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74.5 Connections. (a) All permanent connections may be made tight by soldering or tinning.
(b) Special threads or other suitable means may be used in making the connection between the two chambers, in order to obtain a tight connection without the use of a wrench.**
74.6 Filling the gasoline chamber-(a) Sampling materials from tank cars and open tanks. The empty gasoline chamber, Figure 1, shall be first immersed in the tank or container of the liquid to be tested and allowed to fill while being raised and lowered through the liquid. The first filling is for the purpose of thoroughly washing the chamber, which should then be emptied. The chamber shall then be refilled in the same manner, care being exercised to see that it is completely filled with the liquid.
(b) Sampling materials from systems under pressure. The gasoline chamber equipped with valves, top and bottom, Figure 2, shall be used.
Before drawing the sample, the liquid section of the gasoline chamber shall be cooled to a temperature as low as or lower than that of the gasoline to be tested. The sample shall be taken as follows:
Close both valves and connect the gasoline chamber to the tank, pipe line, or other receptacle holding the liquid to be tested, by means of the lower valve of the chamber. The sample must be taken at a place which will insure that a representative sample of the gasoline is being obtained. With the upper valve closed, open the valve on the tank or pipe line and then open the lower valve on the gasoline chamber, allowing full tank or line pressure to be exerted on the chamber. Open slightly the upper valve of the chamber and allow gasoline to escape until a volume equal to at least twice the capacity of the gasoline chamber shall have been displaced. This procedure insures the removal of all air from the chamber. The operation of purging should be accomplished without allowing the pressure in the chamber to drop below that of the tank or line. After filling the gasoline chamber, close both top and bottom valves in the order mentioned, and disconnect the chamber from the tank or pipe line. After disconnecting the gasoline chamber, attach it to the air chamber immediately; and immediately open the upper valve of the gasoline chamber. If the air chamber is not immediately attached, the gasoline chamber should be kept chilled to prevent rupturing due to expansion of its contents.
(c) Sampling by pouring. A sample shall not be transferred to the gasoline chamber by pouring unless it is impossible to obtain it by either of the methods described in paragraphs (a) and (b). When this is the case, both the liquid and the gasoline chamber shall be cold enough so that no appreciable change in composition of the sample occurs during the pouring. Table I gives the maximum pouring temperatures for gasolines of various vapor pressures. The responsibility for errors incurred by pouring rests solely with the operator. Pouring shall not be used in referee tests.
**For statutory and source citations, see note to 8 74.1. **For statutory and source citations, see note to § 74.1.
TABLE I.-TEMPERATURES TO WHICH GASOLINE AND GASOLINE CHAMBER
SHALL BE COOLED BEFORE POURING SAMPLE
(d) Sampling by water displacement. It is recommended that transfer by water displacement be used when the methods described in paragraphs (a), (b), (c) cannot be employed.**
74.7 Test. Prior to each test, the air chamber, from which the gage has been removed, shall be thoroughly purged of any gasoline vapors that may remain from a previous test. The purging may best be accomplished by filling the air chamber with warm water (90° to 100° F.) and allowing it to drain. This operation shall be repeated at least five times. If the air chamber is purged sometime before the test is made, it should be rinsed with water (temperature optional) just before the test is started. Prior to each test, the gage should be shaken to dislodge gasoline that may remain from a previous test. The gage shall then be attached to the air chamber.
The temperature of the air and water vapor in the air chamber shall be determined by inserting a thermometer of suitable range and accuracy inside the chamber just before making the connection with the gasoline chamber. The thermometer shall be allowed to remain in the air chamber for not less than 5 minutes before the temperature is read. This temperature reading shall be recorded as the “initial air temperature.” It is necessary that the initial air temperature shall be the actual temperature of the air in the air chamber when attached to the gasoline chamber. Immediately, after determining the initial air temperature, the air chamber with gage in place shall be attached tightly to the gasoline chamber containing the sample to be tested (see g 74.6). If a threaded coupling is employed, a little shellac or other suitable sealing material on the threads may assist in making a tight connection.
The vapor-pressure bomb shall then be turned upside down and shaken vigorously in this position. This operation shall be repeated several times. The bomb shall then be immersed at least up to the
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bottom of the pressure gage in the water bath, which shall be held at a constant temperature of 100° F., plus or minus 0.5° F. While the bomb is immersed in the bath, the apparatus shall be closely observed for leaks. The coupling device is normally in the liquid section of the apparatus and, because it is most in use, shall be given particular attention. Liquid leaks are more difficult to detect than vapor leaks. If a leak is detected the test shall be discarded. After 5 minutes, the bomb shall be withdrawn from the bath, inverted, shaken vigorously, and replaced in the bath. Thereafter, at intervals of 2 minutes, the bomb shall be withdrawn from the bath, inverted, shaken vigorously, and replaced in the bath. These operations shall be done quickly to avoid cooling the bomb and its contents.
Prior to each removal of the bomb from the bath, the gage reading shall be observed. The gage should be tapped lightly prior to each reading. The temperature of the bath shall also be closely observed to make certain that it is within the specified limits. When consecutive gage readings thus observed become constant, the value of the reading shall be recorded as the "uncorrected vapor pressure" of the sample under test.*+
74.8 Calculations. To calculate the “Reid vapor pressure" of the sample under test, the "uncorrected vapor pressure" must be corrected for the change in the pressure of the water vapor and air in the air chamber due to the differences between the “initial air temperature" and the temperature of the bath. Table II shows the corrections to apply for "initial air temperatures” ranging from 32° to 110° F. This corrected value shall be recorded as the "Reid vapor pressure.
NOTE: Example: If the initial air temperature is 82° F. and the uncorrected vapor pressure is 24.6 pounds, the Reid vapor pressure is 24.6 pounds minus 0.9 pound or 23.7 pounds.
Due to the fact that the atmospheric pressure provided in the air chamber counteracts the external atmospheric pressure, the only pressure acting internally on the pressure gage is the absolute vapor pressure of the liquid in the bomb, so that the gage reading is the absolute vapor pressure of the liquid in pounds per square inch at 100° F. For purposes of brevity the "Reid vapor pressure" shall be recorded in pounds, without reference to temperature used in the test or to unit of surface.**
74.9 Accuracy. With proper care and attention to detail, duplicate results obtained for Reid vapor pressure values should not differ from each other by more than 0.5 pound.
**For statutory and source citations, see note to 8 74.1. **For statutory and source citations, see note to $ 74.1.
TABLE II.-CORRECTIONS TO CONVERT UNCORRECTED VAPOR PRESSURES TO
REID VAPOR PRESSURES
(14.4-P.) (t-100) Correction factor:
t=air chamber temperature at beginning of test, degrees Fahrenheit.