Design of a multichannel outdoor data logger for precise temperature measurements

Abstract

In this thesis, we present the prototype of a multichannel data logger (the Logger) for precise temperature measurements. Its intended application is to take soil (permafrost) temperature measurements with a thermistor probe or thermistor string. However, its hardware and firmware architectures are quite flexible, so it can be used in other applications. The Logger has 16 channels. In addition to the internal memory, it supports microSD-cards of up to 2 GB, which allows it to store up to 41.9 million measurements for each channel. The Logger's estimated battery life is 8 years when making measurements once per hour. It has a radio transmitter, which will allow it to download data wirelessly and potentially participate in a wireless sensor network once the appropriate firmware is developed. Currently, only the communication protocol with the radio is implemented, while the radio-to-radio protocol is under development. The Logger is small - only 6 x 1 x 1 inches and the final product will be even smaller. Components are rated down to -40° C and the Logger successfully passed testing at -30° C. After the extensive testing to ensure performance it has been shown that the Logger outperforms the Campbell Scientific, Inc. CR1000 logger and exceeds the design requirements. Measured temperature resolution of the Logger is below 2.5 mK in the entire temperature range. The Logger's equivalent temperature accuracy, which was determined using a known resistive input, is below 10 mK within -25° C to 40° C and below 20 mK elsewhere. The developed calibration technique provides the equivalent accuracy below 0.3 mK within -40° C to 40° C. To provide an accuracy of ±0.01° C when making temperature measurements with thermistors, the Logger should be calibrated against a thermometer that has been calibrated as a secondary standard, which will be done in the future.