Capillary Viscometry; Measurements Principles - Fisher AVS 470 Manual De Instrucciones

35
1.2

Capillary viscometry

Capillary viscometry is the most accurate method for the determination of the viscosity of liquids with a
Newtonian flowing behaviour. The measurement as such consists in a time measurement. The time
measured is that which a specific quantity of liquid requires to pass through a capillary having a defined
width and length. Conventionally, this process is watched with the human eye, and the flow time is
measured manually using a stop watch.
In the case of the AVS 470 Viscosity Measuring Unit, as with all viscometry measuring devices from
Schott Instruments, the liquid meniscus is captured on the measurement planes, either in an
optoelectronic manner by means of light barriers, or else on a thermo-resistivity basis by thermistors.
1.3

Measurements principles

Optoelectronic sensing of the liquid meniscus
The near-infrared light which is generated in LEDs located in the upper section of the measurement
stand is conducted through a glass-fibre light-conductor cable onto the measurement planes. The light
shines through the viscometer before it arrives at another light-conductor cable located on the opposite
side; inside this second cable, the light is conducted to a receiver in the upper section of the
measurement stand.
While the liquid meniscus passes through the measurement planes, the lens-like effect of the meniscus
causes a short-term darkening of the light beam, followed by a magnification. This process generates a
measurement signal which can be evaluated accurately.
Fig. 1 Viscometer for optoelectronic measurements
Viscometer with thermistor sensors (TC viscometer)
In the case of TC viscometers, glass-coated thermistors serving as sensors are molten in on the level of
the measurement planes. While the meniscus passes through the measurement planes, the differences
in the thermal conductivity properties of air and liquid lead to a change in the heat balance. The
thermistors of the TC viscometers are molten hermetically tightly into the glass coating of the viscometer,
so that the viscometers located inside are chemically resistant to all kinds of substances other than
strong leaches, fluoric solutions, or concentrated hot phosphate solutions.
TC viscometers are protected under patent right by the German Design Patent no. 85 04 764.3 and
US Patent no. 4 685 328.
Chapter1 The AVS470 Viscosity Measuring Unit