A reference sensor system is provided for detecting the presence and/or measuring the concentration of analyte particles in a sample stream. The system includes: a) a laminar flow channel; b) three or more inlet means in fluid connection with the laminar flow channel for respectively conducting into the laminar flow channel (1) an indicator stream which may include an indicator substance which indicates the presence of the analyte particles, (2) the sample stream, and (3) a reference stream, which can be a control stream and/or internal standard stream; and, c) wherein the laminar flow channel has a depth and/or width sufficiently small to allow laminar flow of said streams and a length sufficient to allow particles of the analyte to diffuse into the indicator stream to form a detection area. Branching channels may be provided as outlet means for conducting the streams out of the laminar flow channel. Also provided are methods of using the device to improve accuracy and increase convenience of measuring the concentration of analytes in a sample. Monitoring reference stream(s) in laminar flow with sample stream(s) allows for calibration or for monitoring or correcting for the effects of experimental conditions which might otherwise degrade accuracy. A reference stream which serves as both a control stream and an internal standard stream is convenient and economical. The device of this invention also needs less frequent calibration than previously-known fluid analysis devices because the devices and methods of this invention provide for continuous and simultaneous quality control and internal standardization.