Update/replace channel value?

I know how to write data with MATLAB Analysis to channel, in my case I read the absolute value from my electricity counter (which updates every 1min) and calculate the consumption of the last 20min.

datetimeStop = datetime('now');
datetimeStart = datetime('now')- minutes(20);
data = thingSpeakRead(readChannelID, Fields=[1], DateRange=[datetimeStart,datetimeStop], ReadKey=readAPIKey);

analyzedData = data(length(data)) - data(1);

thingSpeakWrite(writeChannelID, analyzedData, WriteKey=writeAPIKey);

I would like to write to a second channel the average consumption for each hour of the day, e.g. 01:00, 02:00, ...

My problem: With thingSpeakWrite I add values to a channel, but if I calculate the running average for each hour at every new day - with (n*avg_old + new_value)/(n+1) - I want to update/replace the average. How is that possible?

BR

Bernd

Answer 1

[Vinod]

The data in a ThingSpeak channel is a time series. Think of a value in the field as a measurement made at a certain point in time. If I understand what you are trying to do, it is "changing history" in that timeseries by going back and updating/replacing a previous measurement.

I'd recommend a better approach is to only update the value in the channel when you have all the data for the period. So, let's say you're doing a 4 hour running average, then wait until the 4 hour window is complete to calculate and update the value in the field.

[Bernd]

Thanks Vinod for your answer, but I think I do not understand your recommended approach.

So I understand that a thingspeak channel is only ment to be a time series and does not work to hold a single value (which would be the case if I want to store a rolling average), right?

What could be a workaround? For sake of simplicity, let's assume I want to compute each day the rolling average for the daily consumption (I know this use cases does not make sense, but let's keep it simple) like this:

Day 1 consumption: 10 --> Avg = 10

D2 consumption: 20 --> Avg* = 15

D3 consumption: 3 --> Avg* = 11

....

I know how to read at the end of a day the consumption of the last 24h hours, but where could I "store" the newly calculated average, because I am not interested in a series plot but in the single average consumption value?

BR

Bernd

[Vinod]

Thanks for clarifying that Bernd. I think I understand your use case a bit better now.

Every ThingSpeak channel has a metadata field. You can use the REST API to update the metadata of the ThingSpeak channel on a rolling basis. See here for more info:

https://www.mathworks.com/help/thingspeak/writesettings.html