Chemglass AF-0350 Welch Vacuum Pump - Manual (Page 5)

1.8 Vacuum Gauges (See Accessory section in the back of this manual.)
The type of vacuum gauge to be used is determined largely by the pressure range to be measured. Pressures
in the ranges produced by DUOSEAL pumps can be covered by McLeod, Pirani or Thermocouple gauges.
The McLeod gauge is used where high accuracy of measurement is required. The Pirani and Thermocouple
gauges are electrical and give continuous readings of the total pressure. There are preferred where rapid
pressure changes occur. The McLeod gauge does not measure condensable vapors; therefore, if vapors are
present it will generally read lower in pressure than electrical gauges. For higher vacuums in systems employ-
ing diffusion, turbo-molecular or ion pumps, a hot or cold cathode gauge is used.
1.9 Traps (See Accessory section in the back of this manual.)
1.9a The need for a Trap
Where corrosive vapors or large quantities of condensable vapors are evolved from vacuum processing, a cold
trap may be used in the connecting line to the pump. It will help prevent damage to the pump mechanism and
reduce oil contamination. The cold trap, immersed in a suitable Dewar flask, is installed to that the vapors may
come in contact with the surfaces of the trap and condense. Commonly used refrigerants are liquid nitrogen
or dry ice and acetone. The refrigerant to be used depends upon the freezing point of the contaminations. A
variety of cold traps are available from Welch Vacuum, Thomas Industries, inc.
1.9b The care of a Trap
When using a cold trap the refrigerant should be maintained at a high level in the flask to keep the trap at a
uniformly low temperature. If the trap is re-warmed it may allow re-evaporation of the condensate. The
refrigerant add tube on the liquid nitrogen trap should not be obstructed as the refrigerant boil-off can produce
dangerously high pressures. If the trap becomes saturated it should be disconnected from the system, drained
and cleaned. An increase in pressure in the vacuum system will normally indicate that the trap has become
saturated. To clean the trap, remove the trap from the system, allow the trap to warm up and rinse off the
condensate with a suitable solvent in a fume hood. Thoroughly clean and dry the trap before reinstalling in the
system.
1.10 Types of Lubricants (See Accessory section in the back of this manual.)
All DUOSEAL mechanical vacuum pumps are normally tested with DUOSEAL® oil and shipped with a full
charge to prevent unnecessary contamination. An additional supply of oil is furnished with each pump with
instruction to drain and discard the oil contaminated in the pump and replace with the fresh oil. DUOSEAL oil
has been especially prepared and is ideally suited for use in mechanical vacuum pumps because of its desirable
viscosity, low vapor pressure and chemical stability.
The vacuum guarantee on all DUOSEAL pumps applies only when DUOSEAL oil is used. Other lubricants
for special applications are available including various lubricants compatibility, lubricants for use with diffusion
pumps as well as other special requirements.
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