ASTM D5191 Test Method for Vapor Pressure of Petroleum Products (Mini Method)
9. Preparation of Apparatus
9.1 Prepare the instrument for operation in accordance with the manufacturer's instructions.
9.2 Clean and dry the test chamber as required to avoid contamination of the test specimen. Prior to sample introduction, visually determine from the instrument display that the test chamber pressure is stable and does not exceed 0.1 kPa (0.01 psi). When the pressure is not stable or exceeds this value, check that the chamber is clean of volatile materials remaining in the chamber from a previous sample or check the calibration of the transducer.
9.3 If a syringe is used for introduction of the sample specimen, chill it to between 0 and 4.5°C (32 and 40°F) in a refrigerator or ice bath before drawing in the sample. Avoid water contamination of the syringe reservoir by sealing the outlet of the syringe during the cooling process.
10. Calibration
10.1 Pressure Transducer:
10.1.1 Perform a calibration check of the transducer at least once every six months, or sooner, as indicated from the quality control checks (see Section 11). The calibration of the transducer 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 test chamber (Note 8). Apply vacuum to the test chamber. When the vacuum measuring device registers a pressure less than 0.1 kPa (0.8 mm Hg), adjust the indicator to zero or to the actual reading on the vacuum measuring device as dictated by the instrument design or manufacturer's instructions.
NOTE 8 - Refer to the annex section on Vacuum Sensors (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 test chamber of the apparatus to atmosphere 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 against the value obtained from the pressure measuring device, 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. (Warning - Many aneroid barometers, such as those used at weather stations and airports, are pre-corrected to give sea level readings; these must not be used for calibration of the apparatus.)
10.1.3.1 For mercury barometers used as the pressure measuring device, the barometric pressure reading shall be corrected for the change in the density of the mercury column between 0°C and the operating temperature and converted to the same units of pressure as the vapor pressure apparatus display. After making the density correction, the conversion for the height of a mercury column at 0°C to kPa or psia is made as follows: 1 in. (25 mm) Hg at 0°C = 3.3865 kPa or = 0.49116 psia.
10.1.4 Repeat 10.1.2 and 10.1.3 until the zero and barometric pressures read correctly without further adjustments.
10.2 Thermometer - Check the calibration of the platinum resistance thermometer used to monitor the temperature of the test chamber at least every six months against a National Institute of Standards and Technology (NIST) traceable thermometer.
11. Quality Control Checks
11.1 Use a verification fluid with known volatility as an independent check against the instrument calibration each day the instrument is in use. For pure compounds (see 7.1), multiple test specimens may be taken from the same container over time, provided the pure compound test specimen is air saturated according to the procedure given in 8.4, and the spent test specimens are not re-used, in whole or in part. Record the dry vapor pressure equivalent value, and compare this to the statistical value of the control sample from your laboratory. If the difference exceeds your control limits, check the calibration of the instrument.
11.2 Some possible materials and their corresponding vapor pressures, as found in ASTM DS 4B, include: cyclopentane 68.3 kPa (9.91 psi); 2,2-dimethylbutane 68.0 kPa (9.86 psi); 2,3-dimethylbutane 51.1 kPa (7.41 psi); 2-methylpentane 46.7 kPa (6.77 psi); and toluene 7.1 kPa (1.03 psi).
NOTE 9 - It is recommended that at least one type of control sample used in 11.1 be representative of the fuel(s) regularly tested by the laboratory. The total 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.
