Can you rephrase your question or give more details?
The input is a temperature. A thermocouple generates a very small voltage that is proportional to voltage. http://en.wikipedia.org/wiki/Thermocouple
a thermocouple is a single capacity system..so when immersed in water bath what would be the input to the thermocouple at that instant...is thAT a ramp input or step input??
An instant change must be a step. A thermocouple has thermal mass so it will take time to exponentially settle towards a final value. That is closer to a ramp than a step. What is the time constant of the thermal change?