1314-900 Revision BA
250W Precision Calorimeter
6
flow (gm/sec) and specific heat of the transfer medium cp (J/gm-°C):
Watts = J/sec = m
ass 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 1314 block diagram is shown in Figure 2. 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. To allow for some
variability in the flow rate for the system, an adjustable flow valve is made available, and is
inserted in series with a fluid line connecting the inlet stream directly to the outlet stream. This
valve is normally set to produce a flow rate of .5 GPM for the system, as this is the recommended
minimum flow rate required by the RF load to achieve the system specification of 250 watts.
