Can I use the Acaia Lunar scale to measure 100 mg precisely? - Page 2

#11: Post by **Jeff** » April 13th, 2022, 1:40 pm

I checked again and it doesn't seem to "pick a round number" when declaring stability of the reading.

#12: Post by **jpender** » April 13th, 2022, 3:19 pm

Thanks for checking. Maybe I'll try that one. My current scale only weighs up to 20g and sometimes I want greater range. It's repeatable to ±2mg when I'm careful. How precise is your scale?

#13: Post by **Jeff** » April 13th, 2022, 4:51 pm

No clue. It weighs the two, supplied 20 g weights as the same. It has a two-point cal (20 and 40 g).

I'd guess it's good to 10 mg. I have a hard time believing any of the sub-$100 scales are any better than 0.1%, if that.

#14: Post by **jpender** » April 13th, 2022, 6:19 pm

Mine (originally $28, now $50) is definitely a lot better than 10mg. I would throw it away if that's all it could do. I tested it against an analytical balance in my wife's lab. Someone else on Amazon did the same thing. Good to 3mg conservatively and 2mg with some care.

I'm surprised that you have two (no doubt inexpensive) 20g calibration weights that are within 1mg of each other. It's actually a little suspicious, but also possibly a coincidence.

#15: Post by **boren** » April 13th, 2022, 6:32 pm

Derryisreal wrote:However, concentrates further confuse me. I am not sure what I should aim for. I am terrible at math.

Using a concentrate can't be easier. That's how I do it:

To make the RPavlis concentrate: Mix 5 gram potassium bicarbonate into a 500 ml bottle (good for making 12.5 gallons)
To make the machine-ready RPavlis water: Mix 40 ml of concentrate into a gallon (or 4 liter) of water

April 13th, 2022, 6:54 pm

Thank you, I am going to give it a try

#17: Post by **Jeff** » April 14th, 2022, 1:16 pm

20.006 and 20.014 g this morning. That seems consistent with M2 or M3 calibration weights with 8 mg and 25 mg tolerances, respectively. No easy access to an analytical balance, so I'll stick with the "10 mg" in my head. The 0.1% figure comes from reading over several load-cell spec sheets. 0.1% of 20 g would be 0.02 g.

#18: Post by **jpender** » April 14th, 2022, 1:46 pm

I have three $35 and under scales (gram, centigram, milligram) and each of them has a precision that is far better than 0.1% of FS. They are closer to 0.01%. I don't know if that's due to better hardware or oversampling or both but it's readily apparent. The least expensive of the three actually includes repeatability in the specification as ±2d. That works out to 0.004% of FS.