Lake Shore Model 340 Temperature Controller User’s Manual
8-4
Instrument Programming
8.2 SOFTCAL™
The Model 340 performs inexpensive sensor calibrations with a two algorithms called SoftCal. These
algorithms work with DT-400 Series Silicon Diode sensors and Platinum Sensors. They create a new
temperature response curve from the standard curve and known data points entered by the user. The new
curve loads into one of the 40 user curve locations (21 thru 60). These paragraphs describe the data points
needed from the user and the expected accuracy of the resulting curves.
Both DT-400 and Platinum SoftCal algorithms require an existing standard curve in the Model 340. When the
user enters the sensor type to calibrate, the correct standard curve is selected. When calibration is complete,
the user selects the new curve for an input; the Model 340 does not automatically choose the newly
generated curve for either input.
Enter calibration data points (normally measured at easily reached temperatures). Each algorithm operates
with one, two, or three calibration points. Accuracy increases with more points.
There are two ways to get SoftCal calibration data points:
n
The user record the response of an unknown
sensor at well controlled temperatures, or
o
The user buys a Lake Shore SoftCal calibrated sensor. There
are advantages to both methods.
n
User:
When the user 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 measurement errors can be significant. Calibration can be no better than
user-supplied data.
o
Purchased:
Lake Shore sensors with SoftCal calibration include a set of calibration points in the
calibration report. Lake Shore generates SoftCal calibration points in a controlled calibration facility for best
accuracy. The user enters the calibration points into the Model 340 to generate a curve. If the user buys
the CalCurve service with the calibrated sensor, the factory generates the curve for later entry like any
other curve.
8.2.1 SoftCal and 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-470 Series of silicon
diode sensors has a repeatable temperature response from 2 K to 475 K. These sensors closely follow the
Standard Curve 10 response and routinely interchange with one another. SoftCal is an inexpensive way to
improve the accuracy of an already predictable sensor.
NOTE:
Standard Curve 10 is the name of the temperature response curve, not its location inside
the Model 340. Standard Curve 10 is stored in Curve Location #1 in the Model 340.
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 algorithm 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 8-1.
Point 1:
Calibration data point at or near the boiling point of helium, 4.2 K. Temperatures outside 2 K to 10 K
are not allowed. This data point improves between the calibration data point and 28K. Points 2 and 3 improve
temperatures above 28 K.
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.
Point 3:
Calibration data point near room temperature (305 K). Temperatures outside the range of 200 K to
350 K are not allowed.
Liquid Nitrogen
Boiling Point
77.35 K
Room
Temperature Point
305 K
Liquid Helium
Boiling Point
4.2 K
SoftCal Point 1
SoftCal Point 2
SoftCal Point 3
0
25
50
75
100
125
150
175
200
225
250
275
300
325
350 kelvin
Acceptable Temperature Range for Silicon Diode SoftCal Inputs
2 - 10 K
50 - 100 K
200 - 325 K
Figure 8-1. SoftCal Temperature Ranges for Silicon Diode Sensors