Slippery slope. More information than anyone probably wants or cares about. I like weighing and can't fathom doing anything other by weight. Spirits by volume? You are wasting your time and are highly inaccurate.
The scale probably doesn't need to be NTEP, but it should be. Non-NTEP scales generally can't be calibrated, and the TTB wants your measuring equipment calibrated. Given this is used for tax determination, it could be arguable that this is a value exchange and NTEP should apply. Dunbar probably has a good handle on this.
NTEP scales are typically higher quality than non-NTEP scales. It doesn't mean a non-NTEP scale isn't good, it can be better than an NTEP scale, but generally, NTEP is there for a reason. Generally you don't make a junk NTEP scale, but lots of people make junk non-NTEP scales.
Non-NTEP scales are typically sold based on readability - the display accuracy, the number of digits on the scale display. However, you need to realize that showing more numbers on the display doesn't mean the scale is accurate to the digit of the display. This is a massive misconception. Just because the display shows it, don't mean it's so.
You could make a 1000 pound scale with a display that reads 999.99 - but it doesn't mean that the scale is accurate to 0.01 pounds. In fact, you have no idea at all if the scale is accurate to that level, because there are no rules to mandate that it is. The numbers after the decimal point could be complete nonsense. You think it's highly accurate because it shows more numbers, but that ain't the case.
That's where NTEP comes in. Among other things, NTEP defines the number of "DIVISIONS" that the scale is capable of accurately resolving. Legal for Trade means that the the display accuracy is equal to the accuracy that is defined by the division in one of these classes. NTEP also means that the scale is independently verified to read accurately across a range of voltages, temperatures, and other operating conditions.
NTEP CLASS I - 100,000 Divisions and UP (Precision Laboratory Use)
NTEP CLASS II - 10,000 to 100,000 Divisions (Lab Use, Precious Metals, etc)
NTEP CLASS III - 1,000 to 10,000 Divisions (Commercial legal for trade)
Accuracy/Readability = Maximum weight / Divisions
So, you can have an NTEP Class 3 scale, 1,000 pounds, with 1,000 divisions. The display should read 0000 (1000/1000 = 1). Nothing after the decimal point. You would assume it is accurate to the pound only.
You can also have an NTEP Class 3 scale, 1,000 pounds, with 10,000 divisions. The display should read 0000.0, and the scale will increment in .1 pound steps. 0000.1, 0000.2, 0000.3. You would assume that it is accurate to a tenth of a pound.
So what's the difference? The 10,000 division NTEP scale is going to be more expensive than the 1,000 division NTEP scale. What makes scales more expensive than others? Not the total weight capacity, no no no. It's the divisions. The more divisions a scale can accurately measure, the more complex the circuity, the higher tech the load cells, the tighter the manufacturing tolerances, the more substantial the frame needs to be, and the more expensive the scale.
That all said, the scale used for a specific operation needs to be suitable for that operation. Lets say you are proofing 50 pounds of 120proof spirit to 80 proof for bottling, that's going to be 28.154 pounds of water for a total final blend volume of 78.154 pounds.
If you have a 5000 pound NTEP pallet scale with a 1 pound accuracy, your display weight of 78 pounds is everything from 77.5 pounds to 78.4 pounds. So you add water until your display reads 78 pounds.
In proof terms, it means you are anywhere from 79.7 proof to 80.4 proof, you'll have no idea unless you gauge again.
If you read 80.4 - you'll need to slowly keep adding water and gauging, over and over, in little steps. A waste of time. If you read 79.7 proof. Sorry to hear it, hope you have more spirit on hand to raise the proof, which you'll need to do slowly, re-mixing and gauging every time.
Now, if you had a 150 pound scale with an accuracy of 0.05lb (NTEP Class III - 3000 Divisions, actually LESS ACCURATE THAN THE 5000lb Scale). You would add water to 78.15 pounds.
If proof terms, you are going to be better than 79.95 to 80.05. Do you gauge again? Of course you do. But you'll be dead on, no fiddling around with trying to add an unmeasurable amount of water or spirit (proofing by trial and error).
I just hope someone bothers to get this far and at least got some bit of useless trivia knowledge out of this.
That said, EVERYTHING BY WEIGHT, NO OTHER WEIGH ... err WAY.