Several considerations when choosing a thermometer
When performing temperature calibration, it is critical to select the correct thermometer for the reference probe and the device under test. Here are some factors to consider: Accuracy Many thermometers for resistance thermometers offer ppm, ohms, and/or temperature specifications. The conversion from ohms or ppm to temperature depends on the thermometer used. For a 100Ω probe at 0°C, 0.001Ω (1mΩ) equals 0.0025°C or 2.5mK. 1ppm is also equivalent to 0.1mΩ or 0.25mK. You also need to pay attention to whether the technical indicator is "reading" or "range".
Measurement error
When making high-accuracy resistance measurements, ensure that the thermometer is able to eliminate thermal EMF errors caused by dissimilar metal connections in the measurement system. A common technique for eliminating thermal EMF errors is to use a switched DC or low frequency AC current source.
resolution
Be careful with this indicator. Some thermometer manufacturers confuse resolution with accuracy. A resolution of 0.001°C does not mean an accuracy of 0.001°C. Generally speaking, a thermometer with an accuracy of 0.001°C should have a resolution of at least 0.001°C. Display resolution is very important when detecting small temperature changes - for example, when monitoring the solidification curve of a fixed-point vessel, or when checking the stability of a calibration bath.
linearity
Most thermometer manufacturers provide accuracy specifications at one temperature (usually 0°C). This is useful, but you are often measuring a wide range of temperatures, so it is important to know how accurate the thermometer is over its operating range. If a thermometer is very linear, its accuracy specifications will be the same throughout its entire temperature range. However, all thermometers have some degree of nonlinearity and are not completely linear. Make sure the manufacturer provides accuracy specifications over the operating range or linearity specifications that you use when calculating uncertainty.
stability
Since measurements are made over a wide range of environmental conditions and over various lengths of time, reading stability is very important. Make sure to check the temperature coefficient and long-term stability specifications. Make sure that changes in environmental conditions do not affect the accuracy of the thermometer. Reputable manufacturers all provide temperature coefficient indicators. Long-term stability metrics are sometimes combined with accuracy metrics-for example, "1ppm, 1 year" or "0.01°C, 90 days." Calibrating every 90 days is difficult, so a 1-year indicator is calculated and used in the uncertainty analysis. Be wary of providers that offer "0 drift" indicators. Every thermometer has at least one drift component.
