Depth (Pressure)

The depth (pressure) sensor uses quartz crystal oscillation to measure how deep the instrument is within the water column.

How it Works
Depth needs to be determined, particularly because water temperature is influenced by depth. Measuring depth allows one to make calculations to determine the "true" water temperature at that depth.

A quartz crystal oscillates at a known frequency. The more pressure that is applied to the sensor, the more electricity is required to keep the oscillation going. Depth is determined from the amount of electricity.

Resonant-quartz pressure transducers have an accuracy of 100 ppm. This is achieved through the use of a precision oscillator whose frequency varies with pressure-induced stress on the quart crystal resonator. Quartz crystals make excellent sensing elements because of quartz's mechanical repeatability, stability, and low hysteresis. The crystal oscillations are maintained and detected with oscillator electronics similar to those used in precision clocks and counters.

This double-ended tuning fork consists of two identical quartz beams driven piezo-electrically in 180° phase opposition such that little vibration energy is transmitted to the mounting pads. The high-Q resonant frequency, like that of a violin string, is a function of the applied beam-axial load; the frequency increases with tensile and decreases with compressive force. With proper corrections for sea water compressibility, quartz-resonant depth sensors can measure oceanic depths (6000m) to an absolute accuracy of less than one metere. Moreover, these temperature-compensated, low power sensors allow construction of reliable, self-contained wireless devices with internal data recording.

Sensory Variations
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