Most draft beer brewed in the U.S is not pasteurized, so it must be kept cold. The temperature of non-pasteurized Ale & Lager type beers must be maintained between 36-38°F all the way to the point of dispense.
Temperatures even half a degree above 38°F will cause CO2 to break out of the beer and result in foam and promote sour/cloudy beer. When temperature rises above 50-55°F, bacteria growth rapidly begins to spoil flavor and cloud the beer. Simply put, keg beer storage can be compared to milk storage: "if it is not kept cold, it will spoil."
The air temperature in the refrigerator can fluctuate greatly when the door is opened.
Therefore it is important to check the "liquid" temperature of the beer rather then the air temperature in the refrigerator.
The ideal method for monitoring the liquid temperature (inside the keg) is done by use of a liquid thermometer. Such thermometers are enclosed in a casing filled with liquid; this provides a constant accurate reading of the liquid (beer) temperature within the refrigerator.
Another method for monitoring the liquid temperature (beer inside the keg) is to put a glass of water in the refrigerator with a thermometer in it. Allow the water to chill for 24 hours, and then you will have a accurate reading of the "liquid" temperature.
Additionally, if your draft beer is being dispensed away from the refrigerator, the temperature of the dispensed beer should be monitored. This is to make sure the 36-38°F temperature is being maintained all the way to the point of dispense.
To check the dispense temperature, simply pour beer into a glass and use a thermometer to read the temperature - in a proper system this will match the liquid temperature inside the kegerator.
If a walk-in cooler is being used for keg storage, try to limit the traffic in and out as opening and closing the cooler with result in increased storage temperatures. If this cannot be prevented, it is recommended to install flexible clear plastic air curtains or an electric air curtain to prevent cold air from being lost when the cooler is opened.