Filter
Top Products
page 11
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 Outgassing5.2 Outgassing
5.2 Outgassing5.2 Outgassing
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 Ingassing5.3 Ingassing
5.3 Ingassing5.3 Ingassing
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 5.4 Effects of
5.4 Effects of 5.4 Effects of
5.4 Effects of
TherTher
TherTher
Ther
mal Conductivitymal Conductivity
mal Conductivitymal Conductivity
mal 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 Conductance5.5 Effects of System Conductance
5.5 Effects of System Conductance5.5 Effects of System Conductance
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.