Measuring Darcy Flux and Transient Pore Velocity: Development of a Compact Temperature, Pressure, and Electrical Conductivity Sensor

Abstract:

Our team was asked to make a Flux Tool that could measure pressure, temperature, and electrical conductivity (P/T/EC), at five depths along a 1-m length. The overall goal of the Flux Tool project is to develop a precise battery-powered device to measure transient subsurface conditions that is of low cost to the end user, straightforward to construct, and can operate in the field for up to six months. The key sensing element presented in this poster is a 2-cm3 compact sensor module (pictured below). Sensory modules (“pods”) are inserted in horizontal holes drilled into the vertical 5-cm diameter hollow Delrin shaft of the overall Flux Tool device (shown in CAD drawing of device structure). The framework of the pod is a cylindrical 3-D printed (SLA) plastic protective shell, with connection wires emanating from the inserted end and capped with a perforated titanium disk (on the market as a screen for tobacco pipes). Pressure and temperature are measured using an MS5803-02BA miniature 24-bit altimeter module which draws <1μA and, uncalibrated, can read up to 10m head (+/-2 cm). Temperature resolution is 0.01 ℃ with an accuracy of +/- 0.2 ℃ in the 10-40 ℃ range, with an overall measurement range of -40 ℃ to + 85 ℃. The EC measurement is made using a lab-built analog AC-EC circuit measuring current between a pair of 3-mm long gold-plated pins housed in the pod. The sensor pod is designed to be useful in a wide range of applications where P/T/EC measurements are needed using battery-based hardware. I2C is used to communicate with the altimeter module in the pod, allowing for up to 1-m separation between the pod and data logging device. The EC measurement is an analog signal, which may be routed through an external multiplexor to allow for multiple pods to operate as part of a single application. Flux Tool code, bill of materials, and project progress is located at https://github.com/OPEnSLab-OSU/PNNLFluxV1.