Electrical Conductivity Wenner Circuit
Project Leads: Cameron Whitlow
This Wiki page will describe an in lab developed circuit to measure the Electrical Conductivity (EC) of water with a four pin sensor. Descriptions of why the circuit was created, what is aims to do, and how it was designed to meet these goals are all described below.
- Purpose
- Data and Results
- Using and Tuning the Circuit
- Circuit Schematic and PCB
- Circuit Design and Reasonings
The original EC sensor utilized in our projects was a 2 pin method that proved to be dependent in only specific situations and was not dependable as a broad application. The Wenner styled 4 pin EC sensor was designed to eliminate the issues plaguing the 2 pin solution and serve as a generic sensor that can be used on any project requiring aquatic electrical conductivity measurements. This new sensor style will alleviate the downsides of the 2 pin method, namely biofouling. This is a phenomena where biological material, such as moss, can collect and grow in between the emerged probes effecting the resistivity reading. This also alleviates the same issues with other debris as dust or rock.
A variety of data from lab testing will be shown in this section, both for the initial prototype board designed to test the general functionality of the sensor design and an application of the design on a board that will be implemented into a field sensor.
The initial prototype circuit
This section will go through some of the designs at different levels of development to show some of the jobs of the overall circuit and how they are integrated on a sim level and layout level.
The following figure shows the schematic designed in LTSpice for simulations prototyping.
There are two core operational blocks in the main schematic, the loop on the left and the signal processing on the right. The left loop is abstracted from the oscillator that will require extra chips in the design but simulates the effect. This loop generates a positive 3.3v to negative 3.3v square wave in the voltage source and feeds this signal across the "water" represented by the three resistors in series. As the signal propagates across the water model the output nodes A, M, N, and B branch out into the right circuit that handles the signal processing making the measurement. This loop also consists of a smaller resistor, described as Rref1. This resistor is a current sensing resistor, where the value is always known by the measurement conversion software and allows for the ratio between voltage and current to be read.
The second portion of the circuit, the op-amp circuitry on the right side, handles the signal processing to redefine the multiple differential voltages being read from the water simulated outputs. Each output signal that isn't ground is put through a voltage follower to create a separation of impedances between the resistances being read and the rest of the circuit - making it so these voltages won't be altered by the act of working on these signals. These op amps have a very high input impedance when compared to the resistances being read, drawing such a low current from the signal generating circuit that effectively no voltage is lost. These op amps then have a low output impedance to the next stages allowing for easy signal processing.
After the voltage followers are the differential voltage amplifiers which serve as the first step towards creating a single readable voltage for each measurement (current or voltage). This ground referenced voltage then goes through a 'super diode' circuit topology, another voltage follower design with an included diode to clip any negative voltage. Note that the wave prior to this point is still a square wave oscillating around ground. This diode will remove the negative portion as the utilized ADC does not convert negative voltages. The diode also slightly clips the positive voltage wave but retains most of the signal. The final portion goes through a small filtering stage of a capacitor