ASTM D6897 Standard Test Method for Vapor Pressure of Liquefied Petroleum Gases (LPG) (Expansion Method)
8. Sampling and Sample Introduction
8.1 Samples shall be obtained and stored in accordance with Practice D1265, or IP 181, unless the test samples can be taken directly from the source of the material to be tested. Use a container of not less than 100 mL in size and filled at least 70 % with sample up to a maximum fill density as specified in regulations, which is typically no more than 80 %. Consult the specified regulations for more details.
8.2 Any method of coupling the vapor pressure apparatus to the sample source can be employed. Tubing, 3 mm to 7 mm in diameter, of suitable-working pressure, and made of material corrosion-resistant to the products being sampled, is satisfactory for this purpose. A flexible transparent polyperfluoro-alkoxyethylene (PFA) tubing greatly facilitates the purging and sampling operations.
8.3 Practice D1265 cylinders shall be equilibrated above the sample cell temperature of 5 °C to ensure sufficient sample cylinder pressure to fill the sample cell entirely. Practice D3700 cylinders shall be maintained at a pressure above the vapor pressure ofthe LPG at 5 °C (approximately 655 kPa for special duty propane and approximately 175 kPa for commercial butanes).
9. Preparation of Apparatus
9.1 Prepare the instrument for operation in accordance with the manufacturer's instructions. Since LPG is discharged at the outlet of the apparatus, connect a tubing to the outlet and lead the other end of the tubing to a safe exhaust system or a gas recovery system to protect the environment from LPG.
9.2 If contaminated, clean the measuring chamber with a solvent. Acetone has been used successfully. Cleaning is performed by drawing the solvent into the chamber by the integrated piston and expelling the solvent into a waste container.
10. Calibration
10.1 Pressure Transducer:
10.1.1 Check the calibration of the transducer when needed as indicated from the quality control checks performed according to Section 11. The calibration of the transducer is checked using two reference points, zero pressure (that is, <0.1 kPa) and the ambient barometric pressure.
10.1.2 Connect a McLeod gage or a calibrated electronic vacuum-measuring device to the vacuum source in line with the measuring chamber (Note 7). Apply vacuum to the measuring chamber. When the vacuum measuring device registers a pressure less than 0.1 kPa (0.8 mm Hg), adjust the transducer control to zero or to the actual reading on the vacuum measuring device as dictated by the instrument design or manufacturer's instructions.
NOTE 7 - Refer to Annex A6.3 of Test Method D2892 for further details concerning the calibration of electronic vacuum measuring devices and proper maintenance of McLeod gages.
10.1.3 Open the measuring chamber of the apparatus to atmospheric pressure and observe the corresponding pressure value of the transducer. Ensure that the apparatus is set to display the total pressure and not a calculated or corrected value. Compare this pressure value with the pressure obtained from a pressure-measuring device (see 6.4), as the pressure reference standard. The pressure-measuring device shall measure the local station pressure at the same elevation as the apparatus in the laboratory, at the time of pressure comparison. When the instrument is used over the full pressure range, a calibration with a dead-weight tester shall be carried out. (Warning - Many aneroid barometers, such as those used at weather stations and airports, are pre-corrected to give sea level readings. These shall not be used for calibration of the apparatus.)
10.1.4 Repeat 10.1.2 and 10.1.3 until the zero and barometric pressures read correctly without further adjustments.
10.2 Temperature Sensor - Verify the calibration of the resistance thermometer or equivalent (see 6.1.4) used to monitor the measuring chamber temperature when needed as indicated from the quality control checks performed according to Section 11 against a temperature sensing device which is traceable to National Institute of Standards and Technology (NIST) or national authorities in the country the equipment is used.
11. Quality Control Checks
11.1 Use a verification fluid or gas of known vapor pressure as an independent check against the instrument calibration each day the instrument is in use. For pure compounds, multiple test specimens may be taken from the same container over time.
11.2 A possible pure gas for verification of the instrument and its corresponding vapor pressure at 37.8 °C and a vapor to liquid ratio of 0.5:1 is:
Propane VPtot(37.8 °C) = 1301 kPa
1.2.1 If the observed total pressure differs from the reference value by more than 7.0 kPa, check the instrument calibration (see Section 10).
11.3 A second possible pure gas for verification of the instrument and its corresponding vapor pressure at 37.8 °C and a vapor to liquid ratio of 0.5:1 is:
Butane VPtot (37.8 °C) = 356.5 kPa
11.3.1 If the observed total pressure differs from the reference value by more than 6.0 kPa, then check the instrument calibration (see Section 10).
11.4 If a liquid is used to check the performance of the test, cool and air saturate the liquid according to the corresponding sections in the sample preparation procedure of Test Method D5191.
11.5 A possible liquid for verification of the instrument and its corresponding vapor pressure at 70 °C and a vapor to liquid ratio of 0.5:1, representing 95 % confidence of at least 90 % coverage, is:
Pentane VPtot (70 °C) = 316 kPa +/- 4 kpa
11.5.1 If the observed total pressure falls outside the range specified in 11.5, check the instrument calibration (see Section 10). (Warning - The values given in 11.2, 11.3, and 11.5 for the vapor pressures of propane, butane, and pentane are the total pressure values. If the instrument reading corresponds to the automatically corrected LPG vapor pressure relative to atmospheric pressure (see 13.2), add 101.3 kPa to the value displayed by the instrument before comparing to the above values for pure compounds.) (Warning - The use of single component verification materials such as listed in 11.2, 11.3, and 11.5 will only prove the calibration of the equipment. It will not check the accuracy of the entire test method, including sample handling, because losses due to evaporation will not decrease the sample vapor pressure for single component materials as happens with losses of light ends in multi-component mixtures.)
NOTE 8 - The value for pentane was derived from the 2015 interlaboratory cooperative test program and represents the total pressure value of the air saturated liquid at the specified temperature.
NOTE 9 - It is recommended that at least one type of verification fluid or gas used have a vapor pressure representative of the samples regularly tested by the equipment. The vapor pressure measurement process (including operator technique) can be checked periodically by performing this test method on previously prepared samples from one batch of product, in accordance with the procedure described in Section 12. Samples should be stored in an environment suitable for long term storage without significant sample degradation. Analysis of result(s) from these quality control samples can be carried out using control chart techniques, such as those outlined in Practice D6299.
