I've been building a little Pi network in a box for the past couple of weeks.
3x Pi 4B 4GB in a rack mountable case, I drilled and fitted standoffs and then cut holes for the ethernet ports into the front;
I put a couple of fans in the lid connected to an ESP32 which I've written some fan control code for, it'll be interfaces to the first Pi by UART.
(I know the ESP32 is overkill for this, but it's the only think I had to hand).
So, before I put it to work, I decided to do a bit of experimenting with thermals and had some interesting (to me) results. I thought I'd ask here what people think.
I have made various changes and they are indicated by the dotted vertical lines where I've annotated the graph (this is a Gafana graph, and the Pis are reporting the temperatures (amongst other things) to an Influx DB server using Telegraf.
The dotted lines correspond, from left to right with the following events:
- Start measuring, Lid on the case, fan controller at 50% speed
- Removed lid and added a single Fanshim (Pimoroni) to Pi 1 (That's the right hand Pi in the image)
- Removed Fanshim from Pi 1
- Replaced the lid (and its fans)
- Lowered fan controller speed to 20%
- Removed lid
- Added 4 piece heatsink set to each Pi
- Returned lid
- Removed lid
- Swapped memory cards in Pi 1 and Pi3(note the green and blue lines switch places)
- Physically swapped Pi1 and Pi3 locations in the case
I made a few changes to try and control the thermals a bit; I disabled the WiFi and Bluetooth using dtparams. I disabled the activity and power LEDs in the same way. I updated the VL805 firmware to the latest 1307ad and updated EEPROM to the latest as of two days ago.
At first, I believed the thermal differences were related to the mounting in the case, but physically swapping the positions of Pi 1 and Pi 3 proved that was not the case.
Second I thought perhaps Pi1 and Pi3 had different configurations on their memory cards, so swapping those proved this was not the case (the lines switch places because I swapped the cards between the physical Pis.
Finally, updating all of their EEPROMs and VL805s proved there wasn't some other software/firmware difference causing the discrepancy.
The question I posit is thusly; Are these normal, expected margins for 3 Pis of the same model with their conditions controlled as much as possible. The Pis were purchased at the same time, from the same supplier (Okdo), they have 3 identical, brand new memory cards, 3 identical brand new power cables, have been running (idle) for the same amount of time, and all commands run on each are carried out at the same time using tmux.
If it's not normal, for one of the Pis to be ~2 degrees off from the other two, what else can I investigate?