ASTM D1267 Test Method for Gage Vapor Pressure of Liquefied Petroleum (LP) Gases (LP-Gas Method)
9. Procedure
9.1 Safe means for the disposal of vapors and liquids during this operation and in the subsequent sampling operation shall be provided.
9.2 Purging - With the assembled apparatus in an upright position, connect the inlet valve of the lower chamber to the sample source with the sampling connection (7.2). Open the sample source valve to the apparatus. Cautiously open the bleeder valve on the upper chamber, permitting the air or vapors, or both, in the apparatus to escape until the apparatus is full of liquid. Close the lower chamber inlet valve and open the bleeder valve to its wide open position. Allow the contained liquid to evaporate until the apparatus is covered with white frost (may require more than one chilling), then invert the assembly, and expel any residual material through the bleeder valve. Allow the residual vapors to escape until the pressure in the apparatus is essentially atmospheric, then close the bleeder valve.
9.3 Sampling - Return the apparatus, now containing only vapors, to its normal upright position and open the inlet valve. As soon as the apparatus attains essentially the same pressure as the pressure of the sample source, momentarily open the bleeder valve. If liquid does not promptly emerge, repeat the purging step (9.2). If liquid appears immediately, close the bleeder and inlet valves in that order (Note 1). Close the valve on the sample source, and disconnect the sampling line. Immediately close the straight-through valve between the two chambers and open the inlet valve, with the apparatus in an upright position. Close the inlet valve as soon as no more liquid escapes, and immediately open the straight-through valve.
NOTE 1 - Transfer of the sample is facilitated by chilling the apparatus with a portion of the material under test.
9.3.1 When using the 33 1/3 % lower chamber (A1.1.3) proceed to 9.4.
9.3.2 When using the 20 % lower chamber (Appendix A1.1.4), close the straight-through valve and again open the inlet to permit expulsion of the lower chamber contents. As soon as no more liquid escapes from the lower chamber, close the inlet valve and immediately open the straight-through valve.
9.3.3 The upper chamber, prior to this operation, is liquid full at some temperature that is normally below the environmental temperature. Since any warming of the apparatus would cause expansion of the liquid content of the upper chamber, leading to possible rupture of the chamber, it is necessary that the procedural steps of providing free space in the apparatus be completed promptly.
9.4 Vapor Pressure Determination:
9.4.1 Invert the apparatus and shake it vigorously. Return the apparatus to its normal upright position and immerse it in the constant-temperature water bath maintained at the test temperature (4.1). The apparatus including the bleeder valve coupling, but not the pressure gage, must be immersed. Throughout the determination, the temperature of the water bath shall be checked periodically by means ofthe temperature measuring device.
9.4.1.1 At test temperatures from 37.8°C (100°F) up to and including 50°C (122°F), maintain the bath at +/- 0.1°C (0.2°F). At test temperatures above 50°C (122°F), up to and including 70°C (158°F), maintain the bath at +/- 0.3°C (0.5°F).
9.4.1.2 Observe the apparatus assembly throughout the test period to ensure freedom from leaks. Discontinue the test and discard the results at any time a leak is detected.
9.4.2 After 5 min have elapsed, withdraw the apparatus from the water bath, invert it, shake it vigorously, and then return it to the bath. Perform the shaking operation quickly to avoid excessive cooling of the apparatus and its contents. Thereafter, at intervals of not less than 2 min, withdraw the apparatus from the bath, invert, shake it vigorously, and then return it to the bath. Prior to each removal of the apparatus from the water bath, tap the gage lightly and observe the pressure reading. These operations will normally require 20 to 30 min to ensure equilibrium. After this time, if consecutive observed gage readings are constant, record the pressure reading as the Uncorrected LP-Gas Vapor Pressure of the sample at the test temperature.
9.4.3 If a pressure gage is used that is not calibrated against a dead-weight tester, it is necessary to determine if a gage correction needs to be applied to the pressure gage reading. Without removing the pressure gage from the apparatus or the apparatus from the bath, attach a test gage, previously calibrated against a dead-weight tester, to the bleeder valve outlet and open the bleeder valve. At the end of 5 min, compare the readings of the two gages. Record any correction thus determined as gage correction.
9.4.3.1 Alternatively, if a pressure gage is used that has been calibrated against a dead-weight tester, then the gage correction is zero and it is not necessary to determine a gage correction as per 9.4.3, using a second test gage that has been calibrated against a dead-weight tester.
10. Calculation
10.1 Correct the Uncorrected LP-Gas Vapor Pressure for gage errors (see 9.4.3 and 9.4.3.1).
10.2 Convert the corrected vapor pressure = (test gage reading) + (gage correction) as calculated in 9.4.3 to a standard barometric pressure of 760 mm (29.92 in.) Hg by means of the following equation:
10.2.1 LP-Gas Vapor Pressure:
= corrected vapor pressure, kPa - (760 - P1) 0.1333
= corrected vapor pressure, psi - (760 - P1) 0.0193
where:
P1 = observed barometric pressure, mm Hg.
10.2.2 LP-Gas Vapor Pressure:
= corrected vapor pressure, kPa - (29.92 - P2) 3.3864
= corrected vapor pressure, psi - (29.92 - P2) 0.4912
where:
P2 = observed barometric pressure, in. Hg.
10.2.3 Conversion Factors:
1 kPa = 7.50064 mmHg = 0.295301 in. Hg
1 psi = 51.7151 mmHg = 2.03603 in. Hg
