Uncertainty is more than just the repeatability of the balance it is a summation of all things which contribute to the error in the measurement process. Uncertainty is the root sum square of balance deviation, repeatability on the check standard, variability in the density of the standard, variability in the density of the weight under test, variability of the temperature, variability of the barometric pressure, variability of the humidity, operator skill and the uncertainty of the standard used multiplied by two.