Chemglass CG-4600-01 - Manual (Page 5)

5
Section 3 Operations Guide
3.1 How the Regulator Works
The vacuum regulator consists of 3 important pieces. The
pressure transducer measures the pressure in the piece of
equipment being regulated. The process controller compares
the pressure reading from the transducer with the set point
pressure (i.e. the desired pressure) entered. If the pressure in
the equipment is greater than the set point, the process
controller opens the control valve allowing the vacuum pump
to decrease the pressure in the equipment. When the pressure
in the equipment equals the set point pressure, the process
controller shuts the control valve preventing the vacuum pump
from lowering the pressure any further.
The basic method of operation is very simple. When the
pressure in the equipment is above the set point the controller
opens the control valve connecting the equipment to the
vacuum pump, and when the equipment is at or below the set
point, the controller shuts the control valve, isolating the
vacuum pump from the equipment.
3.2 Factors Affecting Vacuum Regulation Accuracy
a. The rate of evacuation. An important factor affecting stability is the rate the vacuum pump evacuates
the attached equipment. In some cases the rate of evacuation must be manually limited by using a needle
valve. A needle valve is used because many vacuum pumps have such high pumping capacities that
they evacuate equipment faster than the
controller can respond to changes in
pressure (the controller needs 1/10th of
a second to respond to pressure
changes). Particularly in small
equipment, pressure can be lower more
than desired in 1/10th of a second
without a needle valve limiting the
evacuation rate. Regulating the
evacuation rate becomes more
important the smaller the piece of
equipment being evacuated since it’s
evacuated faster (for example consider
a 50 ml flask compared to a rotary
evaporator). The diagram at the right
shows the result of using a needle valve
to limit evacuation rate.
Time (Sec)
Pressure (torr)
100
200
300
400
500
600
700
800
0 10 20 30 40 50 60
Without valve
With valve
Pressure Regulation in a Rotary Evaporator
b. Use a cold trap to condense volatile solvents. At least one dry ice trap should be placed between the
OUTLET of the digital vacuum regulator and the equipment being regulated. This prolongs the life of
both the regulator and the vacuum pump. The dry ice trap available from J-KEM (catalog # RCE1000) is
highly recommended because of its high condensation efficiency, large reservoir and ease of solvent
removal.