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6.3.3.5 Takes Too Long to Cool
FIGURE 6-5 illustrates a typical Model CRX-6.5K sample cooling assembly cooling
curve. The system cools slowly at first because the heat capacity of the materials is
high near room temperature. As the materials cool, their heat capacity drops and the
system cools more quickly. This impacts warm up time and temperature control time
constants.
If the cooling cycle takes significantly longer than the times represented in the
cooling curve (FIGURE 6-5), investigate the possible causes listed in section 6.3.3.2 to
section 6.3.3.4.
6.3.3.6 Erratic Temperature Readings
The Model CRX-6.5K sample stage is a stable temperature control platform and
reasonable temperature control can be achieved over the entire temperature range.
Short term temperature control of a few tens of millikelvins should be expected
around base temperature. As the system approaches maximum temperature, short
term control stability degrades somewhat, but should remain below 1 K in a properly
tuned system.
Considerations when regulating with electronic temperature controls
When operating with the electronic temperature controllers the following problems
may cause unstable or erratic temperature readings.
D
Controller PID parameters are not tuned properly—the controller tuning
parameters listed in section 4.5.5 are a good starting point, but may need to be
modified to achieve optimum control stability based on specific measurement
conditions. Heater range is important to tuning and also must be set properly.
D
Controlling too close to base temperature—attempt to control a few kelvin
higher in temperature, then gradually lower the setpoint to identify the lowest
practical electronic control temperature.
D
Electrical noise—ground loops and other electrical noise can impact the
controller’s temperature readings
FIGURE 6-5
Typical Model CRX-6.5K sample cooling assembly cooling curve with 4 arms
