FED-STD-791D
Method 2508.1
September 30, 1986
THERMAL STABILITY OF LUBRICATING AND HYDRAULIC FLUIDS
1. SCOPE
1.1 This method describes a procedure for determining the thermal stability of fluids.
2. SUMMARY
2.1 In this method, the volatile decomposition products are in continuous contact with
the fluid during test. To eliminate possible catalytic metal effects, the fluid is confined in a glass
apparatus. To remove the variables of oxidation and hydrolysis, air and moisture are removed by
heating and shaking under vacuum prior to the initiation of the test. This method will not
measure the thermal stability threshold (the temperature at which volatile oil fragments begin to
form), but will indicate bulk fragmentation occurring at a specified temperature and testing
period.
3. SAMPLE
3.1 Approximately 20 mL of the lubricant to be tested.
4. REFERENCES, STANDARDS, AND APPARATUS
4.1 Test cell. A borosilicate glass test cell having the dimensions given in Figure 1 is
recommended as the sample container. Any heavy walled borosilicate glass closed end tube of
the proper O.D. having sufficient capacity to hold 20 mL of fluid and of sufficient length to
permit heating and drawing the tube to a sealed end reasonably above the fluid level should be
satisfactory.
4.2 Heater. Aluminum block with at least two openings of the proper diameter
(approximately 2 cm) and depth (approximately 21 cm) so that the sealed test cell is completely
immersed in the heater. A suitable heating bath may be used in place of the aluminum block
heater if desired. The temperature of either the aluminum block or the bath shall be capable of
being controlled within 1C (2F) at the test temperature. The temperature gradient between
fluid sample and the aluminum block or bath shall be kept at a minimum (less than 1C [2F] is
recommended).
4.3 Test cell holder. When a heating bath is used a heavy walled steel sample holder of
the proper size to contain the test cell shall be provided for safety reasons.
4.4 Becton, Dickinson and Co. Yale Luer-Lok (20 YL) or equal 20 mL hypodermic
syringe equipped with a No. 14 square tip needle approximately 21 cm long.
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