The New Water Level Measuring System

Microprocessor-based technologies and a new physical design enable the system to perform a wide variety of functions in service of a larger audience of users. Besides the ability to perform customized data gathering and processing of a large suite of sensors, the system can rapidly disseminate that information via a multitude of pathways.

Foremost is the critical need to accurately and reliably capture water level measurements. Thus, the new system continues the tradition of having primary and backup water level sensors. The primary water level measurement is obtained from a downward looking acoustic sensor. It sends a shock wave of acoustic energy down a half-inch diameter PVC sounding tube and measures the travel time of the reflected signal from a calibration reference point and from the water surface. The calibration reference allows the controller to adjust the measurements for variations in sound velocity due to changes in temperature and humidity. The sensor controller performs the necessary calculations to determine the distance to the water surface. The sounding tube is mounted inside a 6-inch diameter PVC protective well which has a symmetrical 2-inch diameter double cone orifice. The protective well is more open to the local dynamics than the traditional stilling well and thus does not mechanically filter much of the wind waves and chop. In areas of high velocity tidal currents and high energy sea swell and waves, parallel plates are mounted below the orifice to reduce the pull down effects. A schematic diagram of the new system is presented below.

In addition to water levels, the new system has the capability of handling up to 11 different ancillary oceanographic and meteorological sensors. These include such environmental parameters as, wind speed, direction and gust, water current speed and direction, air and water temperature, barometric pressure, water conductivity, dew point, rain fall and solar radiation.

The field units are programmed to take measurements at 6-minute intervals with each measurement consisting of a set of 181 one-second interval water level samples centered on each tenth of an hour. The 181 samples are averaged, a three standard deviation outlier rejection test applied, and the mean and standard deviation recalculated and reported along with the number outliers. The reported measurements have 0.01 foot resolution and are stored in system memory. Timing of the system is controlled by an oscillator located in the GOES satellite transmitter which is accurate to 2 seconds per month. Every three hours the data are transmitted via GOES to the satellite downlink. In addition, telephone connections can be used to retrieve data and to interact with the system. Laptop computers are used by field personnel to check and maintain the system. The backup water level sensor is a strain gauge pressure transducer that records data on a separate data collection platform which is optically coupled to the satellite radio. Yearly second order class I geodetic levels are run from the primary sensor reference point to the local bench mark network. That leveling reference point is a known vertical distance from the calibration point in the sounding tube.