Erin Meister trains baristas for North Carolina-based Counter Culture Coffee and sporadically maintains the blog Meet the Press Pot from her home in New York City. This is part of a series for the caffeine-addicted.

Since brewed coffee is more than 90 percent water, it only makes sense to use the perfect H20 for the job, right?

But what is the "perfect" water for coffee? Are we talking about highfalutin fountains that'll bleed you dry, or straight-from-the creek agua with turtles still in it? Will a Brita filter suffice? Read more after the jump.
Completely pure water doesn't naturally occur -- even the freshest spring water has stuff like calcium, sodium or potassium in it. The percentage of this "other stuff" (that is, anything not hydrogen or oxygen) in water is called TDS, total dissolved solids. The amount in parts per million (ppm) of TDS that a water contains will have a significant effect on your morning cup. As in, the higher your water's TDS, the less that's dissolved from the coffee itself; the lower the TDS, the more concentrated the coffee compounds.

If that's a bit oblique, here's a great analogy that a colleague of mine uses to explain water quality: Think of the water you use to brew coffee as a school bus on its route, and the water's TDS as kids already on that school bus. If the bus can seat 100 people and starts out with 95 on board, there's only room for 5 coffee kids to get on -- not enough to enjoy. If the bus is half full to begin with, that means there's room for 50 coffee kids -- much more harmonious. If the bus rolls out of the depot completely empty, it can fill up with coffee -- follow me here?

What does this all mean? Basically that water with too much or too little TDS does not a good brew make. The coffee beans' dissolvable solids need something to balance them out to prevent them from tasting astringent and overpowering (as when brewed with distilled or pure water), or salty and sour (as when brewed with mineral water). And I can save you the trouble of experimenting with coffee brewed in sparkling water -- it's horrible.

Tap water with something between 150 and 350 ppm TDS and most spring and carbon-filtered waters with 180 to 300 ppm TDS make fine, well-rounded and true-to-flavor joe, which means you don't have to go broke to hydrate your beans. Basically if you find your tap water good enough to drink, go with that. If you use a non-distilled filtered water, that should be fine, too. Easy peasy.

Have you ever bought water especially for use in your morning joe? Tell us about it in the comments.