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This is a great question Sean and a lot depends on what you want to really read it for:

TDS = total dessolved solids

http://www.tdsmet...om/what-is

is a very good site for a fast learn on TDS

the ones I get and use are about 140 bucks and last as long as you take proper care of them...

if you just want to do some quick tests and know that it will not last forever you can get them on amazon for 8/16/20 bucks.

I am not saying they are all junk, just they most likely will not last too many years!

ginny