ASTM D6377 standard test method for determination of vapor pressure of crude oil: VPCR(X) (Expansion method)
10. Calibration
10.1 Pressure Transducer:
10.1.1 Check the transducer calibration on a monthly basis or when required as indicated from the quality control checks (see Section 11). The transducer calibration is checked using two reference points: zero pressure (<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 (see 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 balance shall be carried out.
NOTE 8 - Caution: 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 platinum resistance thermometer used to monitor the measuring chamber temperature at least every six months against a thermometer that is traceable to National Institute of Standards and Technology (NIST) or to national authorities in the country in which the equipment is used.
11. Quality Control Checks
11.1 Use a verification fluid of known volatility 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. Pressurization of the verification fluid may be omitted, provided the pure compound is air saturated between 0 and 1°C, as described in Test Method D5191 (see 8.4). Follow the manufacturer's instruction for sample introduction of verification fluids. The temperature of the verification fluid shall be between 0 and 3°C during the sample introduction, and the measuring procedure shall be in accordance with Section 12 with a V/L of 4:1 and a measuring temperature of 37.8°C. If the observed vapor pressure VPCR4 (37.8°C) differs from the reference value by more than 1.0 kPa (0.15 psi), then check the instrument calibration (see Section 10).
11.2 Some possible materials and their corresponding air saturated vapor pressures at 37.8°C and a V/L of 4:1 (VPCR4) include:
NOTE 9 - The value for 2,2-dimethylbutane in this list was derived from the 1991 interlaboratory cooperative test program5 and represents the total pressure of the air saturated liquid at 37.8°C at a V/L of 4:1. The other values were determined in limited cross check programs.
NOTE 10 - It is recommended that at least one type of verification fluid used in 11.1 have a vapor pressure representative of the crude(s) 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, as per procedure described in 8.1.2. Samples should be stored in an environment suitable for long term storage without sample degradation. Analysis of result(s) from these quality control samples can be carried out using control chart techniques.
NOTE 11 - Caution: The use of single component verification materials, such as those listed in 11.2, 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 that happens with losses of light ends in multi-component mixtures.
12. Procedure
12.1 Set the sample introduction temperature of the measuring chamber between 20 and 37.8°C. For crude oil samples with a pour point higher than 15°C, set the injection temperature at least 5°C above the pour point temperature of the sample.
12.2 Set the V/L to the desired value X:1 (For test results related to Test Method D 323, set the V/L to 4:1).
12.3 Mix the sample in the pressurized floating piston cylinder vigorously with the mechanical stirrer to ensure a homogenous sample, and connect the outlet of the cylinder to the inlet of the apparatus.
12.4 Follow the manufacturer's instructions for introducing the test specimen into the measuring chamber. The volume of the specimen shall be such that after the expansion to the final volume the programmed V/L is achieved.
12.5 After closing the inlet valve, expand the volume of the measuring chamber to the final volume.
12.6 Switch-on the shaker, and leave it on during the entire measuring procedure
12.7 Adjust the temperature control to the measuring temperature (for results related to Test Method D323, adjust to a temperature of 37.8°C). The measuring temperature shall not be lower than at least 10°C above the pour point temperature of the sample.
12.8 Wait for temperature equilibrium between measuring chamber and specimen, and monitor the total vapor pressure every 30 more or less 5 s. When three successive readings agree within 0.3 kPa, record this resulting vapor pressure as VPCRx (Tm°C).