Convert temperature differences between degree_F and degree_C

[Karen K]

I write the driver for Wetternetz Sachsen, and I have to upload some temperature differences there. 

And I wonder what is the intended way to convert those differences from one unit to another.

The template for a restful service driver says that I have to put code to read or calculate additional values into the "get_record" function, where the archive unit system is used. The conversion to the target unit system I have to put into the "format_url" function. 

Temperatures and temperature differences are converted in a different way because of the offset between the units.

But I found no "group_temperaturediff" or "group_temperaturerate" unit group, that could cover that. 

Because I did not know I put the code to read the start and end temperature of the given interval into the "get_record" function. No difference is calculated there. In "format_url" function first the data record is converted into the target unit system and then the temperature difference is calculated. 

Shall I do so or is there a better way?

[Tom Keffer]

As you've discovered, temperature conversion functions are not distributive. That is, F_to_C(68F - 50F) is not equal to F_to_C(68F) - F_to_C(50F).

No, there is no group_temperaturediff.

The tag $trend has a problem similar to yours. It needs to calculate the difference between the temperature some time ago, and the current temperature. It solves the problem the same way you are: perform the conversion just before use. 

I think your approach is very sensible.

-tk

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[Karen K]

Thank you for clarification.