1.  Technical Field
The present invention relates to temperature/time measuring devices and in particular to safety devices for indicating the degree of exposure of perishable goods above a specified temperature for a duration of time and to disposable indicators of this type. 
2.  Background Art
Because of the need to maintain perishable goods below specified temperatures to maintain their value, it is desirable to know the history of such goods regarding their exposure to higher temperatures which might damage the goods; long exposures to temperatures slightly higher than those specified safe and even short exposure to temperatures well above those specified safe.  In transporting foods, drugs or other perishables, it is especially valuable to the recipient of the goods to know that they have been maintained in an appropriate condition during transportation and storage.  Occasionally the transporting vehicles or storage facilities break down and alter the temperatures of perishable goods to spoil them. 
Most inexpensive prior art devices which employ a frozen fluid and an indicator pressing against it do not provide an accurate means of indicating the history of perishable goods because the entire devices are exposed to the atmosphere.  The frozen fluids desolidify much more quickly if the entire frozen fluid is exposed to the ambient temperature and the frozen fluid may melt or sublime around its entire periphery and thereby deform in shape.  Gross distortions of the history of the perishable goods result because the pressure of an indicator against the frozen fluid as it desolidifies may push the remaining solid mass aside, tilt the mass or split the mass giving a very inconsistent reading on the indicator with inconsistent jumps in the movement of the indicator. 
None of the prior art devices provide a specific means for conducting the ambient temperature to the frozen fluid, further decreasing the accuracy of the limit indicator because of random conduction of the ambient temperature. 
Some of the prior art devices do give accurate temperature/time readings but use very elaborate and expensive sensing devices involving instrumentation. 
Few prior art temperature limit indicators provide a security means to prevent tampering with the indicator and thereby insure an accurate reading and none provide combined security with heat flow and insulated fluid. 
Many prior art devices must be assembled after freezing the fluid making activation a complex procedure.