Lake Shore Model 218 Temperature Monitor User’s Manual
2-6
Sensor Considerations
2.3.6 Lead
Wire
Different types of sensors come with different types and lengths of electrical leads. In general a
significant length of lead wire must be added to the sensor for proper heat sinking and connecting to
a bulk head connector at the vacuum boundary. The lead wire must be a good electrical conductor,
but a poor
thermal
conductor, or heat will transfer down the leads and change the temperature
reading of the sensor. Small 30 to 40 AWG wire made of an alloy like phosphor bronze is much better
than copper wire. Thin wire insulation is preferred and twisted wire should be used to reduce the
effect of RF noise if it is present. The wire used on the room temperature side of the vacuum
boundary is not critical so copper cable is normally used.
2.3.7 Lead
Soldering
When additional wire is soldered to short sensor leads, care must be taken not to overheat the
sensor. A heat sink such as a metal wire clamp or alligator clip will heat sink the leads and protect the
sensor. Leads should be tinned before bonding to reduce the time that heat is applied to the sensor
lead. Solder flux should be cleaned after soldering to prevent corrosion.
2.3.8 Heat Sinking Leads
Sensor leads can be a significant source of error if they are not properly heat sinked. Heat will
transfer down even small leads and alter the sensor reading. The goal of heat sinking is to cool the
leads to a temperature as close to the sensor as possible. This can be accomplished by putting a
significant length of lead wire in thermal contact with every cooled surface between room temperature
and the sensor. Lead wires can be adhered to cold surfaces with varnish over a thin electrical
insulator like cigarette paper. They can also be wound onto a bobbin that is firmly attached to the cold
surface. Some sensor packages include a heat sink bobbin and wrapped lead wires to simplify heat
sinking.
2.3.9 Thermal
Radiation
Thermal (black body) radiation is one of the ways heat is transferred. Warm surfaces radiate heat to
cold surfaces even through a vacuum. The difference in temperature between the surfaces is one
thing that determines how much heat is transferred. Thermal radiation causes thermal gradients and
reduces measurement accuracy. Many cooling systems include a radiation shield. The purpose of the
shield is to surround the load, sample, and sensor with a surface that is at or near their temperature
to minimize radiation. The shield is exposed to the room temperature surface of the vacuum shroud
on its outer surface, so some cooling power must be directed to the shield to keep it near the load
temperature. If the cooling system does not include an integrated radiation shield (or one cannot be
easily made), one alternative is to wrap several layers of super-insulation (aluminized mylar) loosely
between the vacuum shroud and load. This reduces radiation transfer to the sample space.
2.3.10 Thermal EMF Compensation with Voltage Excitation
Sensors used at low temperatures must operate with little power dissipated in the sensor. To keep
power low, the voltage across the sensor is kept low. Two major problems occur when measuring
small DC voltages. The first is external noise entering the measurement through the sensor leads
which is discussed with sensor setup. The second is the presence of thermal EMF voltages,
sometimes called thermocouple voltages, in the lead wiring. Thermal EMF voltages appear whenever
there is a temperature gradient across a piece of voltage lead. They can be canceled in the
measurement with a similar temperature gradient in the other voltage lead. Thermal EMF voltages
must exist because the sensor is almost never the same temperature as the instrument. Minimize
them by careful wiring, verifying voltage leads are symmetrical in the type of metal used and how they
are joined, and by keeping unnecessary heat sources away from the leads. Even in a well designed
system, thermal EMF voltages can be an appreciable part of a low voltage sensor measurement.
The Model 218 has no thermal correction algorithm. Other instruments automatically reverse the
current source polarity and average the positive and negative sensor readings to cancel the thermal
EMF voltage. Account for thermal EMF errors when estimating Model 218 measurement accuracy.
