封閉式壓力潮位計大氣壓力、重力、海水密度改正探討

On the Atmospheric Pressure, Gravity, and Sea Water Density for Depth Reduction of Pressures Measured with Sealed Type Pressure Tidal Gauge

設置臨時潮位站時，施作便利是一項重要的考慮要件，「封閉式壓力潮位計」因而成為常用之設置儀器。「壓力潮位計」為以感測水壓力，再藉由包含密度與重力因子之化算，獲得深度。由於「封閉式」與外在大氣環境沒有直接連接，此型潮位計所感測之壓力為大氣壓力與海水壓力之總和。
在現行實務作業上，常以1 decibar之壓力等同1 m之水深值，當增加1 decibar之觀測壓力，感測器水深值視同增加1 m。此一作業方式，忽略重力值的影響與大氣壓值的時空變化。由於「封閉式壓力潮位計」產品輸出數據，常預設將壓力值觀測值扣除標準大氣壓力或某一標準值，若在相同時刻臨時潮位站所在處大氣壓力變異不大且感測器置於水深小於５ｍ時，則此一處理方式實務上大致可行，但是亦可能會有顯著系統性誤差風險。本文目的在探討各項改正之影響，並以實例說明。
總體上，雖然由理論著眼進行改正較為嚴謹，但是影響程度與潮位站所在區域及季節之氣壓特性有關。亦即，若觀測時段中，該站之氣壓為恆定，且接近儀器預設之氣壓，則影響不顯著。反之，則應進行改正。

For the short-term deployment of tidal stations, convenience plays a crucial role. Consequently, the 'Sealed gauge with pressure sensor' emerges as a commonly selected instrument model. The 'pressure sensor' measures sea water pressure and then converts it to depth using sea water density and gravity. Due to the 'sealed' nature of these gauges, the sensor is not directly exposed to the atmosphere, meaning the sensed pressure includes both atmospheric pressure and sea water pressure.
In current common practice, one decibar pressure is considered equivalent to onemeter depth. Although this assumption may hold workable under the condition that the gauge-sensed pressure has been normalized with standard air pressure, no large variation of the atmospheric pressure, and the depth of deployment is under 5 meters, there is still risk potentials. This discussion delves into the influence of the correction items with practical examples.
In general, while it is more rigorous to make corrections from a theoretical standpoint, the level of influence is site-dependent and season-dependent. In other words, if the variation of air pressure during the observation period is significant, conducting corrections is advisable.