Critical temperature indicator device

A critical temperature indicator device for measuring defrost time, comprising:

• an hourglass shaped elongated compartment including a first end and a second end separated by a side wall along a longitudinal axis of the elongated compartment, thereby forming a cavity including a first chamber and a second chamber that are fluidly connected through a narrow section located between the first and second chambers, wherein at least a portion of the elongated compartment is made of a first transparent material;
• a planar permeable structure in the form of a mesh disposed at the narrow section and configured to prevent a movement of a solidified liquid from one chamber to another when the elongated compartment is vertically oriented;
• a liquid, comprising a first phase change material and a second phase change material, disposed in the cavity;
• a substantially planar and rectangular base to which the elongated compartment is releasably and rotatably attached, the base being configured to mount the elongated compartment to a wall, such that the critical temperature indicator device is substantially planar in a plane parallel to the wall;
• and
• a graduation arranged on an outside surface of the elongated compartment suitable for measuring a liquid level in the elongated compartment,
• wherein the elongated compartment is configured to be rotated about a central axis orthogonal to the base, the first and second chambers being symmetric about the central axis,
• wherein the first and second chambers are symmetric to each other, and
• wherein the critical temperature indicator device is configured to determine a critical temperature rise period for two or more temperature intervals in a cooled space.