Several things to think about when selecting a thermometer
When performing a temperature calibration, selecting the correct pyrometer for the reference probe and the device under test is critical. The following factors need to be considered: Accuracy Many thermometers for resistance thermometers provide ppm, ohms, and/or temperature specifications. The conversion from ohms or ppm to temperature depends on the thermometer used. For a probe that is 100Ω at 0°C, 0.001Ω (1mΩ) equals 0.0025°C or 2.5mK. 1ppm is also equivalent to 0.1mΩ or 0.25mK. Also note whether the specification is "reading" or "range".
For example, "1ppm reading" is 0.1mΩ at 100Ω, while "1ppm span" is 0.4mΩ when the full scale is 400Ω. The difference is huge! When examining accuracy specifications, keep in mind that the uncertainty in reading contributes very little to the overall uncertainty of the calibration system, and it does not always make economic sense to purchase the thermometer with the lowest uncertainty. The "Bridge-Super Resistance Thermometer" analysis method is a good example. A 0.1-ppm bridge can cost more than $40,000, while a 1-ppm super resistance thermometer can cost less than $20,000. Looking at the total system uncertainty, it is clear that the bridge improves performance only slightly -- in this case, 0.000006°C -- at a very high cost.
Measurement error
When making high-accuracy resistance measurements, it is important to ensure that the thermometer can eliminate thermal EMF errors generated at the junction of dissimilar metals in the measurement system. A common technique for canceling thermal EMF errors is to use a switched DC or low frequency AC current source.
convenience
The efforts to increase productivity are never-ending. Therefore, you need a thermometer that saves you as much time as possible.
Display temperature directly---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 provide various conversion methods - ITS-90 conversion formula for SPRT, Callendarvan-Dusen conversion formula for industrial PRT ,etc.
Various Input Types ------ You will most likely be calibrating a wide variety of temperature sensors, including 3- and 4-wire PRTs, thermistors, and thermocouples. A thermometer capable of measuring multiple input types provides the best value and the most flexibility.
Learning Curve ------ Adopt a simple and easy-to-use thermometer. Bridges have been used for many years and provide good measurement performance, but require a large investment in operator training (and require an external computer to calculate the temperature obtained from the resistor).
Multiplexer Switches for Channel Expansion – The ability to expand the measurement system with multiplexer switches can also greatly increase productivity when the calibration job includes a constant temperature bath of the same probe type.
Digital Interface - For automated data acquisition and calibration, a computer interface is key. Automated calibration with RS-232 or IEEE-488 interface for connection to thermometer or other system components (thermostatic bath and multiplexer) and calibration software.
