Abstract: A stand-alone electronic simulator device for testing expendable bathythegraph (XBT) systems. The device is inserted into an XBT system in place of an expendable probe for purposes of checking the performance of the rest of the system with an active test signal. The test device simulates an operational XBT probe by switching a highly accurate and stable array of resistance values across its output. The switching rate is controlled by a stable internal time base such that the test pattern plotted on the strip chart recorder and transmitted to an external system will always look the same if the system is functioning properly. Temperature stability in the test device, essential because of the broad ambient temperature range which may be encountered in use, allows checks of the system for probe depth timing linearization in addition to temperature measurement.