Re: [EE] Measuring increasing mass over a 1/2 day period

Many people confuse readability with accuracy.

A 20,000 gram digital scale may give a readability to 1 gram but how 
accurate is that reading?

Load cells that are used to weight have an accuracy of a percentage of 
the capacity of the load cell.

A precision load cell may have an accuracy of +/- 0.25% of the full 
scale value.

Most of the inaccuracy comes from the analog output of the load cell, 
not from the digital circuits.

The output of a 20,000 gram load cell may have a window of uncertainty 
of +/- 50 grams (0.25% of 20,000) due to several factors:

     linearity: the output of the load cell may be greater or less than a 
straight line between min and max.

     Self-heating: A load cell uses the voltage drop across resistors to 
measure the load.  The resistance changes with the resistors heating up 
as time passes.

     Repeatably: The load cell output may not give the exact same result 
with multiple readings.

     Hysteresis: The output of the load cell may not follow the same 
curve when the        weight is decreasing as when the weight is 
increasing.

     Return to zero: The output may not return to exactly zero when the 
weight is removed.

     Long term shift: The output curve may shift as time passes.

When choosing a balance or load cell, one should pick one with a maximum 
reading slightly higher that the anticipated load for the smallest 
window of uncertainty.

Joel T. Curneal
[email protected]


On 2026-03-12 15:18, Chuck Olson wrote:
> James -
> 
> Thanks, I suspected that a re-zeroing might be needed over time.
> 
> I will make up a pedestal for the still which will allow the extra
> height needed to slip my kitchen scale under the water container.
> I'll wait until last last few minutes before doing this - hopefully I
> can try it tomorrow.
> 
> Chuck
> 
> On Thu, Mar 12, 2026 at 3:02 PM James Cameron <[email protected]> wrote:
>> 
>> In my experience with scales, there's a good chance a zeroing will be
>> needed over half a day, depending on how close to the mass you want
>> the reading to be.
>> 
>> There may be other methods other than force of gravity arising from
>> mass pushing on a load cell.
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