Abstract: A power control device for controlling power to a resistance heater in an oven uses a microprocessor, temperature measuring circuit, power measuring circuit, a triac circuit for firing the heater, a power phase synchronizer whereby the microprocessor synchronizes A.C. current signal and allows the triac to fire for a calculated portion of each phase of A.C. current to provide power to the heater such that the oven temperature is accurately controlled. A cooling device for cooling to subambient temperature such as -150.degree. C. also is provided. The power control device and oven are used in conjunction with instruments such as a dynamic mechanical analyzer so that physical data on test samples can be obtained over a wide temperature range.