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Explanation: The thermography website below mentions both thermal variance and thermal deviation. The other website is about a BioScan sensor which is 'capable of measuring thermal variance of less than 9/1000 of a degree'. The difference, if there is one, between thermal variance and thermal deviation is unclear. Thermal variance seems to be used more frequently in a biological context, although thermal deviance is also used. Thermal deviation seems to be used more in a technological or chemical context, so I would be tempted to go for that. I guess it depends what the thermometers in your translation are being used for!