Patent ID: 7836710

Claim:
An apparatus for determining and indicating when to defrost a static freezer, said apparatus comprising: at least one of a housing having a refrigerating surface that periodically freezes during operation and an evaporator that periodically freezes during freezer operation, forming a layer of ice thereon; a sensor for mounting to a predetermined location on the static freezer for substantially continuously measuring whether a predetermined limit quantity of ice has formed at a predetermined location within the static freezer, and being configured to emit a signal when ice has accumulated to said limit, said ice thickness reaching said limit being indicative of the need to commence a manual defrosting procedure; and a warning device in communication with said sensor and configured to emit a warning responsive to said signal indicating ice has formed to said limit; wherein said sensor has a transparent body with a surface disposed on the refrigerating surface or evaporator such that it freezes over with ice together with the refrigerating surface or evaporator, a light source and a photodetector disposed behind the surface such that a light beam emitted from the light source and reflected at the surface does not impinge on the photodector when the surface is free of ice, whereby if a layer of ice forms on the surface, light is diffracted into the layer of ice and diffusely scattered in it such that the scattered light is sensed by the photodetector in a manner wherein the thickness of the layer of ice can be determined from the intensity of the scattered light.