Load Cell Drift in Scales

What is the precision and capacity of the scale?

Given that the test weights are slightly buoyant in air, would a change in air pressure since February, and hence the density of the air, and hence the buoyancy of the test weights in air, change the scale reading by this amount perhaps?

Not quite:

I make it about 0.0003 g for a change of 20 millibars (100 gram weight).

100g capacity, 0.0001 resolution, 0.0006g tolerance.

As long as the scale is enclosed, (and I am assuming that it is), it is not normal for it to be that far off.

How old is the scale?

Hi,

if you are sure about the temperature to be ok and the voltage too, then your scale very likely was overloaded or shocked.

Mechanical overload will cause permanent strain in the flexures that suspend the weight.

This permanent strain is very temperature dependent (in principle near -0.03%/degreeC) but in reality amplified by the principles of the flexures.

You can check this if you make temperature tests with a. no load, b. full load in the center and c. full load in one of the 4 corners. So with these 6 tests in a full temperature cycle you will get either all data within specs. If so your mechanical system of the scale was not overloaded. If not then very likely you had an overload.

Maybe you can relief the stress-strain damage a bit by storing the scale at the highest allowed temperature for some weeks.

Another possibility is that your scale was overloaded at transport. Anti-shock packaging is not at all simple.

If the temp-tests fail then you need a new scale.

Have success

RHABE

Thanks.

More facts: 1. It is three years old. 2.The first and second calibration was good. 3.It was calibrated with the shield. 4. It failed at 70,80 90, 100g.The tech did not pay close attention to the temp, but I monitored the temp and it was 69F/ 36 RH. The report has a check mark next to " ACCEPTIBLE".

This scale had not been used for a year, but could have been overloaded one of the last times it was used, but I cannot verify this.

thanks again

Retry with fresh batteries and known temperature.

After a year out of service I would suspect the power supply and temperature.

(For the good old days of sensitive balance beam scales! 35 years ago we had some marvelous ones at university all enclosed in a glass and wood box so air drafts didn't tip the balance! The scale was used in drying miniscule chemical samples to constant weight.)

Thanks,

The calibration was done in a enclosed room with the shield installed, and an AC adaptor.