Lakeshore 231, User Manual

Page: 20 / 43

Lake Shore Model 231 User’s Manual
2-4 Installation
sensor through the base. A thin braze joint around the sides of the SD
package electrically connect to the sensing element. Avoid contact to the
sides with any electrically conductive material.
When installing the sensor, make sure there are no electrical shorts or
current leakage paths between the leads or between the leads and ground.
If IMI-7031 varnish or epoxy is used, it may soften varnish-type lead
insulations so that high resistance shunts appear between wires if
sufficient
time for curing is not allowed .
Slide Teflon
®
spaghetti tubing 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 could fracture a solder
joint or lead if installed under tension at room temperature.
For temporary mounting in cold temperature applications, apply a thin layer
of Apiezon
®
N Grease between the sensor and sample to enhance thermal
contact under slight pressure. The preferred method for mounting the
DT-470-SD sensor is the Lake Shore CO Adapter.
CAUTION: Lake Shore will not warranty replace any device damaged by
user-designed clamps or solder mounting.
For semi-permanent mountings, use Stycast epoxy instead of Apiezon
®
N
Grease. NOTE: Do not apply Stycast epoxy over the DT-470-SD package
—
sensor stress may shift the readings. In all cases, periodically inspect the
sensor mounting to verify good thermal contact to the mounting surface is
maintained.
For the Model 231P, Series PT-100 Platinum Sensors follow the same basic
procedures for diode type sensors. However, Platinum sensors have no lead
polarity, and some of the materials used at cold temperatures will not
tolerate the high-temperature range of the Platinum sensor.
2.3.4 Measurement Errors Due To AC Noise
Poorly shielded leads or improperly grounded measurement systems can
introduce AC noise into the sensor leads. In diode sensors, the AC noise
shifts the DC voltage measurement due to the diode non-linear
current/voltage characteristics. When this occurs, measured DC voltage is
too low and the corresponding temperature reading is high. The
measurement error can approach several tenths of a kelvin.
To determine if
this problem exists, perform either procedure below.
1. Place a capacitor across the diode to shunt induced AC currents.
Capacitor size depends on the noise frequency. If noise is related to
power line frequency, use a 10 µF capacitor. If AC-coupled digital noise
is suspected (digital circuits or interfaces), use a 0.1 to 1 µF capacitor. In
either case, if measured DC voltage increases, there is induced noise in
the measurement system.