ASTM D3829 Predicting the Borderline Pumping Temperature of Engine Oil
11. Calculations and Determination of BPT
11.1 Yield Stress Calculation and Air Binding BPT:
11.1.1 Calculate the yield stress of the oil with the following equation:
Ys = 3.5 M
where:
Ys = yield stress, mPa, and
M = applied mass, g, at which rotation was observed.
11.1.2 If rotation was observed for the 30 g mass, yield stress will be less than 105 Pa and can be assumed to be zero for purposes of this test method.
11.1.3 If a mass greater than 30 g was needed to start rotation, calculate the shear stress for the largest mass that did not cause rotation, using Eq 9. For purposes of this work, the shear stress calculated in this way is designated the yield stress.
11.1.4 The BPT at which failure to pump by "air binding" of the pump occurs at a critical yield stress of 105 Pa, which corresponds to a 30 g mass.
11.1.4.1 A plot of yield stress versus temperature may be constructed so as to extrapolate or interpolate to 105 Pa for the purpose of estimating the temperature at which to repeat Section 10.
11.1.5 The temperature rounded to the nearest 0.5 °C corresponding to a yield stress of 105 Pa is the BPT for air binding failure mode.
11.2 Viscosity Calculation and Viscosity BPT:
11.2.1 Calculate the apparent viscosity of the oil at each temperature with the following equation:
ηa = C•t•3/r
where:
ηa = apparent viscosity, mPa•s,
C = cell constant obtained in Eq 8
t = time in seconds for number (r) of complete revolutions of the rotor in 10.6.3.6, and
r = number of revolutions timed in 10.6.3.6; either one or three revolutions.
11.2.2 For BPT, from a plot of apparent viscosity versus temperature, interpolate to determine the temperature at an apparent viscosity of 30 Pa•s. This temperature is the BPT for a "flow-limiting" failure mode. Do not extrapolate data; if necessary, repeat Section 10 at another temperature.
11.3 Select the higher of the two temperatures in 11.1.5 and 11.2.2 rounded to the nearest 0.5 °C and the mode of failure (flow-limited or air-binding) as the borderline pumping temperature.
