More testing to be done.

• characterize a single point load cell
• characterize a 5 kg kitchen scale with those four strain gauges on its feet
• acquire correlated measurements with the precision multimeter and HX711 via SPI
• characterize drift over temperature (fridge vs. room temperature)

load cycles: 24h loaded to 100% nominal load, then 24h recovery

temperature cycles: RT (as part of the 24h no-load phase), fridge, RT (~12h each)

seems like now is the time to hook up SPI.

No precision voltage sources will be used this time because HX711 supplies the reference voltage and the measurement scaled to AVDD.

ps. single point cell - temperature drift

The single point cell sense voltage drift looks pretty much the same as that of the 5 kg scale elements one.

Once calibrated for excitation voltage drift however...
Let's try that again for the 5 kg scale.
I guess we'll have to have a look at the compensated scale01 sense voltage in the 68h run next...

pps... some time later....

Let's look at the data again, but this time both dividing the sense voltage by the excitation voltage and then normalizing it to the starting value to display drift.
With twice the acquisition time the single point load cell is still just casually walking away from us while the 5 kg kitchen scale is somewhat inspired by daily cycles. I'm not sure whether to call it a temperature effect yet.

Let's also revisit the prior long dataset with just the 5 kg scale:
With the kitchen scale having an HX711 board glued onto it vs. the single point scale setup more or less floating in the air, some differences in behaviour are to be expected.
Heh Sensepeek, bet you didn't see that coming.
Just doing a very rough calibration of the mains voltage level it becomes evident that it's not exactly stable even over the course of half an hour:
I expect some more drastic day-night variations. What's with the mains voltage? Well.. there's a transformer in the multimeter on which the scale rests, varying in heating power ... I just had to include this data channel. Next I'm pretty sure I'll also have to hook up a hot wire anemometer... driven by an HX711 board... deeper, ever deeper down the rabbit hole :D

I'm beginning to feel what it must have been like to set up the pitch drop experiment.


ppps. waking up at night


it appears I mistakenly closed the console window running the logging application when going to bed. However I woke up around 330h and found, as one does, the measurement to be terminated. A restart was in order - but I forgot about the scale switching off again. Hello, 25-30 µV offset. One might as well just discard the initial chunk of data.

Fuzzy trends:
ignoring the wrong starting time for the normalized plot we're not at a point where the far-fetched mains-induced temperature variations are in the graph as well.. and they tell us... nothing? Mains voltage variation induced temperature changes are still minuscule deviations in the mW range.
Maybe I'll just.. hold on  a a sec...
there, I just removed the kitchen scale (01) from the top of the multimeter around 1534442178.2930505. The single point cell can stay there because it's mostly thermally decoupled from the instrument and in convective exchange with the room air.
The scale is again back to sleep to keep the measurements compatible with prior ones.

This will have to end, one way or another ;-)