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Instructions,
Platinum Resistance Thermometer Installation
HEAT SINKING/THERMAL ANCHORING
1.
Since the area being measured is read through the
body of the sensor, heat flow through the connecting
leads can create an offset between the sensor and
the true sample temperature. Thermal anchoring of
the connecting wires is necessary to ensure that the
sensor and the leads are at the same temperature as
the sample.
2.
Connecting wires should be thermally anchored at
several temperatures between room temperature and
cryogenic temperatures to guarantee that heat is not
being conducted through the leads to the sensor.
Two different sizes of copper bobbins are available
from Lake Shore for heat sinking connecting leads:
P/N 9007-900 (large) and 9007-901 (small).
3.
If connecting wires have a thin insulation such as
Formvar or polyimide, a simple thermal anchor can be
made by winding the wires around a copper post,
bobbin, or other thermal mass. A minimum of five
wraps around the thermal mass should provide
sufficient thermal anchoring. However, if space
permits, additional wraps are recommended for good
measure. To maintain good electrical isolation over
many thermal cycles, it is good practice to first varnish
a single layer of cigarette paper to the anchored area,
and then wrap the wire around the paper and bond in
place with a thin layer of IMI 7031 varnish. Formvar
wiring insulation has a tendency to craze with the
application of IMI varnish. If used, the wires cannot be
disturbed until the varnish is fully cured and all
solvents have evaporated (typically 12 to 24 h).
4.
A final thermal anchor at the sample itself is good
practice to ensure thermal equilibrium between the
sample and temperature sensor.
CALIBRATED SENSORS
As a 2-lead device, the resistance of a platinum sensor's
leads is included in the measured resistance during
calibration. Cutting the leads to a shorter length, tinning a
greater portion of the lead length with solder, or otherwise
changing the point of lead attachment will affect the overall
measurement even if a 4-lead measurement is made to
the point of connection with the sensor leads. Additionally,
the lead material is not pure platinum, but rather an alloy
or coated wire used to increase solderability and provide a
more robust lead. The lead material is Pt-Rh (5%) alloy
wire for the PT-103, platinum-coated palladium wire for the
PT-102, and platinum-coated nickel wire for the PT-111.
During calibration at Lake Shore, contact to each lead is
made within 1 mm of the lead end. An estimate of the
calibration shift for each 1 mm lead length change is given
below for each of the three models. For the PT-103 sensor
Form Number F022-00-00 Revision F — ©2017 Lake Shore Cryotronics, Inc. — 2 May 2017 — Page 4
(which shows a relatively significant lead error below
30 K), the two sensor leads are branched into four leads
prior to calibration, and the sensor is delivered with the
four leads attached. The extensions are bare nickel wire
brazed to the sensor leads using Fusion Inc. LHK-1235-
701 silver brazing paste.
Calibration shift (mK) per 1 mm
Temperature (K)
PT-102 or PT-111
295
77
20
CRYOGENIC ACCESSORIES — Recommended for
proper installation and use of platinum sensors
®
Stycast
Epoxy 2850FT (P/N 9003-020, 9003-021):
Permanent attachment, excellent low temperature
properties, poor electrical conductor, low cure shrinkage.
®
Apiezon
N Grease (P/N 9004-020): Low viscosity, easy
to use, solidifies at cryogenic temperatures, excellent
lubricant.
IMI 7031 Varnish (P/N 9009-002): Nonpermanent
attachment, excellent thermal conductor, easy to apply
and remove.
Indium Solder (P/N 9007-002-05): 99.99% pure, excellent
electroplating material, foil form.
90% Pb 10% Sn Solder (P/N 9008-001): Greater lead
content for higher temperature applications no greater
than 200 °C.
Phosphor Bronze Wire (P/N 9001-00X): Available in
single, duo, and quad strands, no magnetic attraction, low
thermal conduction.
®
Manganin
Wire (P/N 9001-00X): Low thermal
conductivity, high resistivity, no magnetic attraction.
Heat Sink Bobbin (P/N 9007-900 large, 9007-901 small):
Gold-plated oxygen-free high-conductivity (OFHC) copper
bobbins.
Instruments: Lake Shore sells a complete line of
instrumentation for use with platinum sensors, including
current sources, cryopump monitors, temperature
controllers, monitors and thermometers, temperature
scanners and transmitters.
For complete product description and detailed
specifications on the above accessories and instruments,
consult the Lake Shore Temperature Measurement and
Control Catalog, call (614) 891-2243, e-mail
sales@lakeshore.com, or visit our website at
www.lakeshore.com.
lead length change
PT-103
8.9
9.5
2.1
5.4
0.5
24
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