Several considerations for selecting a thermometer[Copy link]
This post was last edited by 灞波儿奔 on 2019-1-6 16:21 When performing a temperature calibration, it is critical to select the correct thermometer for the reference probe and the device under test. The following factors need to be considered:AccuracyMany thermometers for resistance thermometers offer specifications in ppm, ohms, and/or temperature. The conversion from ohms or ppm to temperature depends on the thermometer used. For a probe with a 100Ω value at 0°C, 0.001Ω (1mΩ) is equivalent to 0.0025°C or 2.5mK. 1ppm is also equivalent to 0.1mΩ or 0.25mK. It is also necessary to pay attention to whether the specification is "reading" or "range". For example, "1ppm reading" is 0.1mΩ at 100Ω, while "1ppm range", when the full scale is 400Ω, is 0.4mΩ. This is a huge difference! When reviewing accuracy specifications, remember that the reading uncertainty contributes only slightly to the total uncertainty of the calibration system, and it does not always make economic sense to purchase the thermometer with the lowest uncertainty. The “bridge-to-superresistor thermometer” analysis method is a good example. A 0.1-ppm bridge costs over $40,000, while a 1-ppm superresistor thermometer costs less than $20,000. Looking back at the total system uncertainty, it is clear that the bridge improves performance by only a small amount—0.000006°C in this case—and the cost is very high.
Measurement Error When making high-accuracy resistance measurements, be sure that the thermometer can eliminate the thermoelectric potential errors generated by the dissimilar metal junctions 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 specification. Some thermometer manufacturers confuse resolution with accuracy. A resolution of 0.001°C does not mean 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 important when detecting small temperature changes—for example, when monitoring the freezing curve of a fixed-point container, or when checking the stability of a calibration bath. Linearity Most thermometer manufacturers provide an accuracy specification at one temperature, usually 0°C. This is helpful, but you will usually be measuring a wide range of temperatures, so it is important to know how accurate the thermometer is over the operating range. If the thermometer is very linear, then its accuracy specification will be the same over its entire temperature range. However, all thermometers have some degree of nonlinearity and are not perfectly linear. Make sure the manufacturer provides an accuracy specification over the operating range, or a linearity specification that you use in your uncertainty calculations. Stability Since you will be measuring over a wide range of environmental conditions and for various lengths of time, the stability of the readings is very important. Make sure to check the temperature coefficient and the long-term stability specification. Make sure that changes in environmental conditions will not affect the accuracy of the thermometer. Reputable manufacturers provide a temperature coefficient specification. The long-term stability specification is sometimes combined with the accuracy specification - for example, "1ppm, 1 year" or "0.01°C, 90 days." It is difficult to calibrate every 90 days, so calculate the 1-year specification and use it in your uncertainty analysis. Beware of providers who offer "zero drift." Every thermometer has at least one drift component. Calibration Some thermometers are specified as "not requiring recalibration." However, according to the latest ISO guidelines, all measuring equipment needs to be calibrated. Some thermometers are easier to recalibrate than others. Use a thermometer that can be calibrated from its front panel without special software. Some older thermometers store calibration data in EPROM memory, programmed with custom software. This means the thermometer must be sent to the factory for recalibration—perhaps overseas! Because recalibration is time-consuming and expensive, avoid thermometers that still use manual potentiometers for adjustment. Most DC thermometers are calibrated using a set of high-stability DC standard resistors. Calibrating an AC thermometer or bridge is more complicated, requiring a reference sensing voltage divider and precision AC standard resistors. Traceability Measurement traceability is another concept. Traceability of DC thermometers is very simple with good DC resistance standards. Traceability of AC thermometers and bridges is more complicated. Many countries still do not have established traceability for AC resistance. Many other countries with traceable AC standards rely on AC resistors calibrated with thermometers or bridges that are ten times more accurate in uncertainty, significantly increasing the measurement uncertainty of the bridge itself. Convenience The struggle to improve productivity is endless. Therefore, you need a thermometer that saves you as much time as possible. Direct Display of Temperature - Many thermometers can only display raw resistance or voltage. Temperature is the most useful display, so use a thermometer that can convert resistance or voltage to temperature, and be sure to offer a variety of conversion methods—ITS-90 for SPRTs, Callendarvan-Dusen for industrial PRTs, and so on. Various Input Types—You will likely be calibrating a wide variety of temperature sensors, including 3- and 4-wire PRTs, thermistors, and thermocouples. A thermometer that can measure a variety of input types offers the best value and the most flexibility. Learning Curve—Use a thermometer that is simple to use. Bridges have been used for many years and provide good measurement performance, but they require a large investment in operator training (and require an external computer to calculate the temperature from the resistance). Multiplexers for Expanding Channels—When calibrations include baths of the same probe type, the ability to expand the measurement system with a multiplexer can also greatly increase productivity. Digital Interface - To achieve automated data acquisition and calibration, the computer interface is key. Use RS-232 or IEEE-488 interfaces that can be connected to the thermometer or other system components (thermostat and multiplexer) and calibration software to achieve automatic calibration.
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