Control System - First order system

[kloong]

Q:
1) A thermometer requires 1min to indicate 98% of the response to be a step input. Assuming the thermometer to be a first order system, find the time constant.

2)If the thermometer is placed in a bath, the temperature of which is changing linearly at a rate of 10degrees/min, how much error does the thermometer show?

for the first part, i think that it takes 4 time constant tor for the system to reach 98%. So I am thinking that if i divide the 1min by 4 (i.e, 1min/4), id get the time constant, T. is this correct?

and for the second part, I am not sure how should i tackle it. can you give me an outline of what i should do?

thanks.

 

[CEL]

You got the first part right.The time constant is 15s.
For the second part, your system is of the form G(s)=\frac{k}{s+a}.
You can calculate a from the time constant. K can be determined requiring that the termometer has no steady state error.
Now, what is the response of your system to an input \frac{A}{s^2}?
A = 10/60 degrees/s.