Hastings DV-20, Manual

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101B - PAGE 14
6.0 NOTES ON VACUUM MEASUREMENTS
6.1 Effects of Condensable Vapors:
If the readings of Hastings Vacuum Gauges are to be compared with readings of other types of gauges, consider-
ation must be given to the possible effects of condensable vapors on the other gauges. For example, none of the
many types of the McLeod Gauge give correct readings if condensable vapors such as water, alcohol, acetone, etc.,
are present in the gauge. The McLeod Gauge operates by compressing residual gases and vapors to obtain a
reading, and this compression will tend to condense any vapors that are present. This usually results in the McLeod
Gauge reading a pressure that is too low. Furthermore, two different types of McLeod Gauges may disagree and both
be incorrect, if vapors are present. Unlike the McLeod Gauge, the Hastings thermopile vacuum gauges have the very
useful property of responding to the total pressure of all gases and vapors that are present in the gauge tube.
To exclude vapors from the McLeod type of gauge, it is necessary to employ a trap of some kind that will absorb or
condense the vapors. Water vapor is by far the most common source of this difficulty. A trap cooled by liquid
nitrogen is effective in removing vapors, but even so it may be necessary to keep the McLeod Gauge constantly
under vacuum for several hours, or days, with a trap before it will read correctly. Reference should be made to the
instructions furnished by the manufacturer of the McLeod Gauge to be sure that it is provided with a suitable trap.
6.2 Outgassing:
Hastings Gauge Tubes are made of materials that have been proven by years of usage to be relatively free from outgassing.
However, all surfaces of glass and metal that are exposed to the vacuum system may liberate gases and vapors that were
previously absorbed during exposure to the atmosphere. If the surfaces are contaminated with foreign matter, this outgassing
may be much more persistent than if the surfaces are clean. The possibility of outgassing must be considered in checking the
accuracy of Hastings Gauges or in checking for leaks. This is especially important when working with pressure of less than 10
microns of mercury (or .01 millibars). In this range of pressure, outgassing from surfaces in a newly evacuated system may
flood the enclosure. Also, if the system is being pumped continuously, gauges spaced at different distances from the pump
will register different pressures. For reliable comparison of different vacuum gauges, it is necessary then to insure that the
vacuum system be free of any outgassing or other sources of apparent leaks. This can best be determined by closing the
system off from the pumps and observing if there is any rise in pressure within the range of interest.
6.3 Ingassing:
Ingassing is an effect opposite to outgassing and this can also lead to erroneous readings. Ionization gauges exhibit
a kind of pumping action that tends to clean up residual gases in certain ranges of pressure and thereby lowering the
pressure. Also, if a cold trap is in a closed system, the total pressure may change considerably while condensable
vapors such as water, carbon dioxide and mercury are being condensed.
6.4 Effect of Thermal Conductivity:
All Hastings Vacuum Gauges are originally calibrated in dry air. Since this calibration is a function of thermal
conductivity, any gas having a thermal conductivity different from that of air will also have a different calibration.
Nomograms are provided in Section 8.0 to correct indicated pressures to actual pressures for several of the more
common gases encountered in vacuum work.
6.5 Effect of System Conductance:
Each element that makes up a vacuum system has associated with it a certain conductance (this is the opposite of
resistance). For example, baffles, connecting tubing, and sharp turns can all cause pressure drops throughout the
system during pumping and during the time the system is reaching static equilibrium. It is not an uncommon
occurrence to measure different pressures at different locations in a vacuum system. In checking the calibration of
any vacuum gauge, care must be taken to insure that the gauge and the reference are at the same pressure.