A thermistor-based temperature measurement system applied to an experimental food freeze dryer

This article describes the construction and validation of a system applied to measuring the temperature of shelves and load cells in a freeze dryer using thermistors as sensors. Conditioning and analog-to-digital conversion circuits were built, two linearization techniques were applied to the thermistor response, and calibration was performed. A freeze-drying process was carried out to validate the system using sliced yam samples. The results indicated a maximum linearization error of 0.4 °C in a range of −20 to 50 °C between the thermistors when applying hardware linearization followed by software linearization using the Steinhart-Hart equation. The measurement system presented a maximum standard uncertainty of 0.17 °C for the entire measurement range. A maximum error of 0.3 °C relative to the set-point and an overshoot of around 1% were observed when the system was used in a closed-loop control using the proportional-integral strategy.