Stirring the reaction between hydrochloric acid and a powdered substance For an experiment, I'm adding several substances (as powders) to hydrochloric acid, and measuring the temperature changes to calculate enthalpy. However when adding the powders, the mixture requires some stirring to get all of the substance reacting. 

As I am measuring temperature via a temperature probe hooked up to a data logger (to obtain a temp. against time graph), I need a way to briefly stir the mixture in a controlled way that won't affect the change in temperature (reactions are both endo and exothermic). I have tried stirring with the probe itself, but this is not ideal as the probe is pretty sensitive and friction/bumping with the side of the vessel may affect the recording. 

I initially looked at a magnetic stirring to obtain uniform and controlled stirring, but then I realized the magnet would absorb heat and potentially increase the temperature of the mixture through kinetic energy. So I'm after a stirring method that will not affect temperature, so that the temperature change I record is solely due to the reaction. 

Any suggestions are welcome, because I can't think of any tool or method that would suit.
To minimize thermal coupling between the stirring apparatus and the solution, the best choice is a small, slowly stirred, PTFE or equivalent coated magnetic stir bar. Heat generated by friction should be negligible.

A potential source of heat transfer into the system from this configuration is heat from the stirrer motor. To mitigate this, the surface of the stir plate should be covered with a strip of Styrofoam as thermal insulation. Furthermore, the reaction vessel could be physically separated from the stir plate by using a ring stand and clamp or equivalent. It may be necessary to adjust the thickness of the Styrofoam and distance of physical separation in order to keep the stir bar magnetically coupled to the stir plate magnet.

The setup should first be tested by doing several full runs without adding the acid or the powders. This is essentially an analytical blank. If the temperature changes can be considered negligibly small throughout the duration of an experiment, then you are ready to go. If necessary, any temperature change measured from the blank runs can be subtracted from the actual experimental measurements.