It should be noted that the accuracy of temperature measurement is dependent upon a number of factors.
The Instrument
The device itself will cause errors because of limitations in digital processing and physical issues (eg “cold junction compensation” with thermocouples).
The Sensor
The sensor is often the greatest source of error in the reading. It is also difficult to compensate for sensor errors. It requires that only the sensor ever be used with the instrument and that it be used at a certain temperature. Thermistors and PT100s are interchangeable within 0.4°. Thermocouples, however, have much larger errors. Type K can have errors up to 2.2° and 1° for type T. Special tolerances can reduce Type T’s errors by half and make them acceptable for use with food. Type K should never be used with food. They are outside the 1° accuracy required. Note that the accuracy required is the total of the instrument and sensor error. For example, if an instrument has a 0.4° error and a sensor has a 0.8° error then they are not acceptable because the total is 1.2° error. Many devices fail because they have an accuracy of 1° plus a percentage error of the reading (eg 1° +/- 3% of reading = 1.3° at 10°). It is the total possible error that is important in determining if a system complies with the standards.
Measurement Errors
This is difficult to predict and depends upon the skill of the operator as well as what they are trying to measure. Using the wrong device will produce a false reading. For example an infra-red thermometer measures surface temperature only. The temperature within the item could be significantly different.