Methods for operating multi-probed formation testers to determine a fluid density gradient in the formation. The drain region includes a first doped region having a first conductivity type, a second doped region having a second conductivity type, and a third doped region. The physical object includes information steganographically hidden thereon, and the information includes at least some information indicating an operation of the physical object. A reading section reads desired freeze-frame data from data stored in the EEPROM in accordance with a request from a diag tester.