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Chapter 5: Advanced Operation
5.1 General
This chapter is separated into two parts. Section 5.2 provides advanced operation
procedures, building on the knowledge and experience gained performing those
operations explained in Chapter 4. Section 5.3 explains reconfiguration procedures
that are not typically performed on a day-to-day basis, but which are nonetheless
essential to know.
5.2 Advanced
Temperature
Operation
The material in this chapter is written assuming that the user is an experienced oper-
ator of the Model CRX-6.5K and understands the theory and operation of the
Model CRX-6.5K as discussed in Chapter 4.
5.2.1 Reducing
Condensation on
the Sample
Even when following good vacuum practices with well maintained equipment, a
small amount of residual gas remains in the vacuum chamber after it is evacuated.
The residual gas consists primarily of nitrogen and water vapor. It is generally of little
concern because it is cryopumped onto the refrigerator when it is cooled. However,
problems may occur with some materials if the residual gas condenses on the sample
surface.
5.2.1.1 Heated Cycle Purge
On some vacuum systems, vacuum base pressure can be improved through a “bake
out” process to increase the energy in contaminant molecules so they move away
from the surfaces and can be pumped out more quickly. The Model CRX-6.5K cannot
be baked out, because its components will not tolerate the high temperatures
required; however, controlled warming and cycle purging can improve
vacuum integrity.
The following procedure can be performed with the sample already in the probe station,
as long as the sample will not be affected by the elevated temperature or the gas used.
Follow this procedure to cycle purge the vacuum chamber:
1. Evacuate the chamber as described in section 4.4.2.
2. Warm the refrigerator stages above room temperature; set the setpoints of all
stages to 300 K.
3. Repeat steps a to f three to five times or as needed. Note the reading on the vac-
uum gauge at the end of each cycle; the base pressure should improve with each
cycle. Typically after three to five cycles only marginal improvement in base pres-
sure is achieved and the process can be stopped.
a. Allow the vacuum pump to run for 30 min to 60 min, and record the base
pressure gauge reading.
b. Close the vacuum isolation valve, and shut down and vent the turbo
pumping system.
c.
Purge the vacuum chamber with dry argon or nitrogen gas, following steps 2
to 4 in section 4.4.4. Close the purge valve as soon as the pressure relief valve
opens.
d. Allow the dry gas to remain in the chamber for 30 min to 60 min. The heated
stages will warm the dry gas and internal surfaces of the probe station.
e. Open the vacuum isolation valve.
f.
Start the turbo pumping system to evacuate the chamber.
