TA Instruments AR 2000, Руководство оператора

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AR 2000 Operator's Manual
4 9
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 sample by carrying out an
experiment using no equilibrium time and doing a time sweep experiment (in oscillation 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 additional
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. (Further information on the mapping procedure can be found in the Rheology Advantage Help™
system.)
Controlling Cooling
When you set the control valve on the liquid nitrogen unit, you must compromise between the rate of cooling
(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 necessary.
• 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 opening
and closing of the solenoid valve for more or less equal periods. Start with a setting of "open two complete
turns" and experiment to find the optimum position for your work experiment procedures.
WARNING: The electronic control box supplied with the ETC has no user serviceable
parts inside.
Le cadre de commande électronique fourni avec le four ne contient aucun
consommable.
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 housekeep-
ing, maintenance and periodic examination of cryogenic pressure vessels."