Atago 3749 Pen-Wrestling Digital Hand-Held Urine Specific Gravity Pen Refractometer

I have 2 Miniature Schnauzers - both have bladder and kidney stones. One of the ways they're medically managed is to check their urinary pH and Specific Gravity at least once a week; it was easy to find the correct strips for the pH testing, but I originally didn't purchase a refractometer 1.) because there are MANY of them available for a variety of uses and I found it very confusing to try to decide which one was best for my needs, and 2.) there are WIDE price variations. But I needed to decide on one because I had to pay their vet $22 for each Specific Gravity check.After hours of research I finally decided on this PEN  Atago 3749 PEN-Wrestling Digital Hand-Held Urine Specific Gravity PEN Refractometer . I hoped it would be the right choice because it cost almost $400; it was the right decision. The cheaper ones needed solutions to calibrate them, and some warned about getting certain areas wet - when this is an instrument that is used in fluids! This one is very easy to use; has easy-to-read instructions; is comfortable to hold; uses only distilled water or even tap water to calibrate; (the only precaution was to make sure the water is at approximately the same temperature as the urine sample); the digital display is large and easy to read, and it came in a safe, secure snap-lock container with the PEN cradled in a foam box insert.Overall an excellent product!UPDATE 10/22/2014: I've wondered if using tap water rather than distilled water would eventually affect the pen's accuracy. While checking Atago's web site for some information I found this is no longer a potential issue; the manufacturer now states only water is needed.