I have not used the SDG, but I've been working with Transtech's PQIs on asphalt projects for a number of years now, and found them to be pretty accurate and very reliable, especially if you correlate the gauge with a core sample. I believe the soils gauge works on the same principle as the pavement gauge. I've heard that most state and federal agencies have not recognized the SDG for acceptance testing yet, so everyone continues to fall back on the nuclear gauges. It would be nice if that changed sometime soon so we wouldn't have to deal with all the NRC red tape and travel concerns.