control of test chambers or specimens to stabilized temperatures at set points over a wide temperature range is accomplished with improved response time by an arrangement of cascaded peltier affect heat pumps . generally , power to a first peltier unit in the arrangement is controlled relatively to the temperature deviation of the specimen from the desired set point , while power to a second peltier unit therein is controlled proportionally to the temperature differential across the first peltier unit . in applicatons where large thermal losses to ambient occur , a portion of the power control signal to the second peltier unit is applied to offset the temperature set point as a compensation for such losses . improvement in approach to stabilized temperature is further attained by applying an anticipation signal in proportion to the temperature gradient between the specimen and the first peltier unit to decrease the monitored deviation of the specimen temperature from the set point . for system protection the power control signals to the peltier units are shut off to preclude thermal runaway in applications where the specimen temperature is monitored by a sensor which must be exposed to ambient conditions during set - up periods .