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TM 10-4310-391 -13&P
I-7. VACUUM CONNECTIONS
I-7a. Choice of Connections
The choice of confections and fittings can have a very marked effect on the pumping speed at the vacuum chamber Any
connection placed between the pump and the chamber creates an impedance to the flow of gas This Is particularly true at low
pressures In the millitorr range where the gas flow s substantially molecular in character The gas flow Is then dependent upon the
kinetic activity of the molecules to bring it to the Intake of the pump.
I-7b The Effects of Conductance
It has been shown that the conductance of a tube s proportional to the cube of bits diameter and inversely proportional to bits length
Therefore it Is Imperative that the connecting lines be as large In diameter and as short In length as practical For best results the
diameter of the connecting tube should be at least as large as the diameter of the pump Intake To avoid a large reduction In
pumping speed at the vacuum chamber, it Is clear that the conductance of the line must be considerably greater than the speed of
the pump.
I-7c Metal Joints
If metal piping or tubing Is used, it Is preferable to solder or braze all the confections Where threaded joints must be used, coat the
threads with Glyptal, LocTite Thread Sealant with Teflon, or Leak Lock and screw together tightly Flanged confections with elastomer
gaskets make excellent demountable Joints.
I-7d. Glass Joints
Where glass tubing Is used between the system and the pump Intake, Joints can be made by butting the ends of the two sections
together in a short section of rubber vacuum hose This type of Joint can also be used with metal-to-metal and glass-to-metal tubing
Worm-screw band clamps are useful for securing the hose to the tubing Whatever the Joint you choose to use cleanliness should be
of utmost importance.
I-7e. Valves and Stopcocks
Metal valves or glass stopcocks may be used In the connecting line between the system and the pump to provide a means of
isolating the pump from the system To minimize the Impedance of flow, the valve openings should be as large as possible Lubricate
the rotating plug of the stopcock with a film of vacuum grease sufficiently thick enough to prevent seizure.
I-8. VACUUM GAUGES
The type of vacuum gauge to be used Is deter mined largely by the pressure range to be measured Pressures In the ranges
produced by DuoSeal pumps can be covered by McLeod, Thermistor, Grant or Thermocouple gauges The McLeod gauge Is used
where high accuracy of measurement Is required The Pirani, Thermistor and Thermocouple gauges are electrical and give
continuous readings of the total pressure They 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 employing diffusion turbo-molecular o, ion pumps, the hot f-lament Ionization of the Philips gauge is
used.
I-9. TRAPS
I-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 so 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 contaminants A variety of glass cold traps and Dewar flasks are available from Sargent-Welch.
I-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 rewarmed it may allow re-evaporation of the condensate The opening of the Dewar flask should not be
obstructed as the refrigerant boil-off can produce dangerously high pressures If the trap becomes saturated it should be disconnect
Ed from the system, brained and cleaned An In crease In pressure In the vacuum system will normally Indicate that the trap has
become saturated To clean the trap, remove the refrigerant and allow the trap to warm up Remove the trap from the system and
Arise off the condensate with a suitable solvent Thoroughly clean and dry the trap before pre-installing In the system.
I- I0. TYPES OF LUBRICANTS
All Sargent-Welch mechanical vacuum pumps are normally tested with DuoSeal ail and chipped with a full
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