YSI 50 Series Meters. YSI 5239 A rugged, field DO probe with easy-to-replace cap membranes, the YSI 5239 DO and temperature sensor is compatible with YSI 50 Series meters (except the YSI 55). Weighted and protected by a heavy-duty guard, the YSI 5239 is optimized for YSI microprocessor-based systems.