Lake Shore Model 321 Autotuning Temperature Controller User’s Manual
Installation
2-7
2.7.2 Thermocouple (Model 321-04) Connections
The thermocouple input has a thermal block for connecting thermocouple wires. The positive and
negative terminals correspond to V+ and V– and should match the polarity of the thermocouple used.
Be sure to tighten the screw terminals carefully. Loose connections will result in unstable readings
and control. For additional information on thermocouple operation, refer to Paragraph 3.5.
2.7.2.1 Thermocouple
Compensation
The thermocouple input has a thermal block for connecting thermocouple wires and for temperature
compensation. The thermocouple response curve tables within the instrument are normalized to the
ice point of water. Consequently, accurate readings can be obtained by one of two methods. An ice
bath with a reference junction can be used with the internal room temperature compensation turned
OFF. The more convenient method is to eliminate the reference junction with its associated ice bath
and use the internal electronic room temperature compensation by turning the internal
compensation ON.
2.7.2.2
Thermocouple Wire Types at Cryogenic Temperatures
Below are recommended thermocouple wire types for cryogenic temperatures. The ANSI color
code for thermocouples is red for the negative lead, while the type of thermocouple determines the
positive lead color: purple (Type E), black (Type J), yellow (Type K), and blue (Type T). For details
on thermocouples or other sensors, see the Lake Shore Temperature Sensor Guide.
Chromel™ vs. Gold with 0.03% or 0.07% Atomic Iron
(0.03% not currently sold by Lake Shore)
Consists of Gold (Au) doped with 0.03*
or 0.07 atomic percent Iron (Fe) as the negative
thermoelement and a Ni-Cr alloy (Chromel™) as the positive thermoelement. This thermocouple
has relatively high temperature sensitivity below 25 K, and usable sensitivity below 10 K. It is
widely used in cryogenic applications due to its relatively high thermoelectric sensitivity (>15 µV/K
above 10K). Recommended useful temperature range for the 0.03% Fe is 4 K to 325 K, and for
the 0.07% Fe is 1.4 K.
Type E (Chromel™-Constantan)
Type E is a thermocouple pair consisting of a Ni-Cr alloy (Chromel™) as the positive
thermoelement and a Cu-Ni alloy (Constantan) as the negative thermoelement. It has the highest
sensitivity of the three standard thermocouples (E, K and T) typically used for low temperature
applications: 8.5 µV/K at 20K. This thermocouple is best for temperatures down to 40 K. It is
recommended for oxidizing or inert environments. Do not use it in sulfurous or reducing
atmospheres, or environments that promote corrosion. Recommended useful temperature range
is 3 K to 475 K.
Type K (Chromel™-Alumel™)
Type K is a thermocouple pair consisting of a Ni-Cr alloy (Chromel™) as the positive
thermoelement and a Cu-Al alloy (Alumel™) as the negative thermoelement. It may be used in
inert environments, but not in sulfurous or reducing atmospheres, or environments that promote
corrosion. Sensitivity at 20K: 4.1 µV/K. Recommended useful temperature range is 3 K to 575 K.
Type T (Copper-Constantan)
Type T is a thermocouple pair consisting of Cu (Copper) as the positive thermoelement and a Cu-
Ni alloy (Constantan) as the negative element. It may be used in a vacuum as well as oxidizing,
reducing or inert environments down to 90 K. At temperatures below 80 K, the thermoelectric
properties of the positive thermoelement depend largely on the impurity of iron. The high thermal
conductivity of the copper element makes this thermocouple the least usable for cryogenic
applications. Sensitivity at 20 K: 4.6 µV/K.
Chromel™-CuFe (0.15%)
The Chromel™-Copper/Iron thermocouple consists of a Ni-Cr alloy (Chromel™) as the positive
thermoelement and a Copper/0.15% Iron alloy as the negative thermoelement. Sensitivity at
4.2K: >11 µV/K. Less expensive than Gold-Chromel™ thermocouples and physically stronger.
Recommended useful temperature range is 4 K to 300 K.