Important Consideration in Viscosity Measurement of Jet and Diesel Fuels According to ASTM D1655 and ASTM D975: Direct Determination Versus Indirect Calculation

Cannon Instruments (USA) - Specifications for ASTM D1655 (jet fuel) and D975 (diesel) both reference kinematic viscosity limits.  ASTM D445, the stated referee method, provides a direct determination of kinematic viscosity relying on the constancy of gravity as the driving force. ASTM D7042 merely provides an indirect calculation of kinematic viscosity – requiring both a rotational viscometer and density meter. While D7042 manufacturers claim “precision equivalent to D445” with faster run times, it is important to understand the details behind these claims to make an informed decision when considering the use of either D7042 or D445. 

Precision, a measure of the degree of scatter between data points, is an important consideration in any test method. Equally important is the accuracy of that data. Unlike precision, accuracy refers to the data’s proximity to a reference value. ASTM round-robin studies demonstrate that automated D445 accuracy is far superior to data obtained either manually or through use of D7042. Precision without accuracy simply means that results are “consistently” off-target.        

D7042 also promises “up to 4 times faster” measurement as compared to D445.  While this claim is true when comparing to manual D445 methods, it is not valid when comparing to modern, automated D445 viscometers which also provide up to 4 times faster cycle times. As a result, time savings can be recognized without having to rely on “calculated” kinematic viscosity values that require data bias corrections. Automated D445 delivers accurate direct kinematic viscosity results in a timeframe equivalent to the “calculated” results obtained from the indirect D7042 method.  /-