present. Hastings thermopile gauges however, have the useful property of responding to the total pressure of all gases and vapors that are present in the gauge tube.
To exclude vapors from a vacuum system, it is necessary to employ a trap of some kind that will absorb or condense vapors. Water vapor is by far the most common source of this difficulty. A cold trap cooled by liquid nitrogen is an effective means in removing vapors.
It may be necessary to keep McLeod gauges constently under vacuum for several hours, or days with a trap before it will read correctly. The use of rubber or Tygon tubing connecting the gauge to the vacuum system can lead to gross errors due to excessive outgassing and or adsorbtion by the tube. It is recommended that only glass or metal tubing be used. 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.
5.2 Outgassing
Hastings gauge tubes are fabricated from materials which 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 adsorbed during expo- sure 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 pressures of less than 10 mTorr where atmospheric gases are likely to flood the enclosure due to leaks.
Also, if the system is being pumped continuously, gauges spaced at different distances from the pump will register different pressures. For a reliable comparison of different vacuum gauges, it is necessary then to insure that the vacuum system be free of any outgasses or other sources of apparent leaks. This can best be determined by closing off the system from the pumps and observing if there is any rise in pressure within the range of interest.
5.3 Ingassing
Ingassing is an effect opposite to outgassing and may also lead to erroneous readings. Ionization gauges exhibit a kind of pumping action that tends to clean up residual gasses in certain ranges of pressure and thereby lower the pressure. Also, if a cold trap is in a closed system, the total pressure may change considerable while condenseble vapors such as water, carbon dioxide and mercury and being condensed.
5.4 Effects 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. Contact factory for calibration in gases other than air.
5.5 Effects of System Conductance
Each element that makes up a vacuum system has associated with it a certain conductance (that is the opposite of resistance). For example, baffles, connecting tubing, and sharp turns may cause pressure drops throughout the system during pumping and during the time in which the system is reaching static equilibrium. It is not an uncommen occurence to measure different pressures at different locations in a vacuum system. In checking the calibration of a vacuum gauge, care must be taken to insure that the gauge and the reference are at the same pressure.
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