A DO meter consists of a probe filled with potassium chloride and covered with a semi-permeable membrane, a wire carrying electricity, and two electrodes.  When the probe is placed in the water, the potassium chloride attracts oxygen, just as hemoglobin does in our blood, and pulls the oxygen out of the water and into the probe.  The semi-permeable membrane allows oxygen to pass into the probe but does not allow water in.  

The probe is connected to a wire through which DC electricity flows (the red dashes in the drawing above.)  There is a gold and a silver electrode on the wire, one on either side of the probe.  You can think of these electrodes as devices which measure the amount of electricity passing through the wire at that point.  

Where the probe joins the wire, oxygen mingles with the electricity.  Oxygen is not very ionized, meaning that it does not have a negative charge as electricity does, so the oxygen dilutes the current at the electrode beyond the probe.  The meter measures the difference in current between the two electrodes and uses that difference to determine the amount of oxygen in the water.  

When using a DO meter, the meter must be properly cared for.  The membrane must be kept wet and have potassium chloride solution within it at all times.  The electrode should also be maintained.  The electrode assembly can be cleaned by dipping the electrode tip into ammonium hydroxide.  

The meter must also be calibrated before use to ensure that the reading is accurate.  The meter can be calibrated using water with a known concentration of DO at at set temperature.  This can be done by filling a biological oxygen demand (BOD) bottle halfway full of distilled water.  The water is shaken vigorously to saturate the water with air. Then the probe is placed in the bottle and the reading is adjusted to the known concentration of DO in the bottle. Alternatively, the meter can be calibrated using the Winkler method, which involves adding chemicals to the water and forming a precipitate.