The methods to improve the thermal test equipment and determine the efficiency of solar collectors are proposed. To improve the performance specifications of such equipment, it is proposed to use highly sensitive semiconductor sensors. Based on experimental studies, planar transistors are chosen and sensors that can be used in temperature channels of devices for solar system studies are designed. Series connection of several transistor diodes enables an increase in sensitivity while reducing technological variations and simplifying secondary devices. The experimental studies have shown that the maximum temperature characteristics variation of the nine studied sensors does not exceed ±0.06 °С throughout the measuring range of 0 °С to +80 °С. The structure of a digital temperature difference meter with the studied sensors is designed. The temperature difference meter circuit is based on two current-to-voltage converters, in the feedback of which temperature sensors are enabled. By using a single reference voltage source, the same currents will flow through the sensors. The output signals of both converters are equal to voltage drops on the sensors and are fed to the differential inputs of the ADC, which provides a source code proportional to the measured temperature difference. The meter calibration at any temperature in the measuring range while ensuring the temperature uniformity of both sensors is proposed. After calibration, the estimated error value of the temperature difference meter does not exceed ±0.1 °С throughout the measuring range. The calibration method of precision digital thermometers with the designed sensors in two temperature points is proposed. At 0 °С, the thermometer additive error is determined, which is later used as a correction to all measuring results. The multiplier factor value is expedient to determine near the maximum measured temperature as the ratio of the nominal and the resulting values of the code that matches the calibration temperature. A method for adjusting a nonlinear error component in the whole measuring range is developed. It is based on determining the parameters of sensors approximating dependencies using experimental data. The logarithmic approximation temperature dependence of semiconductor sensors, whose value is equal to zero at both calibration temperature values is proposed. It is shown that this approximation dependence can be easily implemented in the modern microcontrollers base. After calibration, the acceptable error limit of digital thermometers in the measuring range from 0 °С to 100 °С does not exceed ±0.1 °С.