Recently, I was informed of a situation that involved a digital infant scale and a “test weight” also known as a 5 pound consumer barbell weight. Apparently, the barbell weight was used to check the digital scale and it was thought that the scale had not been calibrated correctly due to a scale readout of 5.2 lbs.
Actually, consumer barbell weights like those used in the above scenario are manufactured to tolerances between +/-1% and +/-7%, a significant range of tolerance as compared to NIST certified test weights. Using a consumer barbell weight with a wider manufacturing accepted tolerance than the scale may result in a scale display reading that seems inaccurate. A +/-5% tolerance on a 5 pound consumer barbell weight means the weight can weigh from 4.75 lb to 5.25 lb and still be sold as a 5 pound weight. Compare that to a 5 pound NIST Class F weight which is manufactured and adjusted to within +/-230 mg tolerance. In the example above, the user contesting the calibration accuracy of the digital scale checked their consumer barbell weight again, this time on a certified Legal for Trade scale and found that it weighed 5.2 pounds, meaning the digital scale was correct.
We recommend using NIST certified Class F Sealed manufactured test weights in most applications to check or calibrate medical scales and industrial scales.