Basic Thermometry Concepts: Reproducibility
The history of thermometry helps draw attention to some of the basic challenges in recording and using accurate temperature information. Adopting a universal scale (whether Fahrenheit, Celsius, Kelvin, Rankine or the more obscure Delisle, Réaumur or Rømer scales) makes the establishment of scientific standards possible, as well as the direct comparison of relative temperature data from place to place and instrument to instrument. It also hints at the importance of "reproducibility" in thermometry.
A thermometer measuring the ice point of water should read 32°F (or 0°C) every time you measure it, not 32°F (0°C) one time, 34°F (1°C) the next time and 30°F (-1°C) the next. That would defeat the purpose of a universal scale used to compare the relative temperatures of dissimilar materials and environments.
Reproducibility, along with accuracy and resolution, are the foundations upon which all good thermometer technology is built, but they are actually three different things. Some expensive thermometers on the market today are quite precise and fairly accurate on occasion but are not reliably reproducible. That means that you may or may not have accurate temperature data depending upon the performance of the thermometer at the particular time you take your measurement.
One common challenge to the reproducibility of thermometers is a phenomenon known as "hysteresis." With hysteresis, the physical properties of an instrument, like a thermometer probe, for example, are temporarily changed by the process of taking a measurement. Thermometers exhibiting hysteresis will display different temperatures in the same material (say, an ice bath) over a short period of time and are therefore not "reproducible."
This problem is common with "mechanical" thermometers like bimetal dial thermometers but can also affect "electronic" thermometers like instant-read digitals. An extended resting period, to allow the physical properties of the instrument to return to normal, can sometimes restore accuracy, but often only temporarily. Or, as with dial thermometers, they may have to be re-calibrated regularly.