Thermocouple Sensor Inputs (Thermocouple Model 3060); Sensor Input Terminals; Thermocouple Installation; Grounding And Shielding - Lakeshore 335 User Manual

3.6 Thermocouple
Sensor Inputs
(Thermocouple
Model 3060)
3.6.1 Sensor Input
Terminals
3.6.2 Thermocouple
Installation
3.6.3 Grounding and
Shielding
The information in this section is for a Model 335 configured with thermocouple sen-
sor inputs. Thermocouple inputs are not installed on the standard Model 335, but can
be added by purchasing the Model 3060 dual thermocouple input option. Refer to
section 8.12 for installation of the Model 3060.
Do not leave thermocouple inputs unconnected. Short inputs when they are not in use.
Sensor connection is important when using thermocouples because the measured
signal is small. Many measurement errors can be avoided with proper sensor installa-
tion. The ceramic terminal block has two thermocouple inputs and each input has
two screw terminals; one positive , one negative (FIGURE 3-7).
Attach sensor leads to the screws on the ceramic terminal block. Remove all insula-
tion from the ends of the thermocouple wires then tighten the screws down onto the
thermocouple wires. Keep the ceramic terminal blocks away from heat sources
including sunlight, and shield them from fans or room drafts.
FIGURE 3-7 Thermocouple input definition and
common connector polarities (inputs shown shorted)
Thermocouples are commonly used in high-temperature applications. Cryogenic use
of thermocouples offers some unique challenges. A general sensor installation guide-
line is provided in section 2.4. Consider the following when using thermocouples at
low temperatures:
Thermocouple wire is generally more thermally conductive than other sensor
D
lead wire. Smaller gauge wire and more thermal anchoring may be needed to
prevent leads from heating the sample.
Attaching lead wires and passing them through vacuum tight connectors is often
D
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
D
difficult after the sensor is installed. When possible, keep a piece of scrap wire
from each installation for future use.
Thermocouples can be spot-welded to the cryostat for good thermal anchoring as
D
long as the cryostat has a potential close to earth ground.
Care must be taken to minimize the amount of noise contributed by ground loops,
when grounding thermocouple inputs. For lowest measurement noise, do not ground
thermocouple sensors. The instrument operates with slightly more noise if one of the
thermocouples is grounded. Be sure to minimize loop area when grounding both
thermocouples. 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 Thermocouple Sensor Inputs (Thermocouple Model 3060)

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