22 cHAPTER 2: Cooling System Design and Temperature Control
Model 336 Temperature Controller
It is possible to choose a heater value that results in a maximum power greater than the
power rating of 50 W for output 2, but doing so can cause the Model 336 to work improp-
erly. In this situation the max user current setting should be used to limit the power. Refer
to section 4.5.1.1.1 for details on using the max user current setting.
The resistor chosen as a heater must be able to withstand the power being dissipated
in it. Pre-packaged resistors have a power specification that is usually given for the
resistor in free air. This power may need to be derated if used in a vacuum where con-
vection cooling cannot take place and it is not adequately anchored to a cooled sur-
face. The Model 336 has a current limit feature which allows you to specify the
maximum output current for each heater output (section 4.5.1.1), which when set
appropriately will help protect the heater from being over heated.
2.5.2 Heater Location
For best temperature measurement accuracy, position the heater so that tempera-
ture gradients across the sample is minimized. For best control the heater should be
in close thermal contact with the cooling power. Geometry of the load can make one
or both of these difficult to achieve. That is why there are several heater shapes and
sizes.
2.5.3 Heater Types
Resistive wire like nichrome is the most flexible type of heater available. The wire can
be purchased with electrical insulation and has a predictable resistance per given
length. This type of heater wire can be wrapped around a load to give balanced, even
heating of the area. Similar to sensor lead wire, the entire length of the heater wire
should be in good thermal contact with the load to allow for thermal transfer. Ther-
mal anchoring also protects the wire from over heating and burning out.
Resistive heater wire is also wound into cartridge heaters. Cartridge heaters are more
convenient, but are bulky and more difficult to place on small loads. A typical car-
tridge is 6.35 mm (0.25 in) in diameter and 25.4 mm (1 in) long. The cartridge should
be snugly held in a hole in the load or clamped to a flat surface. Thermal anchoring for
good thermal contact is again important.
Foil heaters are thin layers of resistive material adhered to, or screened onto, electri-
cally insulating sheets. There are a variety of shapes and sizes. The proper size heater
can evenly heat a flat surface or around a round load. The entire active area should be
in good thermal contact with the load, not only for maximum heating effect, but to
keep spots in the heater from over heating and burning out.
2.5.4 Heater Wiring
When wiring inside a vacuum shroud, we recommend using 30 AWG copper wire for
heater leads. Too much heat can transfer in when larger wire is used. Thermal anchor-
ing, similar to that used for the sensor leads, should be included so that any heat
transfer does not warm the load when the heater is not running. The lead wires
should be twisted to minimize noise coupling between the heater and other leads in
the system. When wiring outside the vacuum shroud, you can use larger gage copper,
and twisting is still recommended.
To Next Page
Loading...
