pH meter is relatively inexpensive device that based on voltmeter. Most digital temperature gauage is based on ohmmeter. You can find them in "controller/monitor" section of most reef hardware stores. Take a look at some of RDO's sponsors' website, for example.
Nitrate meter use expensive electrode (ISE, ion selective electrode), and does not work well in seawater to warrent the price (slightly less than a grand, and it does not last very long), at least that's the case last time I check on this gadget. Same comment for calcium meter based on ISE, and in the Ca case, the other ion in the seawater make it almost totally useless.
The only "digital all-in-one tester" I've seen was something like the Medusa.. It's basically a lot of chemical tests with a computer base that reads the precise colour of the test results and gives you a numerical figure.
As far as a handy little strip you can dip in the water and read off results - nothing yet...
Medusa is just a sophicated "multimeter/controller." It is not a tester per se. The important issue is the sensor/transducer aspect of instrumentation.
invisible chemical/physical property (pH, specific gravity)
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\ / (test agent, physical instrument)
chemical with color that determined by the property
or physical property that is visible (how high is the mercury in the tube)
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\ / (reference)
color chart, graduation
