Capillary viscometer according to Ubbelohde
Purpose
The working group is responsible for the dissemination of the unit of viscosity and maintains several Ubbelohde capillary viscometers of different types as national standards. They are used for the very accurate determination of the kinematic viscosity of Newtonian (reference) fluids and ensure the direct traceability of the calibration of Ubbelohde viscometers performed by us to the National Standard.
Scope
The measurement of the kinematic viscosity of Newtonian liquids with standard capillary viscometers according to Ubbelohde can be carried out in the temperature range from -40 °C to +150 °C. The capillary constants are determined by direct comparison to the respective National Standard at 20 °C. The standards DIN 51562, Part 1 and Part 4 are applied accordingly.
Measuring method
In capillary viscometers, the velocity gradient required for viscosity measurement is generated in the form of a laminar flow within a measuring capillary. The kinematic viscosity is measured from the flow time t of a defined volume of liquid through a measuring capillary and can be calculated using equation (1), taking the Hagenbach correction into account:
n = K × t (1)
Where K is the capillary constant of the respective viscometer.
After being filled with the liquid to be measured, the viscometers are tempered in a water bath to an accuracy of < 0.003 K. With the liquid to be measured, a series of measurements with at least 5 individual measurements each is carried out with each viscometer. The time is measured manually via traceable calibrated stopwatches with a time resolution ≤ 0.01 s. The glass viscometers have burned-in ring measuring marks for this purpose.
Uncertainty
The relative uncertainties achieved for the measurement of the kinematic viscosity are in accordance with DIN 51562-Part 4 (1999) " Viscometry - Measurement of kinematic viscosity by means of the Ubbelohde viscometer ". Depending on the capillary constant and temperature, relative uncertainties of 1.4 × 10-3 to 11.8 × 10-3 can be achieved.
The relative uncertainties for viscometer constants are between 1.0 × 10-3 and 3.9 × 10-3, depending on the capillary constant, and are related to the kinematic viscosity of water at 20 °C of 1.0034 mm²/s (internationally agreed value according to ISO/TR 3666-1988).
The relative standard measurement uncertainties are determined in accordance with the “Guide to the Expression of Uncertainty in Measurement” and apply to the expansion factor k = 2.
SVM 3000 Stabinger Viscometer
Purpose
With the help of the Stabinger viscometer, the dynamic viscosity and density of liquids can be measured digitally at the same time in accordance with the standard ASTM D7042.
Scope
Measurements of dynamic viscosity in the range from 0.2 to 20,000 mPa*s, and density in the range from 0.65 to 3.0 g/cm³. The accessible temperature range is between -40 °C and 105 °C.
Measuring method
The SVM 3000 viscometer (Anton Paar) is a rotational viscometer and operates according to the Stabinger measuring principle. It is designed according to a modified Couette principle, with a rotating outer tube and a slower rotating inner measuring body. A magnetic field, which brakes the rotor in a contactless and defined way via eddy currents, completes the apparatus. A simple formalism allows to calculate the viscosity from the speed ratio of the outer cylinder and the rotor. High-precision digital measurement of the rotational speeds makes it possible to record the measured value precisely.
The traceability of these devices is realized by the PTB calibration liquids of viscosity.
Density values are obtained according to the oscillator principle.
Uncertainty
The measurement uncertainties for the viscosity depend strongly on the liquid to be tested and cannot be given as a general number. Achievable repeatabilities are in the range of 0.1 % (manufacturer's specification).