US Patent 9279732 Internal temperature indicator

A temperature indicator having a tubular structure; a float or floating particles releasably held in place in the tubular structure by a temperature dependent material that is activated at or about a critical temperature, the float or floating particles having a first density; and a fluid within the tubular structure, the fluid having a second density that is less than the first density. When the temperature indicator is heated to the critical temperature, the temperature dependent material activates and releases the float or the floating particles, which rise in the fluid so as to be visible as in indicator that the critical temperature has been reached.