ASTM D240 for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter
9. Procedure
9.1 Weight of Sample - Control the weight of sample (including any auxiliary fuel) so that the temperature rise produced by its combustion will be equal to that of 0.9 to 1.1 g of benzoic acid (Note 2). Weigh the sample to the nearest 0.1 mg.
NOTE 2 - If the approximate heat of combustion of the sample is known, the required weight can be estimated as follows:
g = 26.454/Qs
where:
g = mass of sample, g, and
Qs = MJ/kg.
Some fuels contain water and particulate matter (ash) that will degrade calorimetric values. If the heat of combustion is required on a clean fuel, filter the sample to remove free water and insoluble ash before testing.
9.1.1 For highly volatile fluids, reduce loss with use of tape or gelatin capsule mineral oil.
NOTE 3 - Acceptable procedures for handling volatile liquids include those described in the reports referenced at the end of this test method. References (1-6) describe glass sample holders: (7) describes a metal sample holder: (8) describes a gelatin sample holder.
9.1.2 Tape - Place a piece of pressure-sensitive tape across the top of the cup, trim around the edge with a razor blade, and seal tightly. Place 3 by 12-mm strip of tape creased in the middle and sealed by one edge in the center of the tape disk to give a flap arrangement. Weigh the cup and tape. Remove from the balance with forceps. Fill a hypodermic syringe with the sample. The volume of sample can be estimated as follows:
V = (W x 0.00032)/(Q x D)
where:
V = volume of sample to be used, mL,
W = energy equivalent of calorimeter, J/°C,
Q = approximate heat of combustion of the sample, MJ/kg, and
D = density, kg/m3, of the sample.
9.1.2.1 Add the sample to the cup by inserting the tip of the needle through the tape disk at a point so that the flap of tape will cover the puncture upon removal of the needle. Seal down the flap by pressing lightly with a metal spatula. Reweigh the cup with the tape and sample. Take care throughout the weighing and filling operation to avoid contacting the tape or cup with bare fingers. Place the cup in the curved electrode and arrange the fuse wire so that the central portion of the loop presses down on the center of the tape disk.
9.1.3 Gelatin/Mineral Oil - Weigh the cup and gelatin capsule. The capsule should only be handled with forceps. Add the sample to the capsule. Reweigh the cup with capsule and sample. If poor combustion is expected with the capsule, add several drops of mineral oil on the capsule and reweigh the cup and contents. Place the cup in the curved electrode and arrange the fuse wire so that the central portion of the loop contacts the capsule and oil.
9.2 Water in Bomb - Add 1.0 mL of water to the bomb from a pipet.
9.3 Oxygen - With the test sample and fuse in place, slowly charge the bomb with oxygen to 3.0-MPa (30-atm) gauge pressure at room temperature (9.3.1). Do not purge the bomb to remove entrapped air (Warning - Be careful not to overcharge the bomb. If, by accident, the oxygen introduced into the bomb should exceed 4.0 MPa, do not proceed with the combustion. An explosion might occur with possible violent rupture of the bomb. Detach the filling connection and exhaust the bomb in the usual manner. Discard the sample, unless it has lost no weight, as shown by reweighing.).
9.3.1 Lower or higher initial oxygen pressures can be used within the range from 2.5 to 3.5 MPa, provided the same pressure is used for all tests, including standardization.
9.4 Calorimeter Water - Adjust the calorimeter water temperature before weighing as follows:
Isothermal jacket method: 1.6 to 2.0°C below jacket temperature
Adiabatic jacket method: 1.0 to 1.4°C below room temperature
This initial adjustment will ensure a final temperature slightly above that of the jacket for calorimeters having an energy equivalent of approximately 10.2 kJ/°C. Some operators prefer a lower initial temperature so that the final temperature is slightly below that of the jacket. This procedure is acceptable, provided it is used in all tests, including standardization.
9.4.1 Use the same amount (+/-0.5 g) of distilled or deionized water in the calorimeter vessel for each test. The amount of water (2000 g is usual) can be most satisfactorily determined by weighing the calorimeter vessel and water together on a balance. The water can be measured volumetrically if it is measured always at the same temperature.
9.5 Observations, Isothermal Jacket Method - Assemble the calorimeter in the jacket and start the stirrer. Allow 5 min for attainment of equilibrium, then record the calorimeter temperatures (Note 4) at 1-min intervals for 5 min. Fire the charge at the start of the sixth minute and record the time and temperature, ta. Add to this temperature 60 % of the expected temperature rise, and record the time at which the 60 % point is reached (Note 5). After the rapid rise period (about 4 to 5 min), record temperatures at 1-min intervals on the minute until the difference between successive readings has been constant for 5 min.
NOTE 4 - Use a magnifier and estimate all readings (except those during the rapid rise period) to the nearest 0.002°C when using ASTM Bomb Calorimeter Thermometer 56C. Estimate Beckmann thermometer readings to the nearest 0.001°C and 25-V resistance thermometer readings to the nearest 0.0001 V. Tap liquid thermometers with a pencil just before reading to avoid errors caused by the liquid sticking to the walls of the capillary.
NOTE 5 - When the approximate expected rise is unknown, the time at which the temperature reaches 60 % of the total can be determined by recording temperatures at 45, 60, 75, 90, and 105 s after firing and interpolating.
9.6 Observations, Adiabatic Jacket Method (Note 6) - Assemble the calorimeter in the jacket and start the stirrers. Adjust the jacket temperature to be equal to or slightly lower than the calorimeter, and run for 5 min to obtain equilibrium. Adjust the jacket temperature to match the calorimeter within +/-0.01°C and hold for 3 min. Record the initial temperature and fire the charge. Adjust the jacket temperature to match that of the calorimeter during the period of rise, keeping the two temperatures as nearly equal as possible during the rapid rise, and adjusting to within +/-0.01°C when approaching the final equilibrium temperature. Take calorimeter readings at 1-min intervals until the same temperature is observed in three successive readings. Record this as the final temperature. Time intervals are not recorded as they are not critical in the adiabatic method.
NOTE 6 - These instructions supersede the instructions given in 9.5 when using jackets equipped for adiabatic temperature control.
9.7 Analysis of Bomb Contents - Remove the bomb and release the pressure at a uniform rate such that the operation will require not less than 1 min. Examine the bomb interior for evidence of incomplete combustion. Discard the test if unburned sample or sooty deposits are found.
9.7.1 Wash the interior of the bomb, including the electrodes and sample holder, with a fine jet of water and quantitatively collect the washings in a beaker. Use a minimum of wash water, preferably less than 350 mL. Titrate the washings with standard alkali solution, using methyl orange or methyl red indicator.
9.7.2 Remove and measure the combined pieces of unburned firing wire, and subtract from the original length. Record the difference as wire consumed.
9.7.3 Determine the sulfur content of the sample if it exceeds 0.1 %. Determine sulfur by analyzing the bomb washings remaining after the acid titration, using the procedure described in Test Methods D129, D1266, D2622, D3120, D4294, or D5453.
