When choosing a thermometer, take some precautions
measurement error
When conducting high-precision resistance measurements, it is important to ensure that the thermometer can eliminate thermoelectric potential errors generated at different metal connections in the measurement system. A common technique for eliminating thermoelectric electromotive force errors is to use a switch type DC or low-frequency AC current source.
resolving power
Be careful with this indicator. Some thermometer manufacturers confuse resolution and accuracy. A resolution of 0.001 ° C does not necessarily 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. When detecting small temperature changes, display resolution is crucial - for example, when monitoring the solidification curve of a fixed point container, or when checking the stability of a calibration tank.
Linearity
Most thermometer manufacturers provide accuracy specifications at a temperature (usually 0 ° C). This is useful, but you usually need to measure a wide temperature range, so it is important to understand the accuracy of the thermometer within its working range. If the linearity of the thermometer is very good, then its accuracy index is the same throughout its entire temperature range. However, all thermometers have a certain degree of nonlinearity and are not completely linear. Please ensure that the manufacturer provides accuracy technical indicators within the scope of work, or provides the linearity technical indicators you used in calculating uncertainty.
stability
Due to the need to measure over a wide range of environmental conditions and various time lengths, reading stability is crucial. Ensure inspection of temperature coefficient and long-term stability indicators. Ensure that changes in environmental conditions do not affect the accuracy of the thermometer. Reputable manufacturers provide temperature coefficient indicators. Long term stability indicators are sometimes combined with accuracy indicators - for example, "1ppm, 1 year" or "0.01 ° C, 90 days". It is difficult to calibrate every 90 days, so it is necessary to calculate the 1-year indicator and use it for uncertainty analysis. Beware of providers who provide "zero drift" indicators. Each thermometer will have at least one drift component.
calibration
Some thermometers do not require recalibration according to technical specifications. However, according to the latest version of ISO guidelines, all measuring equipment needs to be calibrated. Some thermometers are easier to recalibrate than other devices. To use a thermometer that can be calibrated through its front panel without the need for special software. Some older thermometers store calibration data in EPROM memory and use custom software for programming. This means that the thermometer must be sent to the manufacturer for recalibration - perhaps overseas! Due to the time and cost involved in recalibration, it is important to avoid using thermometers that still use manual pressure gauges for adjustment. Most DC thermometers are calibrated using a set of highly stable DC standard resistors. Calibrating an AC thermometer or bridge is more complex, requiring a reference induction voltage divider and a precision AC standard resistor.
Traceability
Measuring traceability is another concept. The traceability of DC thermometers is very simple through good DC resistance standards. The traceability of communication thermometers and bridges is even more complex. Many countries still lack established traceability of AC resistance. Many other countries with traceable communication standards rely on AC resistors calibrated by thermometers or bridges with an uncertainty precision of ten times, which significantly increases the measurement uncertainty of the bridge itself.
