Sensor Inputs - Lakeshore 325 User Manual


Operates down to 1.2 K with
appropriate sensor

Two sensor inputs

Supports diode, RTD, and
thermocouple sensors

Sensor excitation current
reversal eliminates thermal
EMF errors in resistance sensors

Two autotuning control loops:
25 W and 2 W maximum

Control loop 2: variable DC
voltage source from 0 to 10 V
maximum

IEEE-488 and RS-232C
interfaces
2
www.lakeshore.com Lake Shore Cryotronics, Inc. (614) 891-2244 fax: (614) 818-1600 e-mail: info@lakeshore.com
Model 325
Temperature Controller
Introduction
The Model 325 dual-channel temperature
controller is capable of supporting
nearly any diode, RTD, or thermocouple
temperature sensor. Two independent
PID control loops with heater outputs
of 25 W and 2 W are configured to drive
either a 50 Ω or 25 Ω load for optimal
cryocooler control flexibility. Designed
with ease of use, functionality, and
value in mind, the Model 325 is ideal
for general-purpose laboratory and
industrial temperature measurement and
control applications.

Sensor Inputs

The Model 325 temperature controller
features two inputs with a high-
resolution 24-bit analog-to-digital
converter and separate current sources
for each input. Constant current
excitation allows temperature to be
measured and controlled down to
2.0 K using appropriate Cernox™ RTDs
or down to 1.4 K using silicon diodes.
Thermocouples allow for temperature
measurement and control above 1,500 K.
Sensors are optically isolated from other
instrument functions for quiet and
repeatable sensor measurements. The
Model 325 also uses current reversal to
eliminate thermal EMF errors in resistance
sensors. Sensor data from each input is
updated up to ten times per second, with
display outputs twice each second.
Standard temperature response curves
for silicon diodes, platinum RTDs,
ruthenium oxide RTDs, and many
thermocouples are included. Up to fifteen
200-point CalCurves® (for Lake Shore
calibrated temperature sensors) or user
curves can be stored into non-volatile
memory. A built-in SoftCal®
can be used to generate curves for silicon
diodes and platinum RTDs for storage as
user curves. The Lake Shore curve handler
software program allows sensor curves to
be easily loaded and manipulated.
Sensor inputs for the Model 325 are
factory configured and compatible with
either diodes/RTDs or thermocouple
sensors. Your choice of two diode/
RTD inputs, one diode/RTD input
and one thermocouple input, or two
thermocouple inputs must be specified at
time of order and cannot be reconfigured
in the field. Software selects appropriate
excitation current and signal gain levels
when the sensor type is entered via the
instrument front panel.
The Lake Shore SoftCal™ algorithm for silicon diode
1
and platinum RTD sensors is a good solution for
applications requiring more accuracy than a standard
sensor curve but not in need of traditional calibration.
SoftCal uses the predictability of a standard curve to
improve the accuracy of an individual sensor around a
few known temperature reference points.
algorithm
1