The temperature in the main or primary flow loop is maintained with the primary or main temperature control loop as shown in the lower right corner of Fig. 3. This loop extracts approximately 300 L/min of fluid from the main or primary flow loop and is pumped through a glycol-cooled heat exchanger and then a 10 kW DC heater, filtered, and then re-injected into the main or primary flow loop. This temperature control system can maintain the fluid temperature in the test section to within ± 0.05 ºC of the specified index-matching temperature. An additional auxiliary flow loop (shown in the upper left corner of Fig. 3, with a similar temperature control system, is used to provide fluid for experiments that require external flow into a model. Fluid is extracted from the primary flow loop and routed to a seven horsepower auxiliary pump that produces flow to the experimental model. To maintain the required working fluid temperature, a portion of this fluid is extracted from the auxiliary loop and routed through a parallel auxiliary temperature control loop. As in the primary temperature control loop, the mineral fluid is cooled and reheated before returning to the auxiliary flow loop and into the model. Control instrumentation includes thermistors, flow meters, data acquisition, and computer controls.