The acceptable deviation of a temperature or humidity probe will depend on; your specific application requirement, legislation for your industry/sector, performance defined internally, by your regulatory body or by your customers. As a manufacture we are commonly asked by customers for temperature probes that are accurate to within ±0.5°C.  This implies that if you have an area that is at 5°C then your happy with a temperature probe reading of between 4.5°C and 5.5°C. Understanding deviation When calibrating a probe at 5°C the calibration house will specify the error seen in the probe and their uncertainty in the measurement they have taken. If the measured reading is 5.3°C and their uncertainty is ±0.3°C then they are saying that they are 95% confident that the accuracy of the reading taken is between 5°C and 5.6°C. At 5°C the deviation would be acceptable, at 5.6°C it would fail the ±0.5°C requirement.  Therefore with this measurement and this uncertainty the probe would be deemed to be beyond the acceptable deviation level. Common mistakes (Based on the above area at a temperature of 5°C) In industry it is often assumed that when an employee tests a probe against a known source (hand held thermometer) and they get a reading of 5.5°C then all is good, the probe is within its deviation limit. However this may not be the case, unfortunately they have not allowed for the uncertainty of the reading being taking and have little or no knowledge of how the uncertainty is calculated. See the next FAQ for more about Measurement Uncertainty. Self checking using a known source is great for checking performance of probes periodically, however to know your true deviation, calibration is required and you should use a UKAS calibration house, like Tek Troniks..