I tested by making solutions of DOT4 brake fluid with 0%, 1%, 2%, 3%, and 4% water. After letting the water dissolve for about 8 hours in sealed containers, I checked each. The tester was spot on for all measurements as long as the probes were fully inserted. If only the tips were inserted, all solutions measured
My only issue with the tester is the labeling is not compatible with brake fluid. I got a little brake fluid on my thumb, and when I touched tester, the white labeling immediately disappeared.

As some have said you need to put some shrink tubing over the probes so that only a little protrudes to get consistent readings. Otherwise it's nice to be able to put a moisture reading on my vehicle inspection reports.

Amazing product. Saved me a lot of time with my three vehicles at home that I do all the maintenance on. I took a brand new bottle of brake fluid and tested for water and the control passed. I then took a car with KNOWN issue with the fluid and tested it--came out as 2% moisture which I expected. I then took a graduated cylinder and added 3% water to the brake fluid and let it sit for a week so it could dissolve properly in the fluid. Once it was fully absorbed (actually took a week +1 day) in a sealed container I tested it again and the fluid tester passed by giving it a 3% reading. In my other two vehicles both fluid changes were performed just 6 months prior and they passed at 1% moisture. When I tested 100% water it obviously gave a 4% DANGER reading. I'm pretty sure this works as designed--I just hope it does so over time or when the battery goes low. I will post another review later.

It appears to work as advertised. I did not have old brake fluid to test but it does give the maximum reading (>4%) when tested with water and fresh brake fluid reads the minimum (<1% moisture.) It was difficult to remove the battery cover to install the battery. Had to really pry hard with a small screw driver. Although battery replacement will be infrequent, I don't like to leave alkaline batteries installed for a long time because they often leak, so I will remove the battery in between uses.

Great tool for providing a second opinion on the repair shops that try to sell you on a "brake fluid flush" every time the car goes in for any service. this gives an easy and quick measurement. The only problem I had was that it took a little bit of work to open up the battery compartment. Great addition to my toolbox!