What are they? Temperature labels are temperature monitors consisting of one or more small heat-sensitive indicators sealed under transparent, heat-resistant windows. Types of labels Reversible - reversible temperature labels can change back and forth as needed to provide an indication of the present temperature. One advantage of the reversible labels is that they can be used over and over again. Irreversible - irreversible self-adhesive temperature monitors consist of one or more heat sensitive indicators sealed under transparent, heat-resistant windows. The centers of the indicator circles will turn black at the temperature ratings shown on the label. A one-time usage indication that the specified temperature has been reached