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Cryogenic Temperature Controllers
Maximum flexibility: Two multipurpose input
channels support Diode, Platinum RTD and
most cryogenic NTC resistive temperature
sensors. A Thermocouple input is optional. Constant-Voltage, AC excitation of resistive
sensors extends their useful temperature range
down to 200mK and improves accuracy at
higher temperatures. Unique synchronous filter significantly
improves control accuracy and stability with
cryocooler systems. Loop #1 Primary control loop: 50 Watt, 50 ,
three-range linear heater output. Loop #2 Secondary control loop of the Model
32B is a 10 Watt, 50 , linear output. The
standard Model 32 provides a second loop
output of zero to ten volts. Fail-safe cryostat over-temperature protection
features protect user equipment from damage. Extremely low noise design allows operation in
the most demanding of cryogenic
environments. CalGen
®
feature allows the user to custom fit
any Diode, Thermocouple or resistor sensor
calibration curve. Proven, tested autotuning optimized for
cryogenic systems. Flexible, easy to configure VFD display. PC Windows
®
based Utility software provides a
graphical control panel to simplify
configuration, integration and testing. Remote interfaces include RS-232 and IEEE-
488.2 (GPIB). Industry standard SCPI command
language. National Instruments, Inc. LabVIEW
drivers are available. NIST traceable calibration. European CE
certified.
Cryogenic Temperature Controllers

Model 32/32B
Major highlights of the Model 32:
The Model 32 sets the standard for simplicity, useable features and price. It is a cost-
effective, two-channel controller designed for fixed cryogenic applications (i.e., cryostat
and cryocooler) where measurement accuracy and control stability are the two primary
requirements. The Cryo-con
®
Model 32 is a precision Cryogenic Temperature Controller
designed for single control loop applications. The Model 32B adds a second, medium
power output for systems that require dual-loop control in a single unit. Input Flexibility
The Model 32 has two sensor inputs, both of which
can be easily configured to support virtually any type
of cryogenic thermometer. Configuration is performed
from the instruments front panel or a remote
interface.
Silicon Diode sensors from Cryo-con
®
or any other
manufacturer are directly supported over their full 1.4
to 500K range using built-in calibration curves and
sensor data. Plus, non-volatile Flash memory is
available for several custom or calibrated sensors.
A unique feature of the Model 32 is the use of a
ratiometric resistance bridge technique to measure
Positive Temperature Coefficient (PTC) resistor
sensors including Platinum and Rhodium-Iron RTDs.
This significantly reduces low frequency noise and
drift to provide rock-solid measurements
Platinum RTD sensors the use built-in DIN 43760
(IEC 750) standard curves for 100 , 1,000 or 10K
devices. The standard curve is used for temperatures
from 70K to 1020K and is extended down to 30K for
cryogenic use. Operation to about 14K is possible with
user-supplied curves.
The Model 32 provides robust support for the
Negative Temperature Coefficient (NTC) sensors
commonly used by cryogenic applications. They
include Ruthenium-oxide, Cernox , Carbon-
Glass , Germanium and several others. Since they
have a negative temperature coefficient, the constant-
voltage measurement method will reduce, rather than
increase, power dissipation in the sensor as
temperature decreases. By maintaining the lowest
possible power level, sensor self-heating is minimized
and useful temperature range is greatly increased.
An additional advantage to constant-voltage biasing is
that NTC resistors lose sensitivity in the upper part of
their range. By auto-ranging excitation current to
maintain a constant voltage, sensitivity and noise
immunity in that range is greatly improved.
Sensor excitation used in conjunction with the
constant-voltage feature is a 1.25Hz bipolar square
wave. This effectively cancels thermal EMF induced
offset errors that sometimes occur in cryogenic
systems. The maximum and minimum sensor
resistance
that can be
read is a
function of
the selected
voltage bias.

Thermocouple measurements are supported via a
factory installed option on a single input channel. This
option replaces the standard input connector with a
spade type thermocouple connector. The Cold-
Junction compensation temperature is measured in an
internal, insulated space so that errors commonly
caused by air currents are eliminated.

Measurement accuracy is obtained by using 24-bit
analog to digital conversion at a minimum sample rate
of 10Hz per channel and is further enhanced by
extensive use of Digital Signal Processing (DSP).
Conversion of a sensor measurement into
temperature is performed by using a Cubic Spline
interpolation algorithm. The Model 32 includes built-in
curves that support most industry standard
temperature sensors. Additionally, four user
calibration curves are available for custom or
calibrated sensors. Each user curve may have up to
200 entries and may be entered from the front panel,
or transferred via any of the available remote
interfaces.
New calibration curves may be generated using the
CalGen
®
feature to fit any existing Diode, Platinum or
NTC resistor calibration curve at up to three user
specified temperature points. This provides an easy
and effective method for obtaining higher accuracy
temperature measurements without expensive sensor
calibrations.
The Model 32 continuously tracks temperature history,
independently on each input channel and provides a
statistical summary that indicates the channel's
minimum, maximum, average and standard deviation.
Also shown are the slope and the offset of the best-fit
straight line of temperature history data.
Dual Control Loops
The Loop #1 Heater channel is a linear, low noise RFI
filtered current source that can provide up to 1.0
Ampere into 50 or 25 resistive loads. Three full-
scale ranges are available in decade increments.
The standard Model 32 offers a 0 to 10 Volt output for
a second control loop. This may be used with a
programmable booster power supply where dual-loop
or high power control is required.
Resistance Range Table
Voltage
Bias
Min.
Resistance
Max.
Resistance
10.0mV
10 1.0M
3.33mV
3.3 430K
1.0mV
1 100K For applications that require dual-loop control from a
single instrument, the Model 32B features a 10W
linear heater output as the Loop #2 output. The Loop
#1 and Loop #2 heaters are completely independent
and either heater can be controlled by either sensor
input.
Control modes are Manual, PID, Ramp and PID
Table. The industry standard Proportional-Integral-
Derivative or PID control loop is implemented as a
DSP algorithm and is enhanced to minimize set-point
overshoot and differentiator noise.
For cryocooler applications, the Model 32 offers a
Cryocooler Filter that will synchronously subtract the
coolers thermal signal from the input in order to
provide a significant improvement in baseline
temperature control. Since classic PID control loops
often track and even amplify the low frequency signal
generated by the cooler, synchronous filtering
provides a major improvement in control accuracy and
stability.
The direction of the control loop can be easily
reversed to accommodate thermoelectric type coolers
where power is applied to cool rather than heat.
The field proven Autotune function of the Model 32
involves the use of a specific output waveform to first
develop a process model, then generate the optimum
P, I and D coefficients.
Cryogenic systems often require stable control over a
wide range of temperatures. Here, control loop tuning
parameters can be significantly different at different
temperatures. For this reason, the Model 32 offers
PID tables that store optimum tuning parameters vs.
setpoint temperature. Six PID tables are available.
Each contains PID and heater range settings for up to
16 temperature set points.
The Model 32 will perform a temperature ramp
function using a specified maximum ramp rate and
ta