Temperature response time is defined as the 63% rise time from an initial temperature to a final temperature.

There are multiple ways to specify temperature response time. For a traditional temperature sensor, there are usually two methods: with stirred liquid (oil), and in air (still or moving). Temperature response time in stirred oil is done by taking sensors that are in an oil bath at one temperature, typically 25°C, then moving them very quickly to another oil bath at a different temperature. Temperature response time in air, is usually tested by plunging sensors from an ambient air environment into an oven at a specific temperature. If the sensors are plunged directly into the oven, that is moving air since the oven is moving air around on the inside. To achieve still air, the user can plunge the sensors into an enclosed box inside the oven, so the air does not circulate and remains still.

However, for an isolated temperature sensor, directional response time is also used. Directional temperature response time tests how the sensor responds when one side is exposed to heat, while the other side is not. This replicates a real use case scenario, where the ISOTMP35 can be connected to a high voltage pad or bus bar, and that high voltage area is generating heat. This test provides a repeatable and simple method to evaluate how an NTC performs while electrically isolated in the low voltage region and how the ISOTMP35 performs while directly connected to a high voltage heat source.

Figure 1-1 is an example of measuring the 63% rise time.

Figure 1-1 63% Rise Example

This example is a generic decaying exponential rise, with a 63% rise time value of 1.26, final value of 2. This curve is given by Equation 1.

The 63% time is 3s, and is determined by the denominator in the exponential. This is considered to be 1 tau on an exponential curve.