Heat Sinking Sensor Leads; Sensor Mounting - Lakeshore 330 User Manual

Lake Shore Model 330 Autotuning Temperature Controller User's Manual
2.6.1.2

Heat Sinking Sensor Leads

Excessive heat flow through connecting leads to any temperature sensor puts the active sensing element is at
a different temperature than the sample to which the sensor mounts. This yields a temperature offset between
what is measured and the true sample temperature. To eliminate such temperature errors, select and install
connecting leads properly.
To minimize any heat flow through the leads, use leads of small diameter and low thermal conductivity.
Phosphor-bronze or Manganin wire is commonly used in sizes 32 or 36 AWG. Though these wires have fairly
low thermal conductivity, the electrical resistance is not large enough to create problems in measurements.
Thermally anchor lead wires at several temperatures between room temperature and cryogenic temperatures
to guarantee minimal heat conductivity through the leads to the sensor.
2.6.1.3

Sensor Mounting

Before installing a diode sensor, identify the anode and the cathode.
When viewed with the base down and with the leads towards the
observer, the positive lead (anode) is on the right and the negative
lead (cathode) is on the left. The figure to the right shows the Lake
Shore DT-470-SD silicon diode sensor lead configuration. For other
sensors, read the accompanying literature or consult the
manufacturer to ensure positive identification of sensor leads. Be
sure the lead identification remains clear even after installation.
Record the sensor serial number and location.
On the DT-470-SD, the base is the largest flat surface. It is sapphire
with gold metallization over a nickel buffer layer. The base is electrically isolated from the sensing element and
leads; make all thermal contact to the sensor through the base. A thin braze joint around the sides of the SD
package electrically connects to the sensing element. Avoid contact to the sides with any electrically
conductive material.
When installing the sensor, verify there are no electrical shorts or current leakage paths between the leads or
between the leads and ground. IMI-7031 varnish or epoxy may soften varnish-type lead insulation so that high
resistance shunts appear between wires if there was insufficient time for curing. Slide Teflon
over bare leads when the possibility of shorting exists. Avoid putting stress on the device leads and allow for
thermal contractions that occur during cooling which may fracture a solder joint or lead installed under tension
at room temperature.
For temporary mounting in cold temperature applications, use a thin layer of Apiezon
sensor and sample to enhance the thermal contact under slight pressure. The preferred method to mount the
DT-470-SD sensor is the Lake Shore CO Adapter.
CAUTION
• Use a heat sink when soldering sensor lead wires.
• Lake Shore will not warranty replace any device damaged by solder mounting or use of a user-
designed clamp.
NOTE: Apply Stycast ®
places stress on the sensor that may cause shifts in readings.
For semi-permanent mounting, use Stycast
inspect sensor mounting to verify that good thermal contact to the mounting surface is maintained.
For the Model 330-2X, Series PT-100 Platinum Sensors follow the same procedures for diode type sensors.
The difference is Platinum sensors have no lead polarity and some materials used at cold temperatures will
not tolerate the high temperature range of the Platinum sensor.
Installation
epoxy only to underneath of the DT-470-SD package. Covering the sensor with epoxy
epoxy instead of Apiezon
®
DT-470-SD
Diode Sensor Leads
Cathode
®
N Grease between the
®
N Grease. In all cases, periodically
®
Anode
spaghetti tubing
2-5