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Device & model you are using (MQTT from Windows/Linux)
What dashboard are you using? (Web)
I have a persistent Thermometer widget on my dashboard. I send temperature in Fahrenheit. My last value sent from my device was 82 F.
I logged in from another web browser and the temperature showed 179.6. (which is 82 Celsius converted to Fahrenheit). Also, I refreshed the browser that showed 82 F right after the update and it changed to 179.6. So it seems Celsius is the default unit but the value is being saved as Fahrenheit so when the widget is resent the value from storage it is being re-converted to Fahrenheit. Seems you want to convert back to Celsius to save values so that devices show the correct last value when data is resent to widgets from your server.