Lake Shore Model 321 Autotuning Temperature Controller User’s Manual
2-6
Installation
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, and all
thermal contact to the sensor should be made through the base. A thin braze joint around the
sides of the SD package is electrically connected to the sensing element. Contact to the sides
with any electrically conductive material must be avoided.
When installing the sensor, verify 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
. Teflon
®
spaghetti tubing is useful for sliding over bare leads when
the possibility of shorting exists. Also, avoid putting stress on the device leads and allow for the
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, a thin layer of Apiezon
®
N Grease may
be used between the sensor and sample to enhance the 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 a user-designed clamp
or damaged by solder mounting.
If semi-permanent mountings are desired, the use of Stycast
®
epoxy can replace the use of
Apiezon
®
N Grease. (Note: Do not apply Stycast epoxy over the DT-470-SD package. Stress on
the sensor can cause shifts in the readings.) In all cases, the mounting of the sensor should be
periodically inspected to verify that good thermal contact to the mounting surface is maintained.
For the Model 321-02, Series PT-100 Platinum Sensors follow the same procedures for diode
type sensors. The difference is 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.
2.7.1.4
Measurement Errors Due To AC Noise
Poorly shielded leads or improperly grounded measurement systems can introduce AC noise into
the sensor leads. For diode sensors, the effect of the AC noise appears as a shift in the DC voltage
measurement due to the non-linear current/voltage characteristics of the diode. When this occurs,
the DC voltage measured will be too low and the corresponding temperature indication will be high.
The resulting measurement error can approach several tenths of a kelvin.
For Series PT-100 Platinum Sensors, the noise will not cause a DC shift, but it can still degrade the
accuracy of the measurement. To determine if this is a problem in your measurement system,
perform either of the two following procedures.
1. Place a capacitor across the diode to shunt the induced AC currents. The size of the capacitor
will depend on the frequency of the noise. If the noise is related to the power line frequency,
use a 10 µF capacitor. If AC-coupled digital noise is suspected (digital circuits or interfaces),
then use a capacitor between 0.1 to 1 µF. In either case, if the resultant DC voltage measured
is observed to increase, there is induced noise in your measurement system.
2. Measure the AC voltage across the diode with an AC voltmeter or oscilloscope. Note that most
voltmeters will not have the frequency response to measure noise associated with digital
circuits or interfaces (which operate in the MHz range). A thorough discussion of this potential
problem, and the magnitude of error which may result, is given in the paper “Measurement
System-Induced Errors In Diode Thermometry,” J.K. Krause and B.C. Dodrill, Rev. Sci. Instr. 57
(4), 661, April, 1986; which is available from Lake Shore upon request.
The potential for this type of error can be greatly reduced by connecting twisted leads (pairs)
between the controller and the diode sensors when an AC noise environment exists. We
recommend the use of Duo-Twist™ Cryogenic Wire, which features phosphor bronze wire, 32 or
36 AWG, twisted at 3.15 twists per centimeter (8 twists per inch). Duo-Twist wire is available from
Lake Shore. Refer to the Lake Shore Product Catalog or contact Lake Shore for further information.
