I don’t know where you are getting info about air pressure affecting the readings as this makes no sense to me. Analytical balances are in an enclosure to guard against wind currents, the air pressure is the same inside the enclosure as outside…maybe you are talking about air currents? As far as this being an issue with weight tolerances of 0.01 ct indoors, I haven’t seen it. I have two small pocket scales which I bought for ~$20 each and they both read to 0.01ct with an accuracy of about 0.02 ct. I weighed a parcel of 23 stones individually about two weeks ago and checked the weights last night and they were all the same down to 0.01 ct. These scales are nice as you can put them in your pocket and take them anywhere on buying and selling trips, etc.
If you are dealing in high end goods at $1000/ct, you might want an analytical balance or expensive legal-for-trade electronic, but for every day use, I can’t see it. To actually get to 0.001 ct, you have to clean every last bit of lint off the stone, make sure your balance is dusted, put the stone on the pan and add weights (which also need to be clean and handled with a tweezer) and then balance the pans with the chain or whatever fine adjustment and wait for the balance to stabilize. And the balance has to be leveled to be accurate and free of vibration…I doubt it would be at its best in my living room with the floor sitting on floor joists.
Pardon me for saying it, but I think this is impractical overkill. You can certainly get lousy pocket digitals (they start at $7), but I have two that work fine, so decent ones are out there. I set mine on a dining room table that is approximately level and it seems to work fine. Calibrate it with the calibration weight that comes with it. -royjohn