A Fast and Accurate Thermistor String1

Abstract

A ‘‘fast thermistor string’’ has been built to accommodate the scientific need to accurately monitor internal wave activity in shelf seas and above sloping bottoms in the ocean. The performance of the thermistors and their custom-designed electronics allow temperature variations to be registered at an estimated relative accuracy better than 0.5 mK with a response time faster than 0.25 s. Quantization noise is less than about 40 mK and dominates instrumental noise. Currently, the string holds 32 sensors, which are sampled within 4 s. When sampling every 30 s, the batteries and the memory capacity of the recorder allow deployments up to 3 months. In all respects, this performance is about an order of magnitude superior to thermistor strings currently available commercially. Moored in combination with an acoustic Doppler current profiler the thermistor string provides data to estimate directly quasi-turbulent (high-frequency internal wave band) vertical temperature fluxes and flux gradients. Examples of field observations are given, which show enhanced levels of temperature variance extending above the canonical internal wave spectral levels near the buoyancy frequency, and detailed variations of high-frequency internal wave variability.