FIG. A6.2 Calibration of Vacuum Gages
desired calibration pressure falls between 10 and 90 % of the scale. Before refilling with clean mercury, heat the empty reference McLeod gage at 250°C for at least 30 min at a pressure below 10 Pa (0.075 mm Hg). Thereafter, carefully protect the reference gage from exposure to moisture such as that from atmospheric air. The use of two reference McLeod gages of different pressure ranges is recommended as a precaution. If they agree at the test pressure, it is an indication that the system is free of moisture and other condensibles.
Note A6.1—The general principles of construction of McLeod gages are well-established. The dimensions and tolerances of such a gage are beyond the scope of this test method.
A220.127.116.11 Alternatively, certified secondary gages, electronic or otherwise, can be used, provided the output can be traced back to a primary standard. Secondary gages shall be recertified at a regular basis, but at least once a year.
A18.104.22.168 Set up the test manifold such as that shown in Fig. A6.2. Ensure that the test manifold is leak-free and can be maintained at a steady pressure at the required level. A suitable leak test is to pump down to a pressure below 0.1 kPa and isolate the pump. Observe the pressure inside the unit for at least 1 min. If the pressure rises no more than 0.01 kPa in that period, the apparatus is considered acceptable.
A22.214.171.124 Connect the reference (primary) vacuum gage(s) and the gage(s) to be calibrated to the manifold. The gages shall have such a range that the desired calibration pressure falls between 10 and 90 % of the scale. Insert a dry ice trap between the manifold and the vacuum pump. Adjust the pressure to the required level for the test and run a final leak test as above.
A126.96.36.199 After steady conditions have been maintained for at least 3 min, readings are made of all gages and compared with the reference gage.
A188.8.131.52 Repeat the above procedure at the other required pressure levels.
A184.108.40.206 Make up a chart of corrections to be added at each pressure level for each gage tested. This can be used for interpolation when necessary.
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