Why do my HX711 load cell readings drift significantly over time?

Question

I bought four 5 kg load cells (link) and connected them all together (all white cables, all green cables, etc.). Then I hooked them up to the HX711:

When I read the values from the Arduino using the HX711 library, I can read the numbers and I can tara the scale. Here the very simple code (tara is done in the client):

#include "HX711.h"
HX711 scale;

const int LOADCELL_DOUT_PIN = 4;
const int LOADCELL_SCK_PIN = 5;

void setup() {
  pinMode(LOADCELL_DOUT_PIN, INPUT);
  pinMode(LOADCELL_DOUT_PIN, INPUT);
  Serial.begin(9600);
  while (!Serial) {
    ;
  }
  scale.begin(LOADCELL_DOUT_PIN, LOADCELL_SCK_PIN);
}

void loop() {
  if (scale.is_ready()) {
    long reading = scale.read_average(10);
    Serial.print(": ");
    Serial.println(reading);
  } else {
    Serial.println("! not ready");
  }

  delay(300);
}

This works really well and I get realistic, precise measurements. However, the problem is, that the readings drift quite a bit over time. The readings vary 10 to 100 gram in a few seconds. Also it seems to be directional. Often it goes down, down, down, down. And only a few times up. Here a chart of the absolute values (a few minutes, drift of up to 12%):

I tried using different Arduinos (Uno & Nano). I tried to use four HX711 and yesterday I soldered all cables of the load cells together (but I am still using a bread board and jumper cables for the rest).

I really have no ideas what to try next. Please help.

Answer 1

You've apparently got some reading issues as evidenced by those spikes. Maybe give Sparkfun's library a try.

The downward trend is a bit like load cell creep, but the amount seems large (though perhaps not for a $4 load cell- 10 grams is only 0.2% of full scale) perhaps try replacing the load cells with similar valued precision resistors to determine the source of the drift.