DHR/AR Series Environmental Test Chamber Getting Started Guide
Page 30
Operating Hints
Although the response time of the temperature control system is rapid, many of the samples that are of
interest at high temperatures (
e.g
., bitumen, molten polymers, etc.) are very poor conductors of heat.
Therefore, the limiting factor in reaching the desired starting temperature is the time it takes for the heat to
be conducted into the sample and for the sample to reach thermal equilibrium. You can investigate a sam-
ple by carrying out an experiment using no equilibrium time and doing a time sweep experiment (in oscil-
lation mode). If you plot a graph of how the properties of the sample vary with time, you can quickly
establish the required equilibrium time.
The tendency of polymers (which are measured while in their molten state) to oxidize can present an addi-
tional complication. This problem is generally sample-dependent, but can be reduced by surrounding the
sample with an inert atmosphere. To do this, use nitrogen gas rather than air as the feed to the ETC. It also
helps if you optimize your test procedures to minimize the amount of time that the sample is held at high
temperatures.
Make sure the upper geometry is in place and free to rotate when you perform procedure for mapping of
the bearing. For best results, perform the mapping procedure at ambient temperature and without purge gas
flowing.
Controlling Cooling
When you set the control valve on the liquid nitrogen unit, you must compromise between the rate of cool-
ing (which is improved by having a large flow rate) and the fineness of control (which is optimized when
there is minimal flow rate from the needle valve.) When only a small amount of cooling is required, the
solenoid valve is able to open and shut frequently. However, if a large surge of coolant occurred every time
the solenoid valve opened, the system temperature would oscillate on either side of the set point.
The setting of the needle valve is affected by the desired set-temperature:
•
If cooling is needed at only a few degrees below ambient, then a very small opening is all that is neces-
sary.
•
If you operate at -100°C, then a correspondingly higher flow rate of nitrogen is required.
As a general rule, the correct needle valve setting for the desired temperature is one that results in the open-
ing and closing of the solenoid valve for more or less equal periods. Start with a setting of “open two com-
plete turns” and experiment to find the optimum position for your work experiment procedures.
Low Temperature System Maintenance
For maintenance instructions of the cryogenic pressure vessel, please refer to the instructions supplied with
the unit. If you purchased the Dewar flask from TA Instruments, the document is titled “Guide to good
housekeeping, maintenance and periodic examination of cryogenic pressure vessels.”
