How to store historical values (i.e. previous 10 temperature readings)

🚧 Developers Coding Questions
1

I want to add a trend indicator / value to my custom SHT30 Temperature Humidity sensor driver.

I need some guidance in how to store the old readings. Currently I'm thinking of storing 10 sets of each reading and perhaps the time of those readings.

All I can see is either:

  • Multiple States
  • Hub Variables

To my knowledge neither of the above can be any type of list or array.

Is there another way I may be missing?

Thanks
John

2

I don’t see why you can’t just keep an array of previous values in your state. Certainly attributes can be a list or array because thermostats have a list or their supported modes. But I could easily be mistaken.

Another potentially-simpler way may be to use an exponential moving average. It’s a way of approximating the average of the last N data points without actually storing all those points.

Let’s assume you want an average of the last 60 points....if “ema” is your current average value “new” is your latest reading, you’d do the following:

ema = (ema * (59/60)) + (new * (1/60))

That should show you the basics of the math, it’s probably written in code as

ema = (ema * 59 + new) / 60

You can easily adjust to keep the EMA over any number of points and you never store more than a single value. If you Google exponential moving average you’ll find more compicated formulas that do a better job or allow you to add more emphasis to more recent points and whatnot...but the gist is the same of approximating the average of the last N points.

Now if you were actually wanting to store all those points to plot a trend line or something, then everything I said is meaningless. =p

4

All "state" are stored as strings - but nothing to stop the driver from making that string "array-like" with a comma delimiter, and making that transformation when reading/writing to state.

2 Likes
5

Thanks I try this approach.

Now I have to find the string length limit for states (or perhaps hub variables).

I wonder if I could create a json like string??

6

Absolutely, could use the groovy libraries JsonSlurper and JsonOutput to facilitate this...

1 Like
7

Thank you. My vision/goal is to have something like:

  • Temperature changed +/- xx° in the last hour (or maybe 2 hours).
  • Max of the day
  • Min of the day
  • Max since reset
  • Min since reset