ASTM D7621 Standard Test Method for Determination of Hydrogen Sulfide in Fuel Oils by Rapid Liquid Phase Extraction
10. Calibration and Standardization
10.1 Ensure that all of the manufacturer's instructions for verification and calibration of the mechanical and electronic systems, and operation of the apparatus are followed. Calibration functions are separated from the usual operator controls and are all implemented electronically using the integral display and keyboard.
10.2.1 Air Flow Rate - Verify that the air flow rate is 375 mL/min +/- 55 mL/min, at least once a month using a suitable calibrated flow meter connected to the air output connection. If the flow rate is incorrect, recalibrate the flow (see 10.3.1).
10.2.2 Test Vessel Heater Jacket - Verify that the temperature of the heater jacket is 60.0 °C +/- 1.0 °C, at least every six months, by inserting a calibrated temperature sensor into the heater jacket. If the temperature is incorrect, recalibrate (see 10.3.2).
10.2.3 H2S Detector - Verify the performance of the detector at least every month, or when a new detector is fitted, at zero using air, and at a nominal 25 µmol/mol (ppm v/v) vapor concentration level using pressurized nitrogen (see 7.2) containing a certified level of H2S. This verification shall be carried out at ambient temperature. If the measurement is not within 10 % of the certified value of the vapor concentration, replace the moisture filter and check the cleanliness of the liquid trap, test vessel and associated tubing. Purge the system with air for 30 min and attempt the verification again. If the measurement is still outside the allowed tolerance follow the manufacturer's instructions in recalibrating or replacing the detector.
10.2.4 Vapor Phase Processor - Verify that the temperature of the filter cartridge housing is -20.0 °C +/- 2.0 °C at least every 6 months. If the temperature is outside the allowed tolerance carry out a calibration of the vapor phase processor cooler temperature.
10.2.5 The overall correct operation of the apparatus shall be checked at least every three months, following maintenance, and when a new detector is fitted, by measurement of a solution of hydrogen sulfide (7.2.2).
10.2.5.1 Stabilized verification solutions are available by means of participation in a proficiency testing scheme.
10.2.5.2 Non-stabilized verification solutions, derived within the testing lab using local procedures, may be used.
NOTE 6 - Non-stabilized verification solutions should be standardized by titration before use and have a very short life of only a few hours.
10.2.5.3 For commercially supplied verification solutions, if the result is not within R/√2 of the accepted reference value, check the validity date. If the validity date has not expired, carry out checks as described in 10.2.5.5. For verification solutions from a proficiency testing scheme (PTS), if the result is not within R/√2 of the consensus or accepted reference value, carry out checks as described in 10.2.5.4.
10.2.5.4 For non-stabilized verification solutions, if the result is not within R/√2 of the calculated value, then make a new batch and repeat the test.
NOTE 7 - This calculation assumes that the uncertainty in the ARV for the commercially supplied verification solution, or consensus value from the PTS, is negligible relative to R/√2. Users are advised to validate this assumption.
10.2.5.5 If it is not possible to meet the criteria in 10.2.5.3 or 10.2.5.4, check the air flow rate (10.2.1) and validate the H2S sensor (10.2.3). If it is still not possible to meet the criteria in 10.2.5 or 10.2.5.4, then follow the manufacturer's instructions regarding fault finding and calibration.
10.3.1 Air Flow Rate - The air flow adjustment is implemented electronically (see 10.1).
10.3.2 Test Vessel Heater Jacket - The temperature adjustment is implemented electronically (see 10.1).
10.3.3 H2S Detector - The detector is factory calibrated using the reference material (see 7.2) (static calibration) and using factory calibration liquids with known mg/kg liquid concentrations of H2S (dynamic calibration). This calibration information is held digitally on the detector assembly and is read directly by the computer in the apparatus. The calibration enables results in mg/kg to be calculated from the test results. The dynamic calibration is a function of the detector and apparatus, and has a fixed value.
10.3.3.1 The static calibration is implemented electronically following a verification (see 10.2.3).
10.3.4 Vapor Phase Processor Temperature - Follow the manufacturer's instructions to calibrate the temperature of the filter cartridge housing.