Wireless Measuring System for Freeze Dryers and Harsh Environments

Temperature measurement is nowadays of fundamental importance for both monitoring and controlling most natural and industrial processes. Thus, it is crucial to have the capabilities to measure temperature in different environments, including the most difficult ones. As an example, industrial lyophilization requires reliable temperature monitoring under harsh conditions. Lyophilization, also known as freeze drying, is the process that is used to remove water from biological and/or pharmaceutical products in order to improve their shelf-life while preserving as much as possible the original characteristics. It takes place inside an isolated stainless steel containers at low pressure and low temperature in three different phases. During the entire process the temperature of the product, but also the temperature of the shelves, has to be accurately monitored in order to have satisfactory outcome at the process conclusion. There are different factors that contributes to make the lyophilization process challenging for a measuring system. The first of them emerges from the extremely low temperatures experienced during the first phase, reaching as low as -50 °C. In addition, the requirement of vacuum for the correct physical transformations makes difficult to introduce cables into the chamber. Furthermore every component introduced inside the freeze dryer must undergo sterilization, in order to prevent the introduction of unwanted microorganisms. In this context, the thesis focuses on the development of a wireless, low cost and high-reliable measuring system, designed for real-time monitoring of the temperature inside freeze-dryers, but also capable of extending its applications to other challenging environments and quantities such in the case of the extensive monitoring of temperature and humidity in the agriculture sector. This is done by creating a sensor node with a modular architecture where most of the components can be reused for different applications and different type of analog sensors. The proposed measuring system is a battery powered wireless node featuring a microcontroller supporting Bluetooth Low Energy, realized on a printed circuit board (PCB) equipped with 6 thermocouples where the acquired data are sent from the sensor's position to a central receiver by a long range radio (LoRa) capable of managing distances up to some kilometers. One of the primary design goal is to make the system low power so that it can be used without recharging for a long period of time. To increase the reliability of the node an on-board non volatile flash memory is used as data logger to save all the measurements simultaneously sent by the radio for post processing purposes. Furthermore the final PCB is designed to be resin coated to withstand sterilization, for this reason inductive charging is selected to recharge the battery that powers the system. The measuring system is complemented by two applications that can run on a PC: the first one for managing the configuration phase while the second one for receiving and visualizing the measurements. The experimental results have shown both a satisfactory accuracy from all the six thermocouples and a remarkably low current consumption, as low as 4 uA during its sleep state. Considering a Lithium battery with a capacity of 400 mAh, the device is expected to operate for a few years in low power mode.