pH Meter and TDS (total dissolved solids) meter combo.  pH meter measures 0 to 14 pH range with a resolution of .1 pH and and accuracy of .1 pH.

Use the pH meter to check the pH of your water, mash, [test Star San effectiveness] and finished beer.  The TDS meter could come in handy for checking your RO filter [See: Review – Portable Reverse Osmosis Water System] or for checking purchased distilled or RO water.  Some water processors add minerals back for flavor.  A TDS meter will allow you to make sure you’re getting what you want.

Update: Since this post was published the price, availability or promotion has changed.  Check the product page for current description, price and availability.  Also: Today’s Deals on Amazon

Ph & TDS Meter Set, Combo of ±0.1ph High Accuracy Ph Meter and ±2% Readout Accuracy TDS Meter

Also Consider: All Grain Tips & Gear || Hands On: 5 Stage Reverse Osmosis Water Filter by Reverse Osmosis Revolution