One day while I was setting up for a lab I noticed that two of my scales did not agree. What was worse, the discrepancy was significant, enough to make a system charge way off. One of the advantages of using electronic charging scales is the confidence that you have put the correct amount of refrigerant in the system. If you don’t trust your scale, you lose that advantage. I got a third scale to try and determine which was correct, but then I thought, how do I know ANY of these are right? Maybe I have two incorrect weights, not one. So I visited my friend Jimmy in the physics lab and asked for a weight. He gave me a 2 kilogram weight. I placed the 2 kilogram weight on the scale that I suspected of being off, and it weighed 2.3 kilograms. I then moved the weight to the scale that I thought was correct, and it read exactly 2 kilograms. This restored my confidence in my good scale and verified my suspicions about the bad scale. While this simple test does not replace NIST certification, it is a simple way to keep an eye on the accuracy of your digital scales and maintain your confidence in their readings. I do not believe most digital charging scales can be calibrated in the field, you have to send them into the manufacturer. But you can periodically check their accuracy. You may be wondering why I chose a 2 kilogram weight. Well, physics teachers work in SI, not inch pounds. Jimmy does not have any 2 pound weights. Nearly all science today Is done in SI units, not English units. If your school has a physics lab, you might want to pay them a visit. They have lots of cool toys. After all, air conditioning is really just applied physics.