Dissolved oxygen is the amount of free oxygen in water or any other liquid. Sensors on the other hand are devices/equipment used to measure the amount of the dissolved oxygen in an aqueous solution. Sensors allow automatic detection. There are different types of sensors varying on how they detect and some of them include, optical, galvanic, electrochemical and polagraphic sensors. All these sensors defer in their uses like for instance, optical sensors do not require stirring of the solution in order to get the measurements while electrochemical sensors require stirring. Due to this variation in uses, a person may choose the one that serves him best.
Taking an example of two polagraphic sensors which include YSI PRO 20 and YSI 55.These two differ in their uses and calibration. For instance the YSI PRO 55 has an inbuilt barometer thus making it quick and accurate unlike YSI 55 which doesn't have inner barometer.