3-24
Figure 3-21.—Differential-air, thermometer-type calorimeter.
Flow Calorimeters
Flow calorimeters are classified by the type of circulating method used (open or closed), the type of
heating used (direct or indirect), and the type of measurement performed (true calorimetric or
substitution). Water or other calorimetric fluid is used only once in an open system. An overflow system
is used to maintain a constant rate of flow. Closed systems recirculate the fluid continuously by means of
a pump, and a cooling system restores the fluid to ambient temperatures prior to its return to the
calorimeter. Closed systems are more elaborate and permit the use of fluids other than water.
Flow calorimeters provide the primary standards for the measurement of high power levels; and, in
conjunction with calibrated directional couplers, attenuators, power dividers, or other similar devices
serve to standardize medium- and low-power measuring instruments. The measurement time depends on
the required time for the entering fluid to reach the outlet, where the rise in temperature is measured. The
circulating fluid may serve in a dual capacity as the dissipative medium and coolant, using the direct
heating method, or solely as a coolant, using the indirect heating method. Because of its excellent thermal
properties and high dielectric losses at 1 GHz or higher, water is normally used in both heating methods.
Water is rarely used as the fluid at frequencies lower than 100 MHz, because of insufficient dielectric
losses. The indirect heating method offers a wider frequency and power-range coverage and can be used
in substitution-type measurements.
True calorimetric measurements contain appreciable error, because of nonuniformity of flow rate, air
bubbles, flow-rate measurement inaccuracies, and temperature rise. Flow regulators, bubble traps, and
good thermal insulation are required to eliminate the majority of these errors. Substitution methods do not
involve direct heat dissipation measurement of moving fluid. Greater accuracy is obtained because known
low-frequency power is substituted for the unknown rf power, with all other measurement parameters
remaining constant. The accuracy depends on the exactness of the low-frequency power determination
and the degree to which factors remain fixed during the substitution of one type of power with another.
Figure 3-22 illustrates a flow calorimeter using low-frequency power substitution. Two different
measurement techniques are possible with this type of meter: the calibration technique and the balance
technique. The CALIBRATION TECHNIQUE uses an adjustable known power to exactly reproduce the
same temperature indication originally obtained by the unknown rf power measurement. The BALANCE
TECHNIQUE uses an initial low-frequency power (P1) to provide a steady-state temperature rise in the
calorimetric fluid. When unknown rf power is applied, the original power (P
1) is reduced to a new power
(P2) to maintain the same temperature indication. Therefore, the actual power equals P1 minus P
2. Figure
3-23 illustrates a widely used method of power measurement using a balanced-flow calorimeter.
Temperature-sensitive resistors are bridge-connected as the thermometric elements and are balanced at
ambient temperature prior to the application of power. Low-frequency balancing power and the unknown
