Troubleshooting: Scale

Thermal drift or thermal instability is when the weight changes with temperature.

Symptoms

Weight readings may swing widely with temperature changes:

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  Weight swings with temperature.

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  Weight swings with temperature.

Weight readings are noisy or erratic. There may be positive or negative spikes during the hottest part of the day:

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  Noise on weight.

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  Weight spikes with temperature.

Inspect the load cell wiring and HX711 ADC connections.

1. Inspect RJ plugs on load cells. All four pins should be the same level
2. Inspect load cell cables for rodent damage.
3. Inspect RJ Jacks on hive interface board for bent contacts.
4. Replace RJ plugs on load cells with screw terminal connections.
5. Check scale feet for rocking.
6. Test system using the Test Plug described below.
7. Inspect solder connections on the Hive Interface Board and HX711 board.

Inspect RJ plugs on load cells

Unplug the load cells from the Hive Interface Board. Carefully inspect the RJ-11 plugs. All four pins should be the same level. If one or more pins are higher than the rest, it was not crimped correctly. If it was not crimped correctly, be sure to check for bent pins in the RJ11 jack on the Hive Interface Board (see next step below).

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  Pin on the left is higher than other 3.

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  Same connector, different view. Pin in the back is too high.

Inspect RJ Jacks on hive interface board for bent contacts.

Look inside each RJ jack on the Hive Interface Board. Make sure none of the pins are bent. A bent pin can be carefully pulled back into place with very thin needle nose pliers.

Inspect load cell cables for rodent damage

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  Rodent damage to unprotected load cell wires.

Replace RJ plugs on load cells with screw terminal connections.

The round black cable on the load cell was not intended to be used with RJ crimp connectors. They almost work but if the wires get crossed under the strain relief, the pressure may cause the wires to short or break. This problem may take weeks or months to show up.

Use screw terminal to RJ11 adapter

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  These are $2 each on EBay but you must buy 10 (free shipping to the US).

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  These are $2.24 each on EBay.

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  From Mouser

I have not tried the Keystone Jack RJ connectors with punch down termination. They are less expensive than the screw terminal adapters and are more easily available.

Solder a RJ11 jack on the load cell wires

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  https://www.sparkfun.com/products/14021

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  https://www.sparkfun.com/products/132

Test system using the Test Plug described below.

The Test Plug shorts the two inputs on the HX711 and biases them to the middle of their operating range. This will test everything up to and including the RJ jack on the hive interface board.

1. Unplug all the load cells from the hive interface board.
2. Plug in the Test Plug into any of the four load cell jacks.
3. Let it record data for at least 24 hours.

Depending on how much the temperature of the HX711 changes, you should see weight changes of +/- .01 kg (+/- .03 lbs). If the load cells were ideal (no temperature dependence and no noise) this is the best the present system can do.

Test Plug Schematic

Test Plug pictures

If the weight changes much more, there is a problem with the HX711 or Hive Interface Board. If this tests good, the problem is with the RJ-11 plugs, the load cells or the mounting.

Jumper Cables

Make sure the telephone cables are wired correctly.