ASTM D5191 Test Method for Vapor Pressure of Petroleum Products (Mini Method)
6. Apparatus
6.1 Vapor Pressure Apparatus - The type of apparatus suitable for use in this test method employs a small volume test chamber incorporating a transducer for pressure measurements and associated equipment for thermostatically controlling the chamber temperature and for evacuating the test chamber prior to sample introduction.
6.1.1 The test chamber shall be designed to contain between 5 and 50 mL of liquid and vapor and be capable of maintaining a vapor-to-liquid ratio between 3.95 to 1.00 and 4.05 to 1.00.

NOTE 3 - The test chamber employed by the instruments used in generating the precision and bias statements were constructed of stainless steel or aluminum.

NOTE 4 - Test chambers exceeding a 15 mL capacity can be used, but the precision and bias statements (see Section 15) are not known to apply.

6.1.2 The pressure transducer shall have a minimum operational range from 0 to 177 kPa (0 to 25.7 psi) with a minimum resolution of 0.1 kPa (0.01 psi) and a minimum accuracy of more or less 0.8 kPa (more or less 0.12 psi). The pressure measurement system shall include associated electronics and readout devices to display the resulting pressure reading.

6.1.3 A thermostatically controlled heater shall be used to maintain the test chamber at 37.8 more or less 0.1°C (100 more or less 0.2°F) for the duration of the vapor pressure measurement.

6.1.4 A platinum resistance thermometer shall be used for measuring the temperature of the test chamber. The minimum temperature range of the measuring device shall be from ambient to 75°C (167°F) with a resolution of 0.1°C (0.2°F) and an accuracy of 0.1°C (0.2°F).

6.1.5 The vapor pressure apparatus shall have provisions for introduction of the test specimen into an evacuated test chamber and for the cleaning or purging of the chamber following the test.

6.2 Vacuum Pump, capable of reducing the pressure in the test chamber to less than 0.01 kPa (0.001 psi) absolute.

6.3 Syringe, (optional, depending on sample introduction mechanism employed with each instrument) gas-tight, 1 to 20 mL capacity with a more or less 1 % or better accuracy and a more or less 1 % or better precision. The capacity of the syringe should not exceed two times the volume of the test specimen being dispensed.

6.4 Iced Water Bath or Air Bath, for chilling the samples and syringe to temperatures between 0 to 1°C (32 to 34°F).

6.5 Pressure Measuring Device, capable of measuring local station pressure with an accuracy of 0.20 kPa (0.03 psi), or better, at the same elevation relative to sea level as the apparatus in the laboratory.
6.5.1 When a mercury manometer is not used as the pressure measuring device, the calibration of the pressure measuring device employed shall be periodically checked (with traceability to a nationally recognized standard) to ensure that the device remains within the required accuracy specified in 6.5.

6.6 McLeod Vacuum Gage or Calibrated Electronic Vacuum Measuring Device for Calibration, to cover at least the range from 0.01 to 0.67 kPa (0.1 to 5 mm Hg). The calibration of the electronic vacuum measuring device shall be regularly verified in accordance with the annex section on Vacuum Sensors (A6.3) of Test Method D2892.

7. Reagents and Materials
7.1 Purity of Reagents - Use chemicals of at least 99 % purity for quality control checks (see Section 11). Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Lower purities can be used, provided it is first ascertained that the reagent is of sufficient purity to permit its use without lessening the accuracy of the determination.

NOTE 5 - The chemicals in this section are suggested for use in quality control procedures (see 11.2) and are not used for instrument calibration.

7.2 Cyclohexane, (Warning - Cyclohexane is flammable and a health hazard).

7.3 Cyclopentane, (Warning - Cyclopentane is flammable and a health hazard).

7.4 2,2-Dimethylbutane, (Warning - 2,2-dimethylbutane is flammable and a health hazard).

7.5 2,3-Dimethylbutane, (Warning - 2,3-dimethylbutane is flammable and a health hazard).

7.6 2-Methylpentane, (Warning - 2-methylpentane is flammable and a health hazard).

