Accurate water level measurement is fundamental to hydrographic surveying, where pressure loggers are widely used as tide gauges to support sounding reduction, tidal analysis, and vertical reference modelling. Although pressure measurement is a mature technology in oceanographic instrumentation, different sensor technologies exhibit distinct performance characteristics that can influence the quality and stability of hydrographic observations. Two sensing approaches dominate modern oceanographic pressure loggers: piezoresistive pressure sensors and quartz resonant pressure sensors. Understanding the differences between them is important when selecting instrumentation for hydrographic applications that require stable measurements over multi-week deployments.
Piezoresistive pressure sensors measure pressure through the deformation of a silicon diaphragm, which changes the electrical properties of strain gauges embedded in the sensor structure. These sensors are widely used in oceanographic instruments because they are robust, compact, and available across a wide range of pressure ranges. Typical datasheet specifications cite accuracies around ±0.05% of full scale — sufficient for many hydrographic and oceanographic deployments — though it is worth noting that published datasheet figures do not always reflect real-world performance and should be interpreted with some caution. Their combination of cost-effectiveness, small size, and reliable performance makes piezoresistive sensors the most commonly deployed option in compact oceanographic loggers. However, because the measurement relies on mechanical strain within the sensor structure, these devices can exhibit long-term drift as materials relax over time or respond to temperature cycling.
It is also worth noting that sensor technology in this space continues to advance. RBR has recently introduced a piezoresistive pressure sensor rated to ±0.01% of full scale — a specification that narrows the performance gap with quartz sensors considerably, and one that challenges some of the assumptions traditionally made about the accuracy ceiling for this sensor type. A zero dbar version has also been introduced to allow convenient measuring, and subsequent removal of, atmospheric pressure from the readings.
Quartz resonant pressure sensors operate on a different principle: applied pressure alters the resonant frequency of a quartz crystal. Because frequency can be measured very precisely, quartz sensors are capable of extremely fine resolution and excellent long-term stability. Paroscientific quartz sensors commonly used in oceanographic pressure loggers typically exhibit average drift of around only 30 parts per million per year, with a specified maximum of approximately 140 ppm per year. This low-drift behaviour makes quartz sensors particularly attractive for applications where pressure must remain stable over long periods, or where small changes in water level need to be resolved with high confidence.
The practical differences between these technologies become important in hydrographic surveys requiring continuous water level observations over several weeks. During long deployments, sensor drift can appear as a slow bias in the pressure record, which can propagate into tidal reductions and ultimately into vertical uncertainty estimates. Understanding how drift behaves and accumulates over time is therefore critical when selecting a pressure logger for a particular survey task.
Choosing between piezoresistive and quartz sensors is often a balance between deployment duration, stability requirements, and practical considerations such as instrument cost and availability. Piezoresistive sensors remain the versatile workhorse for many survey deployments, particularly where multiple instruments are required or deployment durations are relatively short. Quartz sensors are often preferred where long-term stability is critical or where the pressure measurement forms a key reference for other observations.
Operational hydrographic survey programmes provide practical examples of these considerations and the presentation will conclude with a summary of how both types of pressure loggers are routinely deployed as tide gauges within the Hydrospatial Industry Partnership Program (HIPP) in Australia to support tidal reduction and hydroid modelling during survey operations.