ASTM D525 Test Method for Oxidation Stability of Gasoline (Induction Period Method)
10. Procedure
10.1 Bring the pressure vessel and the gasoline to be tested to a temperature of 15 to 25°C. Place the glass sample container in the pressure vessel and add 50 +/- 1 mL of test specimen. Alternatively, transfer 50 +/- 1 mL of test specimen into the glass sample container first, before placing the glass sample container into the pressure vessel. Cover the sample container, close the pressure vessel, and using a quick release air coupling introduce oxygen until a pressure of 690 to 705 kPa is attained. Allow the gas in the bomb to escape slowly in order to flush out the air originally present (Release the pressure at a slow uniform rate through the needle valve at a rate not to exceed 345 kPa per min). Introduce oxygen again until a pressure of 690 to 705 kPa is attained and observe for leaks, ignoring an initial rapid drop in pressure (generally not over 40 kPa) which may be observed because of the solution of oxygen in the sample. When the rate of pressure drop does not exceed 7 kPa in 10 min, assume the absence of leaks and proceed with the test without repressuring.
10.2 Place the charged pressure vessel in the vigorously boiling water bath, or appropriate liquid bath equipped with mechanical stirring, being careful to avoid shaking, and record the time of immersion as the starting time. Maintain the temperature of the liquid bath between 98 and 102°C. Observe the temperature to the nearest 0.1°C at intervals during the test, and record the average temperature to the nearest 0.1°C as the temperature of the test. Make a continuous record of the pressures in the pressure vessel, or if an indicating pressure gage is used, take pressure readings at 15-min intervals or less. If, during the initial 30 min of the test, a leak develops (as indicated by a steady drop in pressure considerably in excess of 14 kPa in 15 min) discard the test. Continue the test until reaching a point preceded by a pressure drop of exactly 14 kPa in 15 min and succeeded by a drop of not less than 14 kPa in 15 min or until the induction period exceeds the product specification.
NOTE 7 - Caution: If a boiling water bath is used and the test is made in a region where the atmospheric pressure is consistently below normal (101.3 kPa), it is permissible to add a liquid with higher boiling point, such as ethylene glycol, to the water in order to maintain the operating temperature of the bath in the neighborhood of 100°C. If a liquid other than water is used, be sure the liquid is compatible with the seals in the pressure vessel.
10.3 Record the number of minutes from the time the pressure vessel is placed in the bath until the breakpoint has been reached as the observed induction period at the temperature of the test.
10.4 Cool the pressure vessel to approximately room temperature within 30 min after removal from the bath, using ambient air or water not more than 35°C, before releasing the pressure slowly from the pressure vessel through the needle valve at a rate not exceeding 345 kPa per minute. Wash the pressure vessel and sample container in preparation for the next test.
11. Calculation
11.1 The number of minutes from the time the pressure vessel is placed in the bath until the breakpoint has been reached is the observed induction period at the temperature of the test.
11.2 Method of Calculation - Calculate the induction period at 100°C from one of the following equations: (a) When the test temperature is above 100°C:
Induction period at 100°C = (IPt)(1 + 0.101(ta - 100))
(b) When the test temperature is below 100°C:
Induction period at 100°C, min = (IPt)/(1 + 0.101(100 - tb))
where:
IPt = induction period, min, at the temperature of the test,
ta = test temperature when above 100°C, °C, and
tb = test temperature when below 100°C, °C.
