How to choose the right pyrometer
degree of accuracy
Many thermometers of resistance thermometers provide ppm, ohm and/or temperature specifications. The conversion from ohm or ppm to temperature depends on the thermometer used. For a probe that is 100Ω at 0 C, 0.001Ω(1mΩ) is equal to 0.0025°C or 2.5mK. 1ppm is also equivalent to 0.1 mω or 0.25mK. It is also necessary to pay attention to whether the technical index is "reading" or "range". For example, the "1ppm reading" is 0.1 mω at 100ω, while the "1ppm range" is 0.4 mω at 400ω. The difference is very big!
When checking the accuracy technical indicators, it should be remembered that the reading uncertainty has little influence on the total uncertainty of the calibration system, and it is not always economical to buy a thermometer with the lowest uncertainty. The analysis method of "bridge-super resistance thermometer" 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 obvious that the bridge can only improve the performance to a small extent-in this case, it is 0.000006 C-and the cost is very high.
measurement error
When carrying out high-accuracy resistance measurement, it is necessary to ensure that the thermometer can eliminate the thermoelectric potential errors generated at different metal connections in the measurement system. A common technique to eliminate thermoelectric electromotive force error is to use switching DC or low frequency AC current source.
resolution ratio
Be careful about this indicator. Some thermometer manufacturers confuse resolution and 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.. When detecting small temperature changes, the display resolution is very important-for example, when monitoring the solidification curve of fixed-point containers, or when checking the stability of calibration tanks.
linearity
Most thermometer manufacturers provide technical indicators of accuracy at a temperature (generally 0 C). This is very useful, but you usually have to measure a wide temperature range, so it is very important to know the accuracy of the thermometer in the working range. If the linearity of the thermometer is very good, its accuracy index is the same throughout its temperature range. However, all thermometers are nonlinear to some extent and not completely linear. Please make sure that the manufacturer provides the accuracy specifications within the working range or the linearity specifications you use when calculating the uncertainty.
stability
Because it is necessary to measure in a wide range of environmental conditions and various time lengths, reading stability is very important. Make sure to check the temperature coefficient and long-term stability index. Ensure that the change of environmental conditions will 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 one-year index should be calculated and used for uncertainty analysis. Beware of providers who provide "0 drift" indicators. Every thermometer will have at least one drift component.
calibrate
Some thermometers are specified by technical indicators as "no need to recalibrate". However, according to the latest ISO guide, all measuring equipment needs to be calibrated. Some thermometers are easier to recalibrate than others. Use a thermometer that can be calibrated through its front panel without special software. Some old thermometers save calibration data in EPROM memory and program with customized software. This means that the thermometer must be sent to the manufacturer for recalibration-perhaps abroad! Because recalibration is very time-consuming and expensive, it is necessary to avoid using a thermometer that is still adjusted by a manual potentiometer. Most DC thermometers are calibrated by a set of DC standard resistors with high stability. Calibration of AC thermometer or bridge is more complicated, which requires a reference inductive voltage divider and precision AC standard resistor.
Traceability
Measurement traceability is another concept. Through good DC resistance standard, the traceability of DC thermometer is very simple. The traceability of AC thermometer and bridge is more complicated. Many countries still do not have the established traceability of AC resistance. Many other countries with traceable AC standards rely on AC resistors calibrated by thermometers or bridges whose uncertainty is ten times more accurate, which will obviously increase the measurement uncertainty of the bridge itself.
convenience
Efforts to improve productivity are endless. Therefore, you need to use a thermometer that saves as much time as possible.
Direct display of temperature-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, Callendarvan-Dusen conversion formula for industrial PRT, and so on.
Various input types-You are likely to calibrate various temperature sensors, including 3-wire and 4-wire PRT, thermistors and thermocouples. Thermometers that can measure various input types can provide the best value and the greatest 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 needs a lot of investment in operation training (and an external computer is needed to calculate the temperature obtained from the resistor).
Multiplex switch for expanding channels-When the calibration work includes constant temperature tanks of the same probe type, the productivity can also be greatly improved if the measuring system can be expanded with multiplex switch.
Digital interface-In order to realize automatic data acquisition and calibration, computer interface is the key. Automatic calibration is realized by using RS-232 or IEEE-488 interface and calibration software which can be connected with thermometer or other system components (thermostatic bath and multiplex switch).
