What is an acceptable amount of drift for unloaded scale

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wwfoste
What is an acceptable amount of drift for unloaded scale

I’m working on getting my hive scale back up and running. I'm updating the software for my scale to the new release and recalibrating it. What would an acceptable amount of drift for an unloaded scale be considered?

It started out at -0.02 lbs but has drifted up to -0.13 lbs over the course of 3 hours. It’s currently unloaded and just sitting in my house. What range would you consider acceptable for it to bounce around in before I call it good enough? What would be the cause of that, and is there a way to fix it?

Paul
Paul's picture
What is an acceptable amount of drift for unloaded scale

When a scale is first turned on, I often see about 0.2 lbs drift in the first 2 to 3 hours. This is true also with the Adam Equipment scale we started with. I am not an expert but I think it is from the heating of the strain gauge when it is excited. Even though that is only for a few milliseconds every 5 minutes in our case, that is enough to cause a thermal drift.

It could also be the electronics (voltage reference in the ADC) moving around.

For the worst case (if you believe the spec sheet), see this link for a quick (and possibly flawed) theoretical thermal analysis based on the spec sheets for the load cells:
http://hivetool.org/w/index.php?title=Interface_the_HX711_to_Pi#Temperat...

You can also check the long term drift, creep, and return to zero. Load it with something heavy that is not affected by humidity. Leave it for 24 to 48 hours and look at the drift. If held at a constant temperature, the change would be creep. Then unload it and see how close to the original setting (return to zero) it is.

The most important is the temperature drift after it has warmed up. You can measure the thermal drift by leaving it outside with a constant weight on it. I like to see less than +/-0.01 lbs over a normal daily temperature swing.

wwfoste
Scale drifting up

My scale has constantly drifted lower as the day has gone on. It's now around -0.30 after starting at -0.02 lbs. Since it's just indoors, I'm assuming it's something with my calibration? Would recalibrating it help fix this or is this indicative of a different issue? When calibrating and using the hx711, the numbers i get back constantly move around. I usually just wait until I get several numbers close together back and go with that. Is that what you normally see too?

wwfoste
Scale still drifting

It's just doing a steady move. Every 5 measurements or so, it moves another hundredth. It's made it's way to -0.87 lbs now. I assume something is wrong with my slope. Thoughts?

wwfoste
Fixed - loose connection

Since the scale continued it's downward trend, I thought about trying to troubleshoot it. I disconnected one of the load cells to see what would happen. Since I hadn't recalibrated, it started being erratic, but when I reconnected it, it seems to have fixed the issue. It's held steady for over an hour, and mostly steady for 3. So it's seems to have just been a loose connection.

wwfoste
Still getting a good amount of drift and movement in scale

I'm still getting a good amount of drift and movement in the unloaded scale. It continues to rise in weight slowly and is more erratic in its readings. Would poor electrical connections specifically for the RJ14 connectors be a cause of this? If so, what would the fix be? Using conducting gel or just rework the connections?

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