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5.3
Periods without operation
If the capillary viscometers are not used over a long period of time, the liquids contained in the system, in
particular aggressive solutions, have to be drained. If the liquid is left in the system, one has to reckon
that the solutions used will alter in the course of time and attack the glass, in particular the capillaries.
Cleaning agents should be matched to the previous samples or impurities.
In many cases, an aqueous cleaning agent (glass cleaners, detergents) or organic solvents (such as
acetone or hydrocarbons) are sufficient.
Strong oxidizing cleaning agents such as chromic acid may only be used by trained personnel and
must be suitably disposed for safety and environmental reasons - the current guidelines for handling
hazardous materials must be observed.
In the last rinse cycle, the viscometer should be rinsed with a suitable solvent with a low boiling point
(such as acetone), and dried by an air flow, which is preferably generated by negative pressure (for
example, water jet pump). The viscometer is dry and dust-free by this treatment and can thus be used for
manual and automatic measurements.
5.4
Reproducibility of results
The measurement or analysis results depend on a variety of factors. Please check the plausibility of the
measurement results or analysis results at regular intervals, and carry out the required reliability tests. In
this regard, please adhere to the usual validation procedures and especially to the "Viscometers within
quality assurance systems" chapter.
5.5
Viscometers within quality assurance systems
Recommendations for companies that have introduced a quality management system (OM system)
according to DIN EN ISO 9001: In this quality assurance system, an inspection of the measuring
equipment is planned. The intervals and required accuracy can be defined by each company according to
its own requirements. The standard DIN/ISO 10 012, Part 1 serves as a guideline in this matter. We
recommend regular inspection of the viscometers in defined intervals.
Inspection of the viscometer constants:
a) Calibration using comparative measurements with reference measuring standards
Comparative measurements must be performed with a viscometer (reference measuring standard) which
was tested at the PTB (Federal German Physical-Technical Institute) and provided with a constant.
During this comparative measurement, the viscometer to be inspected and the PTB - tested viscometer
were placed simultaneously in the same thermostat bath. The test liquid tested, the viscosity of which
must not be known exactly, is filled into both viscometers, tempered and the flow-through time then
measured. The constants of the viscometers to be inspected are then calculated according to the
following equation:
K
constant of the tested viscometer
K
constant of the standard reference viscometer
t
low time (HC) of the tested viscometer (corrected by Hagenbach-Couette)
t
flow time (HC) of the standard reference viscometer (corrected by Hagenbach-Couette)
Within the quality management system in accordance with DIN EN ISO 9001, traceability of the
measuring equipment to national measuring standards is demanded. This traceability can be achieved by
inspecting the comparative viscometers (reference measuring standards) at regular intervals at the PTB.
The time intervals are defined according to the specifications made in the quality assurance quality
management system of the user.
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