannon Instrument CMRV-4500 - Manual (Page 117)

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CANNON ® Mini-Rotary Viscometer CMRV-4500 Instruction & Operation Manual
Version 1.1e—October, 2011 • CANNON ® Instrument Company
2139 High Tech Road • State College, PA • 16803 • USA
CHAPTER
C
APPENDIX C —
THERMOMETRY
Kinematic viscosity and temperature
Kinematic viscosity is an extremely temperature-sensitive measurement -
a change of 1°C can sometimes lead to a viscosity change of 10 percent
or more. Therefore, it is not surprising that temperature measurement and
control are the most common problems encountered by laboratories
performing accurate kinematic viscosity measurements.
Although capillary viscometers typically measure kinematic viscosity
with a precision of several tenths of one percent, measurements accurate
to within one tenth of one percent (0.1%) are possible. To achieve this,
temperatures must be measured with an accuracy of 0.01°C, and be
maintained within a range of ± 0.01°C.
Thermometers The ideal reference thermometer is a high-quality standard platinum
resistance thermometer with a precision bridge. Because many laborato-
ries cannot justify the cost of such a thermometer,
CANNON ® Instru-
ment Company recommends the use of a calibrated ASTM kinematic
viscosity thermometer.
ASTM Thermometers Each ASTM kinematic viscosity thermometer measures only 3 degrees
on a scale subdivided into 0.05°C units (equivalent thermometers are
available with Fahrenheit scales). These thermometers contain an ice-
point scale which allows recalibration by determining the ice-point
temperature.
Thermometer Calibration Calibration of the thermometer is very important. Often the true tempera-
ture of a liquid differs markedly from that shown on the thermometer
scale. It is not uncommon for kinematic viscosity thermometers to give
readings varying as much as 0.1°C from the actual temperature. The true
liquid temperature is obtained by applying the proper correction (as noted
on the original calibration certificate) to the reading showing on the
thermometer scale and including any difference obtained in a recent ice-
point measurement of your thermometer.
Thermometer Immersion Proper thermometer immersion is critical for viscosity measurements.
Even a calibrated thermometer will read incorrectly if it is improperly
immersed in the bath. “Total immersion” kinematic viscosity thermom-
eters should be used with the bulb and entire mercury column beneath the
surface of the liquid, but with the emergent stem above the surface at
ambient temperatures.