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1.3.4
Temperature Response Curves and CalCurves™
A temperature response curve is needed to convert a sensor readings to temperature. Some types of sensors
share the same response and have a standard curve. The Model 340 includes standard curves for several
sensors. If an application requires more accuracy or a standard curve is not available, a specific curve must
be loaded into the instrument for an individually calibrated sensor. Lake Shore offers a broad range of
calibrated sensors and a CalCurve service that helps get calibration data into the instrument. The Model 340
provides several ways to load curves from Lake Shore calibrations or user supplied sensors. Any curve can
be entered from the front panel or over computer interface. CalCurves hold a copy of a curve called a
breakpoint interpolation table and are installed at the factory when purchased with a Model 340, or they can
be field installed using the data card slot.
1.3.5
SoftCal™
SoftCal is a good solution for applications that need more accuracy than a standard curve, but need no
traditional calibration. SoftCal is an algorithm that utilizes the predictability of standard curves to improve the
accuracy of an individual sensor around a few known temperature reference points. SoftCal generated curves
can be purchased with some types of sensors. The Model 340 also generates SoftCal curves.
1.3.6
Configurable Heater Output
The heater output is the most visible Model 340 control function because heater action is in direct opposition
with the cooling system. The Model 340 can source up to 100 watts of variable DC power to control Loop 1.
For good control, heater power should complement cooling power. For example a heater output may need a
range of 0 to 25 watts to work well with a cooling source that is 10 watts. Heating power is determined by the
resistance of the heater and the current being sent to it. Heater resistance is usually fixed once installed, so
the Model 340 heater output has several current ranges. These ranges allow different powers without the loss
of control resolution. The Model 340 allows for heaters of different resistance, but at highest power the choice
of resistance is limited.
1.3.7
Heater Output Protection
Cryostats have delicate sensors and wiring that can be damaged if over heated. Features have been added
to the Model 340 to minimize the possibility of over heating. These include setpoint temperature limit, heater
current range limit, internal heater diagnostics and a fuse in the heater output wiring.
1.3.8
Second Control Loop
When a instrument can run one closed loop control system it is referred to as a single loop controller. The
Model 340 has a the ability to run a second, independent control loop so it is called a two loop controller. The
second loop is used more often to reduce the temperature gradients in one cooling system than run two
different cooling systems. Even in well designed systems temperature gradients can be too large for some
experiments. The second loop can be used to control the temperature of radiation shields and other physical
structures or to control a small heater close to the sample while a large heater controls the load. The second
loop has much less power than the first loop and a few less software features.
1.3.9
AutoTune
Selecting the appropriate control settings (tuning) is a sometimes frustrating and tedious task. The AutoTune
feature can help by choosing a workable set of control settings on most systems. The AutoTune feature is
only available one control loop at a time.
1.3.10 Setpoint Ramping and Zones
In some applications getting to a controlled temperature is as important as being there. The setpoint ramp
feature allows smooth continuous changes in setpoint. With this feature data can be taken as the system is
changing in temperature for faster experiment cycles. It can also be used to make the approach to a setpoint
temperature more predictable. Ramping over large temperature changes may require different control settings
and the zone feature will change them automatically. Control settings for several different temperature zones
can be loaded in the Model 340 and the proper settings are used for any new setpoint.
Introduction
Lake Shore Model 340 Temperature Controller User's Manual
1-5
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Table of Contents
Troubleshooting
