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Lake Shore Model 325 Temperature Controller User’s Manual
Installation 3-7
3.5.2 Thermocouple Installation
Thermocouples are commonly used in high-temperature applications. Cryogenic use of thermocouples offers some
unique challenges. A general installation guideline is provided in Section 2.3. Consider the following when using
thermocouples at low temperatures:
• Thermocouple wire is generally more thermally conductive than other sensor lead wire. Smaller gauge wire and more
heat sinking may be needed to prevent leads from heating the sample.
• Attaching lead wires and passing through vacuum tight connectors are often necessary in cryogenic systems.
Remember, the thermocouple wire is the sensor; any time it joins or contacts other metal, there is potential for error.
• Temperature verification and calibration of room temperature compensation is difficult after the sensor is installed.
When possible, keep a piece of scrap wire from each installation for future use.
3.5.3 Grounding and Shielding
For lowest measurement noise, do not ground thermocouple sensors. The instrument operates with more noise if one of
the thermocouples is grounded. Grounding both thermocouples is not recommended. The instrument does not offer a
shield connection on the terminal block. Twisting the thermocouple wires helps reject noise. If shielding is necessary,
extend the shield from the oven or cryostat to cover the thermocouple wire, but do not attach the shield to the instrument.
3.6 HEATER OUTPUT SETUP
The following section covers the heater wiring from the vacuum shroud to the instrument for both control loop outputs.
Specifications are detailed in Section 1.2. For help on choosing and installing an appropriate resistive heater, refer to
Section 2.4.
3.6.1 Loop 1 Output
Of the two Model 325 control loops, Loop 1 is considered the primary loop because it is capable of driving 25 W of
heater power. The heater output for Loop 1 is a traditional control output for a cryogenic temperature controller. It is a
variable DC current source with software settable ranges and limits. The heater is configurable for optimization using
either a 25

or a 50

heater resistance. At the 25

setting, the maximum heater output current is 1 A and the
compliance voltage is 25 V. At the 50

setting, the maximum heater output current is 0.71 A and the compliance
voltage is 35.4 V. Heater power is applied in one of two ranges: Low or High. At the Low range setting, the Loop 1
heater will output 10% of the High range power.
3.6.2 Loop 1 Heater Output Connector
A dual banana jack on the rear panel of the instrument is used for connecting
wires to the Loop 1 heater. A standard dual banana plug mating connector is
included in the connector kit shipped with the instrument. This is a common
jack and additional mating connectors can be purchased from local electronic
suppliers, or from Lake Shore (P/N 106-009). The heater is connected
between the HI and LO terminals. The ground terminal is reserved for
shielding the heater leads when necessary.
3.6.3 Loop 1 Heater Output Wiring
Heater output current is what determines the size (gauge) of wire needed to connect the heater. The maximum current
that can be sourced from the Loop 1 heater output is 1 A. When less current is needed to power a cooling system it can
be limited with range settings.
When setting up a temperature control system, the lead wire for the heater must be capable of carrying a continuous
current that is greater than the maximum current. Wire manufactures recommend 30 AWG or larger wire to carry 1 A of
current, but there is little advantage in using wire smaller than 20 to 22 AWG outside the cryostat. Inside the cryostat,
smaller gauge wire is often desirable.
It is recommended to use twisted heater leads. Large changes in heater current can induce noise in measurement leads
and twisting reduces the effect. It is also recommended to run heater leads in a separate cable from the measurement
leads to further reduce interaction.