Improving sensitivity of your Electronic Temperature Controller
Using an Electronic Temperature Controller to precisely control mash temperatures is only as useful as your ETC is accurate. If your ETC is slow to sense the changing temperature in the mash, it will be slow to adjust power to the heating element, allowing for several degrees of temperature swing.
Since the ETC uses a temperature probe that sits inside a thermowell (new models use a liquid tight sensor), the tiny space of air around the probe inside the thermowell can act as an insulator, slowing down the heat from reaching the probe, and therefore keeping the ETC from adjusting as quickly as it could.
The insulating effect of the air inside the thermowell can be greatly reduced by putting heat conductive compound inside the thermowell before inserting the ETC temperature probe. The heat conductive compound fills up the space inside the theromwell around the probe, and transfers the heat from the theromwell to the probe so that the probe quickly reacts to the changing temperature of your wort.
When putting the compound into the thermowell, it is most important to get the compound all the way to the bottom of the thermowell. Since the ETC probe reads temperature at the tip of the probe it is most important to ensure that the cavity between the tip of the probe and the thermowell is completely filled with compound. The compound can be applied with a barbecue skewer or thin doweling and pushed down to the end by spinning the skewer in your fingers as you slowly prod the compound down the thermowell. (Some heat compounds can also be purchased in a syringe which might make application a little easier.)