When performing a temperature calibration, it is critical to choose the right thermometer for the reference probe and the device under test. The following factors need to be considered:
Many thermometers for resistance thermometers provide ppm, ohm, 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) is equal to 0.0025 ° C or 2.5mK. 1ppm is also equivalent to 0.1mΩ or 0.25mK. Also pay attention to whether the technical indicators are "reading" or "range". For example, "1ppm reading" is 0.1mΩ at 100Ω, and "1ppm range" is 0.4mΩ when the full range is 400Ω. The difference is huge!
When checking the accuracy technical indicators, keep in mind that the effect of reading uncertainty on the total uncertainty of the calibration system is small, and it is not always economically meaningful to purchase a thermometer with the lowest uncertainty. The "bridge-super resistance thermometer" analysis method is a good example. A 0.1-ppm bridge costs more than $ 40,000, while a 1-ppm super resistance thermometer costs less than $ 20,000. Looking back at the total system uncertainty, it is clear that the bridge can only improve performance to a small extent-0.000006 ° C in this example-but the cost is very high.
When making high-accuracy resistance measurements, make sure that the thermometer can eliminate the thermoelectric potential errors generated at different 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.
Be careful with this indicator. Some thermometer manufacturers confuse resolution and accuracy. A resolution of 0.001 ° C does not imply an accuracy of 0.001 ° C. In general, 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 container, or when checking the stability of a calibration tank.
Most thermometer manufacturers provide accuracy specifications at a temperature (generally 0 ° C). This is useful, but you typically measure a wide temperature range, so it is important to understand the accuracy of the thermometer over its working range. If the linearity of the thermometer is very good, its accuracy index is the same over its entire temperature range. However, all thermometers have a degree of non-linearity and are not completely linear. Make sure that the manufacturer provides accuracy specifications within the working range, or linearity specifications that you use when calculating uncertainty.
Since the measurement is performed under a wide range of environmental conditions and for various lengths of time, reading stability is very important. Be sure to check the temperature coefficient and long-term stability indicators. Ensure that changes in environmental conditions do not affect the accuracy of the thermometer. Reputable manufacturers all 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." Calibration is difficult every 90 days, so one-year indicators are calculated and used for uncertainty analysis. Beware of providers that offer "zero drift" metrics. Every thermometer will have at least one drift component.
Some thermometers have technical specifications that require “no recalibration”. However, according to the latest version of the ISO guidelines, all measurement equipment needs to be calibrated. Some thermometers are easier to recalibrate than others. Use a thermometer that requires no special software to calibrate from its front panel. Some older thermometers store calibration data in EPROM memory and are programmed using custom software. This means that the thermometer must be sent to the manufacturer for recalibration-maybe abroad! Because recalibration is time consuming and expensive, avoid using thermometers that still use manual pressure dividers for adjustments. Most DC thermometers are calibrated with a set of highly stable DC standard resistors. Calibrating an AC thermometer or bridge is more complicated and requires a reference inductive voltage divider and precision AC standard resistors.
Measuring traceability is another concept. With good DC resistance standards, the traceability of DC thermometers is very simple. The traceability of AC thermometers and bridges is more complicated. Many countries still do not have established traceability of AC resistance. Many other countries with traceable AC standards rely on AC resistors calibrated by thermometers or bridges that are ten times more precise in uncertainty, which significantly increases the measurement uncertainty of the bridge itself.
The efforts to increase productivity are endless. Therefore, you need a thermometer that saves time as much as possible.
Direct temperature display-Many thermometers can only display the original resistance or voltage. Temperature is the most useful display, so use a thermometer that can convert resistance or voltage into temperature, and make sure to provide various conversion methods-ITS-90 conversion formula for SPRT, and Callendarvan-Dusen conversion formula for industrial PRT. ,and many more.
Various input types-you will most likely calibrate a variety of temperature sensors, including 3- and 4-wire PRTs, thermistors, and thermocouples. A thermometer that can measure multiple input types provides the best value and maximum flexibility.
Learning curve --- using a simple and easy-to-use thermometer. The bridge has been used for many years and can provide good measurement performance, but it requires a large investment in operation training (and requires an external computer to calculate the temperature obtained from the resistance).
Multi-way switch for expanding channels --- When the calibration work includes a constant temperature bath of the same probe type, productivity can be greatly improved if the measurement system can be extended with a multi-way switch.
Digital interface --- In order to achieve automatic data acquisition and calibration, a computer interface is the key. RS-232 or IEEE-488 interface and calibration software can be connected with the thermometer or other system components (thermostatic bath and multiplex switch) to realize automatic calibration.