the problem with the IAT is it's Thermal Time Constant. essentially how fast it reads. most GM IAT's are at least 15 seconds TTC. here's a bit of background on the Thermal Time Constant--
Thermal Time Constant (T.C.):
When a thermistor is being used to monitor the temperature of it’s environment then the accuracy of measurement of the resistance of the thermistor is critical.
While the power dissipated in the thermistor is an important factor in this measurement as discussed in the previous section, the thermal characteristics of the system and the thermistor are important also. This is especially relevant in systems where the temperature is changing with time. The dynamic thermal response of the thermistor must be considered in these situations. To quantify this dynamic response, the concept of a Thermal Time Constant (T.C.) is used in the thermistor industry and it is defined as follows:
The Thermal Time Constant for a thermistor is the time required for a thermistor to change its body temperature by 63.2% of a specific temperature span when the measurements are made under zero-power conditions in thermally stable environments.
This concept is illustrated in the example below:
Example: A thermistor is placed in an oil bath at 25°C and allowed to reach equilibrium temperature. The thermistor is then rapidly moved to an oil bath at 75°C.
The T.C. is the time required for the thermistor to reach 56.6°C (63.2% of the temperature span).
The dominant factors that affect the T.C. of a thermistor are:The mass and the thermal mass of the thermistor itself
The definition of Thermal Time Constant arises from the exponential nature of the rate of transfer of heat between the thermistor and the medium that it is monitoring. It is similar in principle to the definition of time constants in describing the responses of systems where physical effects have an exponential response with respect to time.
