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FAQ | Precision
Questions Answers
How do I check the measuring accuracy of my AVS system? Are there calibration liquids for this? "The calibration constant of the capillary viscosimeters can be checked by means of calibration liquid, which can be obtained from the Deutschen Kalibrierdienst (DKD, ZMK Wolfen), the German calibration service. Another possibility is the direct comparison with master viscosimeters, which were calibrated i.e. by the PTB or the DKD (ZMK Wolfen). Furthermore, the viscosimeters can be recalibrated at SI Analytics GmbH. And, the temperature of the thermostat bath must be checked with an accurate thermometer: The deviation from the nominal temperature must not exceed 0.02°C; this is especially important for absolute measurements of viscosity. Note the correct sample preparation when measuring polymer solutions: Correct concentration, avoiding polymer decrease by excessive heating or dissolution times, observing contained additives when weighing and more"
Can measurements be accurate with run time of less than 200 seconds? "The standards to measure the absolute viscosity with capillary viscosimeters - e. g. DIN 51 562, ISO 3105, ASTM D 446 - require run times of at least 200 seconds. In practice, however, many users work with run times of less than 200 seconds. With viscosity meters with a large capillary diameter (i. e. Ubbelohde type II or larger, all micro Ubbelohde types), the increase of measuring uncertainty at a run time of 100 seconds is only very minor compared to 200 seconds and mostly acceptable in practice. With viscosimeters for low-viscosity liquids (e.g. Ubbelohde types 0, 0c, I, Ic), special flow effects occur with decreasing capillary diameter and decreasing run time, so that the measured run times must be corrected as per Hagenbach-Couette. This correction can only be approximated by means of a formula, therefore, the measuring uncertainty increases with a decreasing run time. Therefore, you must adhere to run times of 200 seconds or longer by any means with viscosimeter types with tight capillaries for precision measurements. With low expectations of measuring uncertainty, you can use shorter measuring times < 200 seconds in practice as well for these tight capillary diameters. In this case, the run times must be corrected as per Hagenbach for sure. These corrections can be found in the instructions for use of the SI viscosimeters (also see download area of the SI home page). For micro Ubbelohde viscosimeters, the Hagenbach correction is generally less than for DIN Ubbelohde viscosimeters, so that micro Ubbelohde viscosimeters can also be used with run times between 50 and 100 seconds. For measurements of relative viscosity of polymer solutions, the standard ISO 1628-1 permits minimum run times of 150 to 50 seconds for the blind value (pure solvent) depending on the capillary size. "
What is the Hagenbach-Couette or HC correction? The Hagenbach-Couette correction takes into account the effects during the entry and exit of the sample into or from the capillary as well as the kinetic energy of the sample; therefore, this correction is called the "kinetic energy correction". The correction is comprised of correction times that must be subtracted from the run time. The correction time increases with a shorter run time and the smaller the capillary diameter. The values of the Hagenbach correction can be found in the instructions for use for the viscosimeters (also see download area).