Bird Technologies 6091P - Manuale (Pagina 18)

The heat exchanger removes excess heat from the coolant. The coolant reservoir
holds most of the coolant volume in the system. The reservoir allows the coolant tem-
peratures to stabilize and helps keep the average coolant temperature down.
An input temperature sensor (T
1
) measures the average coolant temperature just be-
fore it enters the load resistor. As coolant flows through the load resistor, it absorbs
the heat energy. The load resistor is very efficient, allowing nearly all of the RF
power to be converted to heat which is transferred by conduction into the coolant. An
output temperature sensor (T
2
) measures the average coolant temperature as it exits
the load resistor.
To ensure high accuracy of temperature measurements, solid-state thermistor assem-
blies are used. The thermistors are semiconductor devices that respond to small tem-
perature variations by large changes in electrical resistance. The resistance of each
thermistor is measured by the analog circuitry and processed by the micro-computer
(MCU) for use in determining the temperature difference (T
2
-T
1
).
A volume flow sensor measures the flow rate of the coolant through the system. The
flow sensor produces an electrical square wave signal based on the coolant flow rate.
The signal frequency increases as the coolant flow rate increased. The flow rate sig-
nal is sent to the analog circuits. The analog circuits process the square wave, pro-
ducing a flow rate signal for use in the MCU. The MCU calculates the RF power
dissipated in the load resistor. The calculation is based on the coolant flow rate and
the rise in coolant temperature as it flows through the load resistor.
The flow rate and temperature difference measurements are continually corrected to
account for changes in the specific heat and density of the coolant as the overall cool-
ant temperature rises. All temperature, volume, and flow measurements along with
resulting temperature change and power calculations are monitored by the MCU.
This allows the user to display, watts, flow, or temperature difference via the front
panel keyboard or request measurement data over the General Purpose Interface
Bus (GPIB).
Bird Model 6091 Calorimeter
6