Null values in graph data

Situation is this: a new emonTx4 is monitoring the phase currents of a three phase supply. After some initial difficulties with getting the emonTx4 to connect to an emonBase, monitoring appears to be working just fine. I used a USB connection rather than radio.

The power channels are all configured into feeds with a 10s update time.

Here is a typical one hour plot. The data “disappears”, that is, presented as null values across all the channels - for five minutes or so at a time. All channels “stop” at the same time. The csv presentation is as you’d expect, “null” for each of the four values.

emonhub.log is not indicating any form of error. Watching the data on the feed display suggests that logging is OK. The feeds are updated within 10s.

I believe the the data is received and recorded, but there appears to be an issue in how it’s accessed. What is the best way to approach the investigation?

This looks very much like the standard problem: What is the reporting interval of your emonTx, and what is the Feed Interval in emonCMS? You need to look at the source code of the sketch for the first, hover your mouse over the name of the Feed on the Feeds page for the second.

The way I suggest people visualise how the Feed works, is as boxes moving along a conveyor belt. The input process drops a number onto the belt whenever a data item comes in. Meanwhile, the Feed puts boxes on the belt at predetermined intervals (Fixed Interval Timeseries). If no data comes in, or it comes in slowly, you have empty boxes (NULLs). If the data comes in too fast, you can see only the last one on top in the box after it’s moved on down the conveyor belt.

So the idea is to have the data arriving at a slightly faster rate than the boxes. If by accident you’ve set up the feeds to 5 s rather than 10 s (and the data should be coming every 9.6 s or so) then you will get NULL values. The only cure is to delete the Feed and start again - you can’t change it once the Feed has been created.