2.4.9 Thermal Radiation
19
|
www.lakeshore.com
2.4.9 Thermal
Radiation
Thermal (blackbody) 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 sur-
round the sample stage, sample, and sensor with a surface that is at or near their tem-
perature 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.
2.5 Heater
Selection and
Installation
There is a variety of resistive heaters that can be used as the controlled heating source
for temperature control. The mostly metal alloys like nichrome are usually wire or foil.
Shapes and sizes vary to permit installation into different systems.
2.5.1 Heater Resistance
and Power
Cryogenic cooling systems have a wide range of cooling power. The resistive heater
must be able to provide sufficient heating power to warm the system. The Model 335
can provide up to 75 W of power from Output 1, up to 25 W of power from
Output 2 in current mode, and up to 1 W of power from Output 2 in voltage mode.
TABLE 2-2 provides the current and voltage limits, as well as the resulting
maximum power for each output for the 25
)
and 50
)
settings, using nominal
heater load values.
Current and voltage limits with resulting max power
25
)
setting (25
)
heater)
50
)
s
etting (50
)
heater)
Output 1
Current limit
1.41 A
1 A
Voltage limit
50 V
50 V
Max power
75 W
50 W
Output 2
(current mode)
Current limit
1 A
0.71 A
Voltage limit
35.4 V
35.4 V
Max power
25 W
25 W
Output 2
(voltage mode)
Current limit
100 mA
Voltage limit
±10 V
Max power
1 W
Even though the Model 335 heater outputs are current sources (Output 2 in current
mode), they have a voltage limit called the compliance voltage. This compliance volt-
age also limits maximum power. So for heater values other than 25
)
or 50
)
, the
maximum power must be calculated using the following equations: P = I
2
R and
P = V
2
/R, where P is maximum power, I is max current, V is max voltage, and R is the
heater resistance. The current and voltage limits are in place at the same time, so the
smaller of the two computations gives the maximum power available to the heater.
When using Output 2 in voltage mode, the output behaves as a voltage source. In this
mode the output is limited to ±10 V, and ±100 mA. As with the current source mode,
both limits are in place at the same time, and the same equations can be used to
determine the maximum power.
TABLE 2-2
