Lake Shore Model 321 Autotuning Temperature Controller User’s Manual
D-10
Application Notes
DT-470 SERIES TEMPERATURE SENSORS INSTALLATION AND OPERATION
There are three aspects of using a temperature sensor which are critical to its optimum performance. The first involves the
proper electrical and thermal installation of the connecting leads which run to the sensor, while the second aspect is the
actual mounting of the sensor to the sample assembly. The final concern is the measurement electronics used for reading
and recording temperature data from the sensor.
CONNECTING LEADS
Although the majority of the DT-470 series sensors are two lead devices, measurements should preferably be made using
a four wire configuration to avoid all uncertainties associated with the lead resistance. This is done by using four
connecting leads to the device and connecting the V+ and I+ leads to the anode and the V- and I- leads to the cathode as
shown in Figure 1. The exact point at which the connecting leads are soldered to the device leads results in negligible
temperature measurement uncertainties.
In a two wire measurement configuration, the voltage connections (point A in Figure 1) are made near or at the current
source so only two leads are actually connected to the device. Some loss in accuracy can be expected since the voltage
measured at the voltmeter is the sum of the diode voltage and the voltage drop across the connecting leads. The exact
temperature uncertainty will depend on the temperature range and lead resistance. For a 10 ohm lead resistance, the
diode voltage will be offset by 0.1 mV which gives a negligible temperature error at liquid helium temperature but a 50mK
error near liquid nitrogen temperature. Note the DI and CY adapter can be used only in a two wire configuration.
An excessive heat flow through the connecting leads to any temperature sensor can create a situation where the active
sensing element (for the DT-470 this is the diode chip) is at a different temperature than the sample to which the sensor is
mounted. This is then reflected as a real temperature offset between what is measured and the true sample temperature.
Such temperature errors can be eliminated by proper selection and installation of the connecting leads.
In order to minimize any heat flow through the leads, the leads should be of small diameter and low thermal conductivity.
Phosphor-bronze or manganin wire is commonly used in sizes 32 or 36 AWG. These wires have a fairly poor thermal
conductivity yet the resistivities are not so large as to create any problems in four wire measurements.
Lead wires should also 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. A final thermal anchor at the
sample itself is a good practice to assure thermal equilibrium between the sample and temperature sensor. Note that the
CU, CY, BO, and DI mounting adapters serve as their own sample thermal anchor.
If the connecting leads have only a thin insulation such as Formvar or other varnish type coating, a simple thermal anchor
cn be made by winding the wires around a copper post or other thermal mass and bonding them in place with a thin layer
of GE 7031 varnish. There are a variety of other ways in which thermal anchors can be fabricated and a number of
guidelines which may be found in detail in the references given below.
SENSOR MOUNTING
General Comments
Before installing the DT-470 sensor, identify which lead is the anode and which lead is the cathode by referring to the
accompanying device drawings. Be sure that the lead identification remains clear even after installation of the sensor, and
record the serial number and location.
The procedure used to solder the connecting leads to the sensor leads is not very critical and there is very little danger in
overheating the sensor, If for some reason the leads have to be cut short, they should be heat sunk with a copper clip or
needle-nose pliers before soldering. Standard rosin core electronic solder (m.p. - 180 C) is suitable for most applications.
Applications involving the use of the SD package up to 200° C will require a higher melting point solder. A 90% Pb 10%
Sn solder has been used quite successfully with a rosin flux.
For all adapters except the CY, CU, and DI, the leads are a gold-plated Kovar. Prolonged soldering times may cause the
solder to creep up the gold plated leads as the solder and gold alloy. This is not detrimental to the device performance.
When installing the sensor, make sure there are no shorts or leakage resistance between the leads or between the leads
and ground. GE-7031 varnish or epoxy may soften varnish-type 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 contractions that occur
during cooling which could fracture a solder joint or lead if installed under tension at room temperature.
The DT-470 sensor is designed for easy removal for recalibration checks or replacement and the following discussions for
each of the adapters are geared in this direction. If semi-permanent mountings are desired, the use of Stycast epoxy can
replace the use of Apieson N Grease. In all cases, the mounting of the sensor should be periodically inspected to verify
that good thermal contact to the mounting surface is maintained.
