VACUUM GAUGES
Typical in the rough vacuum range, a dial vacuum gauge is used to measure
pressure in mm, Hg or inches of Hg. The dial vacuum gauge gives negative
pressure – that is pressure below atmospheric. The reference point for the
vacuum gauge is atmospheric pressure.
Please keep in mind that atmospheric pressure tends to vary from day to
day. As a result of variability, the dial vacuum gauge will indicate slightly
different maximum vacuum readings from day to day.
TRAPS
The pumps will handle humid air. All wetted aluminum parts are treated for
corrosion protection from moisture. All other wetted parts are stainless steel.
If there is a chance liquid may be drawn from the process under evacuation,
Grobet recommends a liquid trap be placed between the process and the
pump. A simple liquid trap is a filtering flask.
When a heavy load of water vapor is evolved from the vacuum process, a
cold trap is recommended to help prevent damage to the pump mechanism.
The cold trap, immersed in a suitable Dewar flask, is installed so that the
water vapor may come in contact with the surfaces of the trap and
condense. Commonly used refrigerants are liquid nitrogen or dry ice and
acetone or alcohol. Dry ice provides sufficient cooling to freeze out most
heavy water vapor loads. A variety of cold traps are available from Grobet.
The symptom of a high water vapor load is if you have droplets coming out
of the exhaust port. If you see droplets of water, Grobet recommends you
use a cold trap to capture the water before entering the pump or dilute the
water vapor stream by adding dry nitrogen to the gas flow.
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 and 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 into the system.
