Update June 2023: (There is an interesting video review of this tester, indicating abnormally high contamination levels in various samples of new brake fluid. The test appears to use a nearby bottle of ionized electrolyte drinking water in an effort to decontaminate the tester's probes between measurements. The ionized water could have a concentration of ions thousands of times greater than that of the contaminating water in the brake fluid. Microscopic amounts of the ionized water, shedding off the probe tips into the brake fluid samples, could be enough to alter the outcome of the conductance tests. It would be interesting to see that test rerun, with de-ionized water used to clean the probes, in lieu of the ionized water.)Original Review: While I don't have the resources to conduct an exhaustive evaluation, I was able to test the unit to my satisfaction. I started by testing the DOT 4 brake fluid in two high performance automobiles. The testing was performed by completely immersing the unit's probes into the master cylinders. Both vehicles had a brake fluid flush less than a year ago. Before immersing the tester, I switched the tester ON and verified that its green battery check light illuminated. For both vehicles, the tester showed a single yellow light, indicating water contamination of less than one percent. After bringing the tester indoors, I tried to approximate the tester's accuracy using various concentrations of tap and distilled water. When immersed in undiluted DISTILLED water, the tester illuminated only the battery check light (the expected result). When immersed in undiluted TAP water, the tester showed all of the lights, indicating water contamination greater than 4 percent (also expected). When immersed in a mixture of one part tap water and 19 parts distilled water (5 percent), the tester showed the expected result of greater than 4 percent. When immersed in a mixture of one part tap water and 39 parts distilled water (2.5 percent), the tester correctly extinguished the >4 percent LED, showing <3 percent contamination. Based on my evaluation, I have confidence in the levels reported by this tester. However, I'll add a few notes of caution: My testing showed only the contamination of fluid in the brake fluid reservoir. Unlike other automotive fluids, the brake fluid doesn't circulate, and the lack of contamination in one part of the system does not prove a lack of contamination in other parts. In my opinion, to measure the contamination with any degree of confidence, a sample of fluid from each of the brake calipers would need to be extracted and tested. Also, a part of my fairly unscientific testing was done at room temperature, and another part with only a single type of brake fluid. I've yet to determine the tester's consistency over its specified temperature range, or with other brake fluids.