The present invention relates to a hand-held random breath test (RBT) unit including a display module for displaying an indication of blood alcohol content (BAC). A sensor module is also provided for releasably fastening to the display module and housing a sensor for sensing the BAC. Preferably, the sensor module can be released from the display module for calibration, and houses the sensor to impede damage to the sensor or the compromising of its calibration.
