All electronic devices will drift overtime and with temperature. It is inevitable. Thermometers less than $20 or ones from the grocery store are not very accurate. They usually are around 2-3 oF in accuracy. The Checktemp from Hanna is on the expensive side but has the Cal-Check function and an accuracy +/- 0.5 oF (-58 to 199.9 oF range). It is +/- 1 oF outside the range.
http://shop.hannainst.com/hi98501-checktemp-digital-thermometer.html Cal-Check is an internal diagnostic function of the electronics. When switched on it should display "0". Any drift will show as a deviation from "0"
Thermometers fall into three categories:
Thermistors
Thermocouples (K-type being the most common)
PT100
Thermistor is a semi-conductor device that resistivity changes with temeprature
Thermocouples consist of probe with two wires of different metals twisted together (hot junction) that have a potential difference based on temperature
pt100 is like a thermistor in which there is an increase in resistance that is measured but it is a resistance of a metal conductor (RTD) made of platinum with temperature and not a semi-conductor (thermistor - NTC, PTC).
Thermistors have limited ranges of around -50 to 302 oF and offer high accuracy
Thermocouples have very wide range based on type (K,J,T, etc). For example a K-type wire probe spec can be up to 2000 oF but not as accurate as a thermistor.
pt100 have a wide range, up to 1650 oF and high accuracy.
Thermocouples have the fastest response. The fast response time of the thermocouple is one of the reasons it is the technology chosen in restaurants. we offer probes as fast as 4 second response time to reach 90% of the value. Thermistors probes can be fast as well. We have some in the 6 second response time to reach 90% of the value.
The pocket thermistor thermometers are the most common due to price. Portable themometers will start in the +$100 dollar range. With thermocouples there are two types of calibrations. The internal meter calibration is for the "hot junction" which is where the probe connects to the meter. This has to be done with temperature controlled baths at multiple points. There are also meters with probe calibration feature in which the "cold junction" is calibrated by placing the probe in an ice bath. An ice bath should be prepared with crushed ice made from DI water. Any salts will cause freezing point depression in which the water will freeze below 32 oF. Some portable thermistor thermometers also offer a ice bath calibration feature.