Project355
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This is going to be a mult-part thread. Hope you find Part 1 an enjoyable read.
Part 1: The Basics
After going thru quite a few beam scales, and finding the strengths and weaknesses in each, I thought I'd share a bit, and also suggest the correct way of absolutely calibrating the scale. I'm putting all this down because there's a lot of misinformation on the web. I went looking for answers, and had to go a bit deeper into things than average Joe Reloader would probably want to go before throwing his hands up and calling it quits. So here goes!
First thing to realize, is the accuracy of a modern beam scale is its beam. The beam has machined, or punched stops for the various poises. The rest of the scale is just the icing on the cake, and while they don't contribute to the accuracy, they are important for repeatability and ease of use.
Lets start with the concepts.
Consider a beam scale with no stops on its beam. Just a weight. You've got the item you want to weigh on one side of a pivot, and a counterweight on the other. If the beam were equal length on both sides of the pivot, and level, you'd need a counter weight or poise, that was the same weight as what you wanted to weigh in order to keep the beam level. That really is the concept behind the apothecary scale, or perhaps better known to all as the “scale of blind justice”. Hows it work? You want a pound of beans. You put a known pound weight on one side, the scale tips. You pour the beans on the other side until the scale is level again. One pound o' beans comin' right up!
What if the counterweight were able slide along the beam? At that point, you could use a beam that was longer on one side than the other, but you'd need a fixed “balancing weight” on the short side, to keep the beam level when the main sliding counterweight was at some position close to the pivot. Simple math would illustrate how by moving the sliding counterweight along the beam, a certain amount of leverage could be utilized to exactly level the beam when the item to be weighed was placed on the short side of the beam. If one could very accurately measure how far the sliding counterweight had moved, and could account for the actual weight of the counterweight you could determine the weight of the item you wanted to weigh. Back to the beans, you take an unknown scoop of beans, put it on the short side of the beam, then slide the counterweight until things are level again. Measuring from the pivot to the counterweight, you could figure out that you've got 1-1/4 pounds of beans (more or less). Very hard to do accurately, and rather than math, it would probably be on a chart that was figured out before hand. That's complex, so it was made simpler.
Now imagine the modern beam scale. It has fixed stops for the counterweight, which I'll call a poise from now on. By using fixed stops, that have been accurately cut into the beam, the distance is all figured out. The scales designer has calculated the weight needed for the poise, and markings have been made along the beam to let you know at which stop the poise is located. That is the basis for the modern scale.
The problem with what has been described so far, is that however finely the stops on the main beam are cut, they're not fine enough for the precise measurement of very small weights, or small fractions of larger weights. What to do....
There are a few ways to tackle the need for higher precision, but what works best is to make the “balance weight” on the short side of the scale also variable with its own set of markings. The scale designer could have made a second portion for fine tuning on the longer side of the beam, but that would add additional length and also need a greater fixed balance weight. In practical terms, making the balance weight variable is the most efficient means of accomplishing fine tuning. In terms of actual practice, the balance weight isn't completely variable, only a portion of it is. That is to say, the pan, any weight in the pan, the pan's support are all part of the balance weight, but only one (or sometimes two) very light poise comprise the adjustable part of that weight.
As a reference to when I talk about calibration, notice that the large main poise is at “zero” closest to the pivot, and the fine tune poises are at “zero” farthest in their movement from the pivot. This gets a little weird to follow but, if both the main and balance weights were at zero closest to the pivot, the scale would still work if it were level. However, you'd have to “subtract” the markings on the fine tune scales from that of the main scale. That would be confusing, so... its set up to make it all addition, and the fine tune poises are simply added to the weight already shown on the main poise.
Whew. That's a lot to handle for the first installment of this multi part essay.
More to come.....
(There will be more on pivots, pointers, frames, beam construction, maintenance and calibration very soon)
Part 1: The Basics
After going thru quite a few beam scales, and finding the strengths and weaknesses in each, I thought I'd share a bit, and also suggest the correct way of absolutely calibrating the scale. I'm putting all this down because there's a lot of misinformation on the web. I went looking for answers, and had to go a bit deeper into things than average Joe Reloader would probably want to go before throwing his hands up and calling it quits. So here goes!
First thing to realize, is the accuracy of a modern beam scale is its beam. The beam has machined, or punched stops for the various poises. The rest of the scale is just the icing on the cake, and while they don't contribute to the accuracy, they are important for repeatability and ease of use.
Lets start with the concepts.
Consider a beam scale with no stops on its beam. Just a weight. You've got the item you want to weigh on one side of a pivot, and a counterweight on the other. If the beam were equal length on both sides of the pivot, and level, you'd need a counter weight or poise, that was the same weight as what you wanted to weigh in order to keep the beam level. That really is the concept behind the apothecary scale, or perhaps better known to all as the “scale of blind justice”. Hows it work? You want a pound of beans. You put a known pound weight on one side, the scale tips. You pour the beans on the other side until the scale is level again. One pound o' beans comin' right up!
What if the counterweight were able slide along the beam? At that point, you could use a beam that was longer on one side than the other, but you'd need a fixed “balancing weight” on the short side, to keep the beam level when the main sliding counterweight was at some position close to the pivot. Simple math would illustrate how by moving the sliding counterweight along the beam, a certain amount of leverage could be utilized to exactly level the beam when the item to be weighed was placed on the short side of the beam. If one could very accurately measure how far the sliding counterweight had moved, and could account for the actual weight of the counterweight you could determine the weight of the item you wanted to weigh. Back to the beans, you take an unknown scoop of beans, put it on the short side of the beam, then slide the counterweight until things are level again. Measuring from the pivot to the counterweight, you could figure out that you've got 1-1/4 pounds of beans (more or less). Very hard to do accurately, and rather than math, it would probably be on a chart that was figured out before hand. That's complex, so it was made simpler.
Now imagine the modern beam scale. It has fixed stops for the counterweight, which I'll call a poise from now on. By using fixed stops, that have been accurately cut into the beam, the distance is all figured out. The scales designer has calculated the weight needed for the poise, and markings have been made along the beam to let you know at which stop the poise is located. That is the basis for the modern scale.
The problem with what has been described so far, is that however finely the stops on the main beam are cut, they're not fine enough for the precise measurement of very small weights, or small fractions of larger weights. What to do....
There are a few ways to tackle the need for higher precision, but what works best is to make the “balance weight” on the short side of the scale also variable with its own set of markings. The scale designer could have made a second portion for fine tuning on the longer side of the beam, but that would add additional length and also need a greater fixed balance weight. In practical terms, making the balance weight variable is the most efficient means of accomplishing fine tuning. In terms of actual practice, the balance weight isn't completely variable, only a portion of it is. That is to say, the pan, any weight in the pan, the pan's support are all part of the balance weight, but only one (or sometimes two) very light poise comprise the adjustable part of that weight.
As a reference to when I talk about calibration, notice that the large main poise is at “zero” closest to the pivot, and the fine tune poises are at “zero” farthest in their movement from the pivot. This gets a little weird to follow but, if both the main and balance weights were at zero closest to the pivot, the scale would still work if it were level. However, you'd have to “subtract” the markings on the fine tune scales from that of the main scale. That would be confusing, so... its set up to make it all addition, and the fine tune poises are simply added to the weight already shown on the main poise.
Whew. That's a lot to handle for the first installment of this multi part essay.
More to come.....
(There will be more on pivots, pointers, frames, beam construction, maintenance and calibration very soon)