A Rapid and Accurate Approach to ASTM Methods for Viscosity and Density
Jun 07 2017 Read 398 Times
Viscosity measurement specialists, Anton Paar, have listened to end users’ ideas when it comes to important considerations in viscosity and density measurement when comparing ASTM Methods D7042 and D4052 with D445.
Method D445 for calculating kinematic viscosity dates from the 19th century and was first standardised with ASTM as D445 in 1965. It is based on the measurement of time for a fixed volume of liquid to flow through the capillary of a calibrated viscometer under gravity and at a closely controlled and known temperature (ASTM D445-15). The kinematic viscosity is then calculated as the product of the measured flow time and the calibration constant of the viscometer. The method relies on gravity. However, when handling liquids one can rarely rely solely on gravity – other factors, for example pumpability, need to be taken in to consideration. Here, the right parameter for analysing the liquid’s behavior is in fact dynamic viscosity.
Methods D7042+D4052 can be performed in one fast measurement with Anton Paar’s SVM™ Stabinger Viscometer™ which employs a far more contemporary measuring principle with interconnected viscosity and density cells that have revolutionised viscosity measurement. This method was first standardised with ASTM’s D7042-04 Method. The SVM™ simultaneously measures dynamic viscosity and the digital density of the sample and calculates kinematic viscosity by dividing dynamic viscosity by density. One might ask, ‘what is the potential ‘error’ that could come from density when calculating kinematic viscosity?’ The simple answer is thatiIt is negligible, since D4052 states 0.0005 g/ml which corresponds to an approximate kinematic viscosity error of 0.05%. In fact, one measurement of SVM™ 3001 delivers more parameters than any other kinematic viscometer: (Kinematic viscosity - ASTM D7042, EN16896, DIN 51659-2, Density - EN ISO 12185, ASTM D4052, IP 365, Dynamic viscosity - ASTM D7042, Viscosity Index or ‘VI’ - ISO 2909, ASTM D2270, API Grades - ISO 91, API 2540, ASTM D1250, IP 200 and Saybolt viscosity - ASTM D2161)
In terms of accuracy, true value is often not exactly known and is most commonly an agreement on an accepted reference value, according to the definitions and principles set out in ISO 5725 )Accuracy (trueness and precision) of measurement methods and results). Bias to true value has not been determined for either D445 or D7042. The viscosity precision, which is comprised of the repeatability and reproducibility of the measurements, is given for different classes of compounds such as base oils, formulated oils, diesel, jet fuel etc. For large majority of robin tests where D445 and D7042 were tested on the same sample set, both repeatability and reproducibility of D7042 were better than those of D445 (Supporting data has been filed at ASTM International Headquarters and may be obtained by requesting Research Report RR: D02-1555/1742/1750/1776/1873)
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