Lakeshore 336, User Manual

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88 c
HAPTER
5: Advanced Operation
Model 336 Temperature Controller
5.10 SoftCal™ The Model 336 allows you to perform inexpensive sensor calibrations with a set of
algorithms called SoftCal™. The two SoftCal™ algorithms in the Model 336 work with
DT-400 Series silicon diode sensors and platinum sensors. They create a new tem-
perature response curve from the standard curve and known data points that you
entered. The new curve loads into one of the user curve locations (21 to 59) in the
instrument. The following sections describe the data points you need to supply and
the expected accuracy of the resulting curves.
A feature similar to SoftCal™ is available for compensating thermocouples using the
Curve Handler® program.
Both DT-400 Series and platinum SoftCal™ algorithms require a standard curve that
is already present in the Model 336. When you enter the type of sensor being cali-
brated, the correct standard curve must be selected. When calibration is complete,
you must assign the new curve to an input. The Model 336 does not automatically
assign the newly generated curve to either input.
Calibration data points must be entered into the Model 336. These calibration points
are normally measured at easily obtained temperatures like the boiling point of cryo-
gens. Each algorithm operates with 1, 2, or 3 calibration points. The range of
improved accuracy increases with more points.
There are two ways to get SoftCal™ calibration data points: you can record the
response of an unknown sensor at well controlled temperatures, or you can purchase
a SoftCal™ calibrated sensor from Lake Shore. There are advantages to both methods.
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User: when you can provide stable calibration temperatures with the sensor
installed, SoftCal™ calibration eliminates errors in the sensor measurement as
well as the sensor. Thermal gradients, instrument accuracy, and other measure-
ment errors can be significant to some users. Calibration can be no better than
user-supplied data.
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Purchased: Lake Shore sensors with SoftCal™ calibration include a set of calibra-
tion points in the calibration report. The SoftCal™ calibration points are gener-
ated in a controlled calibration facility at Lake Shore for best accuracy. The
calibration points can be entered into the Model 336 so it can generate a curve. If
the CalCurve™ service is purchased with the calibrated sensor, the curve is also
generated at the factory and can be entered like any other curve.
5.10.1 SoftCal™ With
Silicon Diode Sensors Lake Shore silicon diode sensors incorporate remarkably uniform sensing elements
that exhibit precise, monotonic, and repeatable temperature response. For example,
the Lake Shore DT-400 Series of silicon diode sensors have a repeatable temperature
response from 2 K to 475 K. These sensors closely follow a standard curve . SoftCal™ is
an inexpensive way to improve the accuracy of an already predictable sensor.
A unique characteristic of DT-400 Series diodes is that their temperature responses
pass through 28 K at almost exactly the same voltage. This improves SoftCal™ algo-
rithm operation by providing an extra calibration data point. It also explains why
SoftCal™ calibration specifications are divided into two temperature ranges, above
and below 28 K. See FIGURE 5-8.
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Point 1: calibration data point at or near the boiling point of helium, 4.2 K. Accept-
able temperature entries are 2 K to 10 K. This data point improves between the
calibration data point and 28 K. Points 2 and 3 improve temperatures above 28 K.
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Point 2: calibration data point at or near the boiling point of nitrogen (77.35 K).
Temperatures outside 50 K to 100 K are not allowed. This data point improves
accuracy between 28 K and 100 K. Points 2 and 3 together improve accuracy to
room temperature and above.
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Point 3: calibration data point near room temperature (305 K). Temperatures out-
side the range of 200 K to 350 K are not allowed.