If you own an Extech IR thermometer the unit is factory calibrated before it is shipped. Extech recommends that annual calibrations be performed to verify meter performance and accuracy. The accuracy of your meter is the amount of error (plus or minus) that it can drift from actual values and still be considered to be within tolerance of the instrument. Each meter's accuracy specification can be found in the back of the User's Guide that you receive with the instrument under the Specifications section. If you ever find yourself questioning your results then the there are a few things you can do to make sure and verify your meter is within tolerance. First, always make sure you have a good battery. Low battery conditions can cause inaccuracies. Next, refer to the IR Measurment Consideration section of the User's Guide to make sure you are using it properly and within the optimal conditions. Finally, if all these things check out and you are still are finding your readings are off then try compare it to a known by using the simple ice bath test - Using a Pyrex container, create a slushy ice water solution with crushed ice and water. Let the solution stabilize for about 20 minutes, stirring and keeping the solution slushy, with crushed ice present. This will create a system that is 32 deg F. make a measurement with the IR Thermometer. If your unit has an accuracy of +/-2% of reading plus 4 degrees. At 32 deg F, this calculates to 4.6 deg, so a reading in the range of 27.4 to 36.6 degrees would be valid. Make sure to refer to the User's Guide to find the accuracy spec for your particular model. If it is out of tolerance then you can contact Extech to arrange for calibration. I hope the information is helpful!