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Therefore, the fundamental calorimetric relationship between electrical power in watts and the
thermal quantities measured inside the calorimeter are temperature rise ΔT (°C), rate of mass
flow (gm/sec) and specific heat of the transfer medium cp (J/gm-°C):
Watts = J/sec = mass flow rate (gm/sec) x ΔT (°C) x specific heat (J/gm-°C)
The system design utilizes the absolute flow method which means it measures the absolute flow
within the system keeping it nearly constant in order to determine ΔT. A basic absolute flow
calorimeter system in general is typically comprised of a liquid cooled RF load with a closely
coupled thermopile, circulating pump/heat exchanger, and signal conditioning circuits with a
display. The 1316 block diagram is shown in Figure 2a, 3a, and 4a. To begin, RF energy is
converted to heat energy inside a RF load containing a circulating coolant flow. The heat energy
is dissipated in the coolant, carrying this heat away in the coolant. The fluid circuitry causes the
coolant to come in contact with the heat sensing surfaces of a closely coupled thermopile located
between the load coolant inlet and outlet and thereby senses the temperature rise across the
coolant and produces a DC voltage. The resultant voltage is not proportional to the absolute
temperature of the coolant but to the difference in temperatures of the coolant streams. The entire
heated coolant stream is involved in the measurement, not just a fraction. Since temperature is
read out as a difference (rather than an absolute quantity) within the thermopile, the only absolute
measurement required is the rate of flow made by the flow meter- and this can be made
reasonably accurately. More importantly, if the flow meter is highly repeatable, the minor
absolute flow measurement errors can be calibrated out with extreme accuracy. A chiller then
removes the heat picked up by the fluid and exhausts it to the ambient air. The cooled fluid is then
recirculated back to the load in a closed loop. The DC voltage from the thermopile and the flow
meter output pulses are fed into a signal conditioning board and then on to a display, calibrated in
RF watts (also available on the remote computer interface for automated system configurations).
Thermistors are utilized on both the input and output water pipes to sense the absolute
temperatures of the water stream at their respective points and are then used to compensate for
errors introduced in the specific heat and gravity characteristics of the system fluid as a function
of temperature. The system is set to produce a coolant flow rate of approximately 4 gallons per
minute as this is the recommended minimum flow rate required by the RF load to achieve the
system specification of 10k watts.
10kW Precision Calorimeter
1316-900 Revision AA
7
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