you put the sample into a test tube, a reactant, calibrate it, then add a reagent into the test tube. Then it sees how much light gets through and compares it to the light that got through on the calibration sample.
"There is no need to calibrate as the test procedure does this for you. Fill the test tube with a clear pond water sample, insert into meter, press the “zero” button (this calibrates the pond water sample to the meter as a reference), add the reagent to the same test tube, Insert back into the meter, press the read timer button, this starts a timer count down, then the meter shows the result on the digital display. All have a 2 year warranty."
You would have to contact the manufacture to make sure the reagents would work on saltwater, but the sample doesnt touch anything but the test tube. Corrosion would be no problem.
