Meters That Use Light Versus Meters That Use Electrodes

I'm just beginning to understand the differences between meters that measure pH, turbidity, dissolved oxygen, and other components of water. The traditional meter measures light or color. For instance in the Secchi scale, one drops a disk in the water until it just fades from view. What is measured is the length of line played out. The Secchi disk is easy to use by volunteers to monitor inland lakes  (e.g., Indiana Clean Lakes Program, and Citizen Lake Monitoring Program in Minnesota). A modification of that principle is used in the World Water Monitoring Challenge Test Kit in which the distance is controlled by the depth of the container and the clarity of the water is compared to a standard chart. Charts are also used in the Kit for color comparisons when color-activating chemicals are added.

Modified Secchi Scale controls the depth and compares the clarity of the disc on the right to a standard shown on the card to the left. 

Color spectrometry is used to calculate the pH value. The chart on the left is compared to the chemical-filled water sample on the right. 

Although the naked eye can do a good job of comparing standards to samples, there is error introduced from the light source and the human eye. Spectrometers are more accurate. Chemicals are available to colorize for many molecules contained in water (or soil). For instance, Phosphate and Nitrate amounts can be detected using appropriate chemicals and a spectrometer. An example of a spectrometer used to measure swimming pool water is the eXact iDip.

An alternative to measuring color through spectrometry is to measure electrical conductivity with a galvanometer and electrodes. Doing so requires specific electrodes and calculations, but not added chemicals. A simple device is the total dissolved solids meter (e.g., Dr. Meter TDS conductivity meter). To learn more, read about ion selective electrodes on Wikipedia. One advantage of using electrodes is the elimination of chemicals and color analysis. Another advantage is automation. For instance, an electrode-based probe can be connected to a smartphone to read, geolocate, calculated, store, and transmit automatically. For instance, Sensorex makes a general smartphone interface for its probes and pH interface for iPhone (approximately $300). YSI has an interesting multi-electrode product that is portable [Meter ($1500), assembly ($1700), and sensors ($450-$1000].

Sensorex iPhone adaptor and probe.