What sensor would you recommend to be used with the WTC series temp controller: an AD590 temp transducer or a thermistor? Would a larger mass increase the stability?
Choosing the right temperature sensor is an important part of the overall system design, and there are a a number of factors to consider before deciding which sensor to use. To help you through the design process, Wavelength has an application note available for download: TN-TC01 Optimizing Thermoelectric Control Systems
Thermistors are ideal when precision temperature control is required across a narrow temperature control range or at a single setpoint. The resistance across the thermistor changes with temperature, but the nonlinear response means there is a narrow temperature range where the thermistor will afford the greatest sensitivity for your application. Different types of thermistors are available, and the thermistor should be selected according to the target temperature. For more information about thermistors, please see the Thermistors Basics page.
The linear semiconductor sensors (eg. AD590 and LM335) respond linearly, so they are usable over a much wider temperature range. These sensors are not as accurate as thermistors, however, so they are not ideal in applications where high stability and precision temperature reproduceability are required.
A larger load thermal mass will be more stable to transient temperature changes caused by outside factors, but will also be much slower to respond when the temperature setpoint is changed. Again, there is a tradeoff that is application dependent.