What Is Response Time?

Nothing responds in zero time and the response time is a measure of the time that a particular sensor takes to reach the equilibrium of the environment it is placed in. The response time can therefore be defined as the time required for a sensor output to change from its previous state to a final settled value within an acceptable tolerance band. Because sensors generally respond exponentially, they never actually reach their final value they just get closer and closer asymptotically until they are so close as to be regarded as settled. This can be an arbitrary value (say 99% of the change) or the point at which they are so close as to be within the tolerance of the sensor.

This is further complicated because there are two generally accepted ways of stating response times i.e the 63% and the 10-90%. The 63% figure is called the time constant and is the time for the sensor to reach 63% of its final value. As a rule of thumb the sensor reaches 95% of its final value in 3 time constants and 99% in five time constants.

The 10-90% is the time it takes for the sensor to traverse between 10% and 90% of its final value.

Typical Response Time

NB. When looking and overall system response other factors may have to be taken into account
such as any pure time delays , sample times, A/D conversion time etc.