A precision CTD microprofiler

This paper describes a conductivity-temperature-depth (CTD) system designed for precise fine scale measurements of salinity and temperature in the deep ocean. The conductivity sensor is a miniature (8 mm long, 2 mm I.D.) four electrode cell. Temperature is sensed with a combination of a miniature fast thermistor (30 ms) and a platinum resistance thermometer. The combined outputs of the thermistor and the platinum thermometer, with their respective interface circuits, have the excellent long term stability and linearity of the platinum thermometer and the speed of the thermistor but are unaffected by thermistor calibration drift. Pressure is measured using a strain gage bridge transducer. All three sensors with their respective interface circuits and the 16 bit digitizer are excited with 10 KHz sine waves. The D/A converter in the digitizer is an electronically switched binary ratio transformer having negligible errors. The digitizer uses successive approximation to measure the 10 KHz sensor output to input voltage ratio. Digitization time is 10 milliseconds, thus, a complete scan of all three sensors is made in 30 milliseconds. The digital output of each scan is transmitted via armoured cable to the shipboard unit in "TELETYPE" format using frequency-shift-key (FSK) modulation. The shipboard unit demodulates the FSK signal from the sensor unit and provides both serial and parallel digital outputs to a shipboard computer. Digital displays and analog outputs are also provided by the deck unit.