The next discussed sensor, which could also be the last, measures the oxidation reduction potential (ORP) of a liquid. ORP is a value that indicates, in this case, how well water can oxidize or reduce another substance [1]. If the value is positive, the water is an oxidizing agent, if the value is negative it’s a reducing (or antioxidizing) agent. If the water doesn’t have enough electrons, it will try to collect more through oxidation. On the other hand, the water can have too much electrons and will lose some to oxidizing agents, without being destabilized.
For bodies of water, ORP can measure it’s ability to break down contaminants and cleanse itself [2]. We can tell if water is safe to consume or come in contact with humans if it has negative or low ORP. If not, it has to be treated with sanitizing agents. If we routinely monitor the ORP of a body of water, we can determine the effectiveness of these sanitizing agents.
The analog ORP sensor will also be used to determine how healthy a body of water is. An ORP sensor is one of the most expensive to calibrate, the buffer solutions cost twice as much as the probe itself. For this reason, I will not be calibrating the sensor, although luckily, I don’t really have to. This analog sensor will return a voltage for each reading, and the ORP is very commonly measured in millivolts [3]. Therefore, all we really have to do is compare the returned voltage to an average ORP reading for healthy bodies of water.
For example, if we insert the probe in a healthy waters, it should return a value between 300 too 500 mV [2], so all we have to do is compare our sample values to that range. In addition, what we really want to observe with the sensor are trends in the ORP. If the ORP of a lake is lowering over a couple of hours or days, it could mean that there’s decaying fish or materials at the bottom which the bacteria are decomposing, using up a lot of oxygen.
Sources
[1] JENCO, “Water Quality 101: What Is ORP in Water Testing?”, Jenco Water Quality Blog. [Online]. Available: https://blog.jencoi.com/what-is-orp-in-water-testing
[2] Government of Northwest Territories, “Oxidation Reduction Potential (ORP)”. [Online]. Available: https://www.enr.gov.nt.ca/sites/enr/files/oxidation-reduction_potential.pdf
[3] DFRobot, “Gravity: Analog ORP Sensor Meter For Arduino”, DFRobot. [Online]. Available: https://www.dfrobot.com/product-1071.html
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