Short answer: Record your measurements in tenths of a Celsius degree. But, set your code up so you can pick the format you are most intuitively familiar with for viewing charts. If using Celsius means you will be constantly doing conversions in your head to Fahrenheit, why go through that hassle? Granted, metric is better in so many ways. But, in the US, it’s not the standard.
Long answer: About this time last year, I made a data logging system that I used for microgreens and then lettuce. One of my goals was to make a working example of sharing datasets. At the time, people were talking on the forum about sharing data, but nothing much was happening.
Anyhow, I grew my stuff, took a bunch of pictures, logged temp & humidity, made charts, stuck the datasets up on github, and linked to it in a post here. As far as I can tell, about 5 people looked at my data, and maybe about 2 of them engaged me in conversations about it. Beyond that, nothing much happened. At that time, I was using Celsius, but I don’t remember anybody saying anything to indicate that they cared about my units.
The lesson I took from that experience was that I should focus on collecting data in a format that was most useful to me, because I was probably the only one who would ever be looking at it closely.
For the next generation of my data logging system, I switched to doing temperature charts in Fahrenheit because I’m used to it. Also, lots of the hydroponic and horticultural resources written in the US refer to temperatures in Fahrenheit.
I suggest you log data in whatever format is most useful for your own growing. I suggest that you log data according to good scientific practice and then present it according to whatever is most useful for your own growing. At this point, data sharing in the OpenAg community is mostly just hypothetical–an ambition for the future. So far, lack of compatibility isn’t a practical problem because there’s not much of anything to be compatible or incompatible with. For now, it’s kind of a blank slate where you can do what you want.
This is an excerpt of what my current logging looks like (Celsius to 0.1 degree, append records to a text file with “%%” as record separator, field separator is “|”):
And, here’s an example of one of my charts. When I make a chart, I do the Celsius to Fahrenheit conversion in my code.
In my experience, rounding to integer degrees Celsius is not good. Changes of less than one degree are important. A good way to think about it is like using significant figures for chemistry or physics calculations. If the sensor datasheet says it can give you something like 0.04 °C repeatability for a 12-bit measurement (see Si7021 datasheet), then you could make good arguments for recording measurements with either 0.1 °C or 0.01 °C precision. On the other hand, using 0.001 °C would be claiming precision you didn’t measure, and 1 °C would be throwing a lot of your measured precision away.