Warsash Scientific explains the importance of sensor placement in ensuring accuracy and temperature stability.
For instance, if the temperature sensor is placed close to the device under control (B), it will read an accurate temperature. However, if this puts the sensor at a distance from the actuator (thermoelectric or resistive heater), there will be a thermal delay in the sensor feedback, decreasing temperature stability.
The more accurate system usually uses a smaller proportional gain (P) and a large integrator time constant of 5-10 seconds. The more stable system (A), with the sensor placed closer to the thermoelectric is stable over a wide range of proportional gain settings and uses a small integrator time constant of about one second. With this system, one has the freedom to increase the proportional gain to reduce time to temperature.
Adapted from Laser/Tec Connect, January 2014 from Wavelength Electronics.