MIL-PRF-83282D
4.4.1 Bulk modulus.
4.4.1.1 Apparatus. The bulk modulus of the hydraulic fluid shall be determined using a
calibrated, precision capillary pycnometer of the type shown in figure 1 (this is a modified 21 T 50
Jerguson pressure gauge). A sample pressure vessel and auxiliary equipment for this
determination are shown in figures 2 and 3.
4.4.1.2 Procedure. The pycnometer volume to capillary diameter ratio shall be chosen to
provide a precision of measurement for liquid density of ±2 parts in 10,000. The pycnometer shall
be charged with candidate fluid to the top of the capillary at 40°C and atmospheric pressure.
Insert the pycnometer in the high-pressure pycnometer housing such that the capillary tube is
visible through the window of the pressure vessel and parallel to the outside vertical surfaces of
the pressure vessel. Assemble the pressure vessel containing the pycnometer, place it in a 40°C
constant-temperature bath, allow equilibrium to be reached, and take a volume reading at
atmospheric pressure. Since the precision of the unit depends upon visual readings, avoid errors
due to parallax and distortion in the pressure vessel window and the walls of the
constant-temperature bath. Increase nitrogen pressure to a new level and, after a 60 minute soak,
take a third reading. For any pressure range, the secant bulk modulus is defined as follows:
Bulk modulus = [(V x DP) ÷ (DV + DVg)]
Where:
V is the original volume of the fluid,
DV is the observed volume change due to DP increase in pressure,
DP is the pressure change between the two measurements in kPa, and
DVg is the correction factor, which considers the bulk modulus of glass in determining
the true volume of the pycnometers at pressures above atmospheric. The bulk
modulus of Pyrex glass is 3.28 x 107 kPa. DVg = [(V x DP) ÷ 3.28 x 107].
4.4.2 Color of finished fluid. The color of the hydraulic fluid shall be compared with a
standard sample prepared by adding 1 part of dye "Oil Red 235" to 10,000 parts of an oil not
darker than ASTM-D1500, color number 1.0 (see 6.5.2).
4.4.3 Compatibility. Samples of candidate hydraulic fluid in amounts of 20, 100 and 180 ml
shall be separately mixed with samples from each of the fluids previously qualified to this
specification, MIL-H-5606, and MIL-H-87257. Total volume of each mixture shall be 200 ml.
The mixtures of candidate fluid and fluids previously qualified to this specification shall be
agitated and then stored at 205°C for 2 hours. The mixtures of the candidate fluid and fluids
qualified to MIL-H-5606 and MIL-H-87257 shall be agitated and then stored at 135°C for
2 hours. At the end of the storage time, none of the mixtures shall show any signs of sediment,
turbidity, or crystallization. All mixtures shall then be stored at -40°C for 2 hours. After the
-40°C storage, none of the mixtures shall show signs of sediment or crystallization; however, a
slight, non-permanent turbidity is permitted.
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