ASTM D1657 Test Method for Density or Relative Density of Light Hydrocarbons by Pressure Hydrometer
8. Sampling
8.1 Unless otherwise specified, samples of liquid hydrocarbons shall be obtained by the procedures described in Practice D1265. The procedure for sampling for verification of the apparatus and for subsequent testing is described as follows.
8.1.1 Connect the source of supply of the liquid to be tested to the inlet valve by suitable fittings so that a representative sample can be introduced into the cylinder. Ascertain that these connections are free of leaks. Open the outlet and vent valves and purge the sampling connections by opening the inlet valve slightly, permitting the product to flow through the outlet valve at the bottom of the cylinder and the vent valve at the top of the cylinder.
8.1.2 When the connections have been purged, close the outlet and vent valves and open the inlet valve, permitting the liquid to enter the cylinder until it is full. If necessary, the vent valve may be opened slightly to permit complete filling of the cylinder after which it shall be closed. At no time shall the pressure in the cylinder be allowed to rise above a gage pressure of 1.4 MPa (200 psi).
8.1.3 When the cylinder has been filled, close the inlet valve and open the outlet and vent valves, permitting the contents of the cylinder to be withdrawn completely and the pressure inside the cylinder to be returned to atmospheric pressure.
8.1.4 Close the outlet and vent valves and open the inlet valve, filling the cylinder to a level at which the enclosed hydrometer floats freely. If it is necessary to accomplish this filling by venting vapor through the vent valve, repeat the purging to cool the cylinder sufficiently to permit its being filled without venting.
8.1.5 With all valves closed, examine the apparatus for leaks. If leaks are detected, discard the sample, reduce the pressure to atmospheric and repair the leaks. Repeat the sampling procedure.
9. Verification of Apparatus
9.1 Carefully clean and dry the hydrometer and the inside wall of the pressure cylinder.
9.2 Insert the hydrometer in the pressure cylinder and attach the thermometer and cover plate. Connect the source of supply of the reference liquid to the inlet valve and ascertain that the connections are free of leaks. Fill the hydrometer with one of the reference fluids (see Section 7) by the procedure given in 8.1.2-8.1.5.
9.3 Disconnect the hydrometer cylinder from the source of supply of liquid and place it in the constant-temperature bath maintained within 0.2°C of the test temperature until thermal equilibrium has been obtained. To accelerate thermal adjustment, occasionally remove the hydrometer cylinder from the constant-temperature bath, tilt to a horizontal position, rock gently a few times to ensure mixing, and replace in the constant-temperature bath. Exercise care to prevent damage to the hydrometer and thermometer.
9.4 When the hydrometer cylinder and thermometer have reached equilibrium, as shown by the thermometer inside the cylinder, remove the cylinder from the constant-temperature bath, stand it on a firm level surface, and while the hydrometer is floating freely, take the hydrometer reading promptly as described in 9.4.1 and 9.4.2.
9.4.1 Observe a point slightly below the plane of the liquid surface and then raise the line of vision until this surface, seen as an ellipse, becomes a straight line. Record the point where this line cuts the hydrometer scale, estimating to the nearest one fifth of a scale division the observed hydrometer reading. A white card held behind the cylinder just below the liquid level will improve the visibility of the surface.
9.4.2 Read and record the temperature of the liquid to the nearest 0.2°C, immediately before and after reading the hydrometer. If the temperature differs by more than 0.4°C, replace the hydrometer cylinder in the bath to restore equilibrium and repeat the readings.
9.5 Repeat the verification with a second portion of the same reference fluid.
9.6 If the temperatures measured in 9.4.2 are within 0.2°C of the reference temperature and the two determinations are within 0.7 kg/m 3 or 0.0007 relative density, average the two determinations. If the temperatures are not within 0.2°C of the reference temperature, correct the observed hydrometer readings to the reference temperature by use of the appropriate table from Adjunct to D1250 Guide for Petroleum Measurement Tables (API MPMS Chapter 11.1) or API MPMS Chapter 11.2.4 (GPA TP-27). Repeat the verification with two additional determinations.
9.7 Compare the average results with the known density of the reference fluids used (see 9.2).
9.7.1 If the results are within 0.2 kg/m 3 (0.0002 relative density), accept the hydrometer scale as suitable for direct reading.
9.7.2 If the difference is between 0.2 and 1.0 kg/m3 (0.0002 and 0.001 relative density) from the known value, apply this correction to readings obtained on test portions of samples.
9.7.3 If the readings differ more than 1.0 kg/m3 (0.001 relative density) from the known value, replace the hydrometer.
9.8 After each determination, empty the cylinder and vent to reduce the pressure to atmospheric in a safe place. Highly volatile liquids and liquefied petroleum gases shall not be left in the apparatus because, at high ambient temperatures, they might generate sufficient pressure to burst the cylinder.
