Yeah. I think I've mentioned this before.
For a while i thought that it was just a mind trick that when lifting 100kgs, 4x 20kg plates felt heavier than 2x 20kgs, 2x 15kgs & 2x 5kgs plates.
Today, i decided to act upon this oddity, weighed my plates with good scales, and found to my horror that 100kg using 4x of my iron 20kg plates actually equals 104.2kgs!
Of my 4x 20kg plates, two weigh 21.3kgs and two weigh 20.9 kgs;
Of my 2x 15kg plates, one is exactly 15kg, but the other is 14.5 kgs (which means that from now of i have to put a 0.5kg micro plate on one side when i use those plates!)
Quite outrageous discrepancies. However, the positive is that last year when i was trying to attain a 120kg bench press, but 'failed' at 117.5kg, i actually lifted 121.7kgs - mission accomplished!
If you bought cheap iron plates, weigh them. You might be surprised.
Yeah. I think I've mentioned this before.
Well the good news is you can fix the 20kg plates. The 15 that is low may be harder. If your any good at metal work you could add screws or if needed screw some metal to the plate. It won't be balanced anymore but then are your plates balanced anyway? I hope you are not as anal as I am or you'll be out in the shop trying to measure balance on your plates, hahaha.
I used a paint marker to mark the actual weight. Over time it really bothered my OCD so I wound up selling those weights to a friend and getting a calibrated set that is accurate to +/-10 grams. Of course now my OCD makes me wonder how altitude and latitude factors in...
But can you trust your scale? How is it calibrated?
And how do you know the people that calibrated did it correctly? Metrology is a scary subject for the anal retentive.