Bird Technologies 8251 Series-Manual - Manual (Page 13)

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Chapter 2 Theory of Operation
Load Resistor
Bird 8251 series loads consist of a thin-film-on-ceramic resistor immersed in a dielectric coolant. The resistor, indi-
vidually selected for its accuracy, is enclosed in a special housing. When surrounded by the coolant, this produces a
uniform, practically reflectionless line termination over the specified frequencies.
Coolant
The load is cooled by natural fluid and air convection currents. The coolant, chosen for its dielectric and thermal
characteristics, carries heat from the resistor to the walls of the cooling tank, where radiator fins surrounding the
tank transfer the heat to the air.
When the coolant is heated, thermal expansion causes an increase in the internal pressure. An expansion tank on
the upper rear of the load allows for coolant expansion due to the rise in temperature, and is equipped with a vent
plug to relieve this pressure while protecting the opening from dirt or other contaminants.
Thermal Interlock
Loads can be supplied with an optional passive overtemperature thermoswitch. Normally closed, it opens at either
230 or 300 °C (110 or 149 °F), depending on the setting, turning off transmitter power. The interlock system will not
permit use of the transmitter until the load has reached a safe temperature.
Figure 2 Shipping Plug
Figure 3 Vent Plug
O-ring seal
O-ring seal