Hi mbreward
you could use a superheat calculation kit where the thermometer measures the direct temp of the refrigerant.
It is not a stable measurement - it is dynamic and due to the flow and constant adjusting of the metering device. The one is use is about 10 years old and it would be good if a manufacturer would build one that gives a K value with average, min and max readings. I use the yellow jacket ritchie.
I spoke with a boss from CPS in the USA last year and he said they were working on an electronic diagnosis tool which also did data logging.
0°C is a standard calibration temp I think the next standard is around 28°C (mercury) so 0°C is the closest for calibration for measuring superheat. I also have a fluke with a high digit resolution but have found they are not accurate and the range is too high.