A short discussion of weighing precision versus accuracy
There is often confusion between accuracy and precision,
especially when someone asks "how accurate is your scale?"
OHAUS provides specifications that will help you establish the
precision of your scales, so that you know how much a scale can
read and how finely that reading will be. While precision
tells you how finely the scale will read, accuracy is the
combination of the abilities of the scale, how the scale is set-up,
how it is used, and how well it is maintained.
If two identical scales are set-up in different locations, like in
a climate-controlled lab and on the shop floor next to a mixer and
an open door, the precision of the scales are the same but the
accuracy will be different. If the scale in the lab is set on
a stable surface, away from drafts and vibration, calibrated and
used by a trained person, then the accuracy of the weighments will
be higher than the shop-floor scale, which might have not been
calibrated and where the weight readings are taking before the item
is stable. If the scales used have the right features and
care is taken, even the scale on the shop floor can be as accurate
as the scale in the lab.
When setting-up a scale, take the following factors into
consideration:
- Human Error - Was the scale set-up right? Is it used
correctly? Are you maintaining it properly?
- Temperature - Is the scale at the same temperature as the
environment it is in? Are there wide changes in the ambient
temperature? Are the items being weighed radically hotter or
colder than the scale?
- Air Currents or Pressure - Are there large drafts or air
conditioning vents affecting the scale location? Are there
rapid changes in air pressure due to equipment like air
compressors?
- Calibration changes - Was the scale calibrated after you bought
it? Has the calibration been changed, and is the scale
periodically checked for proper calibration?
- Vibration
- Electromagnetic Interference (from motors, pumps, etc.)
- Harsh Conditions - Water, ice, cleaning chemicals
- Legal For Trade - Does your application require
approved/certified scales?