Bird Technologies 8750 Series - Manual (Page 14)

Theory of Operation
4
Cooling Water
The electrical performance of these RF loads is affected by impurities or other
chemical additives in the cooling water. The presence of salts in the water defi-
nitely makes the device unusable because salts cause a rapid increase in VSWR.
Therefore, sea water or silty water should not be used for cooling the loads.
The thermal performance of these loads is also affected by impurities, particu-
larly those impurities that accumulate in the form of scale on the exposed sur-
faces of the fluid paths of the load assembly. These deposits may result in an
increase in the thermal and/or fluid resistance(s) of the load and may in turn
cause the load to overheat and fail.
The following types of water are considered safe for the cooling of the Models
8755 Load Resistor: filtered, city, or soft water. In general, any potable water
is suitable for cooling the load.
Flow Interlock Control Circuit
The interlock control circuit provides instantaneous fail-safe protection of the
transmitter and load in the event of even a momentary interruption of the
cooling water supply. This protection is necessary because dissipation of the
heat generated by the RF power absorption is critically dependent upon main-
taining the required minimum water flow rate regardless of inlet water tem-
perature.
The water flow switch, attached to the water inlet of the load, is factory cali-
brated to open the electrical contacts whenever the water flow drops below
seven gallons per minute and close when the water flow exceeds this value.
When the water flow switch contacts open, the time delay relay switch is deac-
tivated, which in turn opens the interlock switch, causing immediate shut-
down of the transmitter or other signal source. The time delay relay also
keeps the interlock switch open for a preset interval of 12 ±2 seconds after the
minimum flow of seven gallons per minute has been reestablished. This safe-
guarding feature assures proper operation of the cooling system before RF
power can be applied to the load, preventing damage or burnout of the resistor
element.