Lakeshore 325, Руководство пользователя, Страница: 27 / 166

Lake Shore Model 325 Temperature Controller User’s Manual
Cooling System Design 2-5
2.3.5 Contact Pressure
When sensors are permanently mounted, the solder or epoxy used to hold the sensor act as both gasket and adhesive.
Permanent mounting is not a good solution for everyone because it limits flexibility and can potentially damage sensors.
Much care should be taken not to over heat or mechanically stress sensor packages. Less permanent mountings require
some pressure to hold the sensor to its mounting surface. Pressure greatly improves the action of gasket material to
increase thermal conductivity and reduce thermal gradients. A spring clamp is recommended so that different rates of
thermal expansion do not increase or decrease pressure with temperature change.
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 should not be a good
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.
Drawing
Not To Scale
To Room
Temperature
Refrigerator
Expander
Refrigerator
Second Stage
Dental Floss
Tie-Down
Thermal Anchor
(Bobbin)
Thermal Anchor
(Bobbin)
Radiation Shield
Sensor
Cold Stage and
Sample Holder
Optical Window
(If Required)
Cryogenic Tape
Cryogenic Wire
(small diameter,
large AWG)
-or-
Vacuum Shroud
Vacuum Space
Heater
(wiring not shown
for clarity)
P-325-2-2.bmp
Figure 2-2. Typical Sensor Installation In A Mechanical Refrigerator