Milwaukee MW600 LED Economy Portable Dissolved Oxygen Meter with 2 Point Manual Calibration, 0.0-19.0 mg/L, 0.1 mg/L Resolution, +/-1.5 percent Accuracy, 100 Percent Saturation Range

I have multiple aquariums and use this to take the guess work out of knowing if the fish have a reasonable amount of oxygen, and ifvI can add more fish to a given tank without the silly rules of thumb the kid at the pet store give you. I tried lower cost meters and found trying to save a few dollars leads to poorly translated manuals and poorly made products. This one is a few dollars more than the bottom of the line junk but provides good value for your dollar. Given that a Dissolved Oxygen reading is not an instantaneous process like reading values with a voltmeter, you must wait for a membrane to become saturated in an electrochemical process, this meter has provided consistent values in test solutions of known oxygen content, so my confidence is high that it is giving me good readings in my environment, as long as I am patient enough to let the probe go the saturation before taking a reading.

Works very well

Great purchase for use in high school biology class. Have to constantly warn to not screw off the cap, but pull. Calibration was easy and fast.

Never worked. Not even once. I calibrated it, following the manual's instructions. Then I put the cap on it, again following instructions. The cap would not come off. While trying to remove the cap (twisting and tugging gently), the sensor snapped off. This is a worthless piece of crap. I am very angry to have been duped out of $200. I am not in a part of the world where it is easy to get equipment. It will take me months, months of lost data, to replace this meter. I hope the manufacturers die in a fire.

Very nice, professional-quality unit but the instructions leave alot to be desired. I am a field biologist used to operating various devices for measuring environmental parameters but I found the instructions for calibrating, operating and storing this unit confusing at best and simply non-existent at worst. There is a small additional sheet of instructions for unit calibration that differs quite a bit from the included "manual" (which consists of a small 5"x 6", 5-page booklet). Furthermore, the included protective cap designed to be placed over the diode doesn't fit at all. I have contacted the company to hopefully clarify some of my questions and their website is okay in that it does include a video showing some calibration techniques but the FAQ section is dismal. On the bright side, the unit included 2 more replacement membranes than it listed in the product description and most importantly, it does appear to accurately measure the D.O. in my koi pond.