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John Dorling has worked with WASP (Windrush Against Sewage Pollution) to develop a system to measure river nitrate concentrations at static locations (typically near sewage outfalls) over periods of several days.
This page outlines an initial design for a smaller system which can both be used at fixed locations and for measurements along a track (e.g. from a boat or canoe).
The system consists of a Nitrate sensor, interface circuitry, and an Arduino microcontroller to display and record results.
The Nitrate probe is 155 mm long, and the waterproof box containing the interface circuitry and the Arduino microcontroller and display is 102 x 70 x 52 mm.
Once every 10 seconds the system takes 120 ADC readings of voltage at intervals of 10 ms (taking ~1.2 secs). The system then computes, displays and saves the average, standard deviation, maximum and minimum of these values. This data is saved to a microSD card.
Before data is collected the probe is calibrated using two (or three) standard solutions with concentrations of 10 and 100 ppm (and optionally 1,000 ppm) of nitrate. The left hand side of the display contains three buttons, these are used to indicate which calibration solution is being used. Pressing any two of these buttons in quick succession returns the system to data collection mode.
The above graph shows the voltage measured during the calibration of a new ('dry') sensor. The different colours correspond to different measurement modes (e.g. blue equals 100 ppm calibration solution). This graph shows the extended time (~ 1 hour) required for the probe to first settle down. Subsequently it is able to respond in a minute or two to changes in concentration.
At present the system simply records the voltage delivered by the probe. Subsequent analysis and conversion to nitrate concentration is performed off-line
The Nitrate measurements can then be presented either as a time-series
or, by combining the data with locations from a GPS, overlaid on background mapping.
An interactive map is used to display Nitrate concentrations. The map can be scrolled and zoomed, and clicking on a specific measurement shows a popup giving the detailed measurement at that point.
Both systems use an Ion Selective Electrode (or ISE) to measure the Nitrate concentration.
ISEs output a voltage which depends on the concentration. ISEs have a very high output impedance, and for Nitrate concentrations typically encountered, produce output voltages in the range 100 to 400 mV.
One issue with ISEs is that they require frequent calibration. Typically a fresh calibration is required before and after a measurement session, and in particular after a sensor has been stored dry. After dry storage the sensors have to be soaked in a calibration solution for an extended period before they settle down (up to 24 hours or more).
The interface circuit has to have a high input impedance (> 5 MΩ), be able to measure relatively small voltages (< 400 mV) accurately and with a precision of better than 0.1 mV. Its output has to be in a digital format suitable for input to a microcontroller.
The interface circuit contains
The interface circuit makes use of the MAX406B reference circuit shown in the MAX406B datasheet. Its output is fed into the 16-bit Adafruit ADS1115 where it is first amplified by a factor of four which allows it to measure input voltages in the range of -1.024 to +1.024 volts. Thus 2.048 volts are represented by 65,536 values, so that one bit represents 31.25 µV or 0.03125 mV.
The ADS1115 output is presented to a digital I2C interface which is connected to the Arduino microcontroller.
Because of the small voltages being measured and in order to reduce the potential for interference a custom PCB was manufactured for the interface.
The Arduino microcontroller connects to the interface board using an I2C (digital) interface. This is used to set the amplification of the Adafruit ADS1115 and to initiate, and receive, ADC measurements.
The complete microcontroller is constructed using the following components.
Indicative costs (as of 2023) are as follows. The sensor contributes over 70% to the overall cost of the system.
