Viscosity Measurement So Easy, Yet So Difficult by Janet L. Lane and Kenneth O. Henderson
Viscosity Measurement So Easy, Yet So Difficult by Janet L. Lane and Kenneth O. Henderson
Viscosity Measurement So Easy, Yet So Difficult by Janet L. Lane and Kenneth O. Henderson
Temperature
As the saying goes, “The most important factor affecting the value of a
home is location, location, location.” In terms of viscosity
measurements, “The most important factor affecting the quality of a
viscosity measurement is temperature, temperature, temperature.”
Viscometers
The size of the viscometer must be chosen such that the flow time is at
least 200 seconds for manual determinations, otherwise eye-hand
coordination will become a significant factor. The operator waits for the
meniscus of the fluid to pass the timing lines and uses a clock or
stopwatch to measure the flow time. Flow times over 200 seconds are
required to eliminate possible operator variation.
It is obvious that a clean viscosity tube must be used for the analysis,
but what is actually meant by “clean”? The tube must be free from dust
or other particles and the fluid should clearly wet the surface of the
glass. Typically, several rinsings with a sample solvent such as
naphtha, followed by a drying solvent such as acetone and then a
purging with a dry, dust-free gas such as air or nitrogen, is sufficient.
The solvents should be free of any particles, so filtering is required.
The solvents also must be residue-free on drying. This is of particular
concern if commercial or practical grade solvents are used.
The residue of previous samples coating the inside of the viscometer
can be removed with a chromic acid (1) cleaning solution or a
nonchromium- containing, strongly oxidizing cleaning solution. (2)
Never use an alkaline liquid detergent or any cleaning solution with a
pH above 8.0 because they will actually change the dimensions of the
capillary by dissolving the glass walls and thus change the calibration
constant. These types of liquids are often found throughout an
analytical laboratory and can be used in other applications, but if one is
used to clean a tube, the tube will need to be re-calibrated. Another
way to change the calibration constant of the smallest size viscometers
significantly is to soak the tube overnight in boiling water. The tube
might be clean but it needs to be re-calibrated before its next use.
The tube must also be dry before using. The use of dry, dust-free air or
nitrogen is recommended.
Timing Device
The timing device must have an accuracy of within +/- 0.07 percent of
the reading and must be capable of taking readings within a
discrimination of 1 part in 2000 or 0.1 seconds for a 200-second flow
time. Care must be taken when using electrical timing devices because
alternating currents may not provide the required
accuracy. Timing devices should be checked against a National
Institute of Standards and Technology (NIST) reference or other
accepted timing reference on a regular basis. Yearly verification is
recommended.
Sample Handling
Automated Viscometers
Summary
Notes
1 Chromic acid is a health hazard and must be handled with
precaution. It is toxic, a recognized carcinogen, highly corrosive, and
potentially hazardous when in contact with organic materials.
2 Nonchromium-containing, strongly oxidizing cleaning solutions are
also highly corrosive and potentially hazardous when in contact with
organic materials