I have both a cheap Chinese one and a Tritex. A key difference is that the Chinese one reads a single echo, whilst the Tritex reads multiple echoes. The Chinese one will be cofused by paint or corrosion on the surface the probe is placed against, while the Tritex won't (usually) be. So to use the single echo type effectively you must clean paint, corrosion or any other crud off down to clean metal, but if that is done they are reasonbly accurate, bearing in mind the stated error range (as mentioned in #6 above). Also make sure the meter is calibrated correctly before use as the echo travels at different speeds through different metals.