7.7 Toluene, (Warning - Toluene is flammable and a health hazard).

8. Sampling
8.1 General Requirements:
8.1.1 The extreme sensitivity of vapor pressure measurements to losses through evaporation and the resulting changes in composition is such as to require the utmost precaution and the most meticulous care in the drawing and handling of samples.

8.1.2 Obtain a sample and test specimen in accordance with Practice D4057, except do not use the "Sampling by Water Displacement" section for fuels containing oxygenates. Use a 1 L (1 qt) sized container filled between 70 and 80 with sample.

NOTE 6 - The present precision statement was derived using samples in 1 L (1 qt) containers. However, samples in containers of other sizes, as prescribed in Practice D4057, can be used with the same ullage requirement if it is recognized that the precision can be affected.

8.1.3 In the case of referee testing, the 1 L (1 qt) sample container is mandatory.

8.1.4 Perform the vapor pressure determination on the first test specimen withdrawn from a sample container. Do not use the remaining sample in the container for a second vapor pressure determination. If a second determination is necessary, obtain a new sample.

8.1.5 Protect samples from excessive temperatures prior to testing. This can be accomplished by storage in an appropriate ice bath or refrigerator.

8.1.6 Do not test samples stored in leaky containers. Discard and obtain a new sample if leaks are detected.

8.2 Sampling Handling Temperature - Cool the sample container and contents in an ice bath or refrigerator to the 0 to 1°C (32 to 34°F) range prior to opening the sample container. Allow sufficient time to reach this temperature. Verify the sample temperature by direct measurement of the temperature of a similar liquid in a similar container placed in the cooling bath or refrigerator at the same time as the sample.

8.3 Verification of Sample Container Filling - With the sample at a temperature of 0 to 1°C, take the container from the cooling bath or refrigerator, and wipe dry with absorbent material. If the container is not transparent, unseal it and using a suitable gage, confirm that the sample volume equals 70 to 80 % of the container capacity (see Note 7). If the sample is contained in a transparent glass container, verify that the container is 70 to 80 % full by suitable means (see Note 7).
8.3.1 Discard the sample if the container is filled to less than 70 %, by volume, of the container capacity.

8.3.2 If the container is more than 80 % by volume full, pour out enough sample to bring the container contents within the 70 to 80 % by volume range. Do not return any sample to the container once it has been withdrawn.

8.3.3 Reseal the container if necessary, and return the sample container to the cooling bath or refrigerator.

NOTE 7 - For non-transparent containers, one way to confirm that the sample volume equals 70 to 80 % of the container capacity is to use a dipstick that has been pre-marked to indicate the 70 and 80 % container capacities. The dipstick should be of such material that it shows wetting after being immersed and withdrawn from the sample. To confirm the sample volume, insert the dipstick into the sample container so that it touches the bottom of the container at a perpendicular angle, before removing the dipstick. For transparent containers, using a marked ruler or by comparing the sample container to a like container which has the 70 and 80 % levels clearly marked, has been found suitable.

8.4 Air Saturation of the Sample in the Sample Container:
8.4.1 Transparent Containers Only - Since 8.3 does not require that the sample container be opened to verify the sample capacity, it is necessary to unseal the cap momentarily before resealing it, so that samples in transparent containers are treated the same as samples in non-transparent containers.

8.4.2 With the sample again at a temperature of 0 to 1°C, take the container from the cooling bath or refrigerator, wipe it dry with an absorbent material, remove the cap momentarily, taking care that no water enters, reseal and shake vigorously. Return it to the cooling bath or refrigerator for a minimum of 2 min.

8.4.3 Repeat 8.4.2 twice more. Return the sample to the cooling bath or refrigerator until the beginning of the procedure.

8.5 Verification of Single Phase Samples - After drawing a test specimen and injecting it into the instrument for analysis, check the remaining sample for phase separation. If the sample is contained in a transparent container, this observation can be made prior to sample transfer. If the sample is contained in a non-transparent container, mix the sample thoroughly and immediately pour a portion of the remaining sample into a clear glass container and observe for evidence of phase separation. A hazy appearance is to be carefully distinguished from separation into two distinct phases. The hazy appearance shall not be considered grounds for rejection of the fuel. If a second phase is observed, discard the test and the sample. Hazy samples may be analyzed (see Section 14).