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Old March 25, 2020, 12:10 PM   #1
Metal god
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What does it mean +/- .1 grain ?

I know that sounds like a dumb question but …

I had always thought that +/- .1gr meant the scale arm may read a tad under or over but it's still giving the correct weight . Meaning If you zeroed the scale at 5.0 grains and your charge weighs 4.9gr it could still be 5.0gr or 4.9gr but its not likely 4.8gr . Thinking it is the scale arm that is off rather then the weight it self .

After reading another thread I now think it means no matter what your scale is saying the charge is . It could be +/- .1gr from "that" reading ? So a 5gr zero and the charge reads 4.9gr . The actual charge could be 4.8 , 4.9 or 5.0gr ?

That's a big deal , especially for those weighing there charges in .3gr increments or less in rifle loads and almost seems impossible to get accurate weights when working up pistol loads when there is less the a grain of charge weight between min & max ?? I mean there are some pistol loads that are only .5gr between min & max . If your scale could be off that much why bother working up a load between the .5gr difference ? Just pick the middle charge and go shoot ???

Quote:
Originally Posted by Unclenick
Never fail to fire the minimum load or a load 15% below maximum, whichever is higher, as your first test round. I've run into starting loads that were already too warm for the particular gun I was trying them in.
Did that firearm show similar signs on other load work ups or was this the first time loading for that firearm . I ask because if you firearm has not shown a tendency to produce high pressure in multiple other loads . Can one infer it's not likely going to in the next combo you try assuming you are with in the established min & max charges ??

Not sure it changes much for me but is interesting when I think I likely have been wrong this whole time when it comes to how I understood the +/- .1gr variance .
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Last edited by Metal god; March 25, 2020 at 12:24 PM.
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Old March 25, 2020, 01:09 PM   #2
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I don't think you are wrong, but I think what the variance means is a function of the context.

Measuring devices are never perfect. What they report is subject to variations, and as the price of the equipment increases, hopefully the tolerances get tighter. For a scale (whether mechanical or digital), a tolerance of +/- 0.1 grain means just that -- whatever the scale reads, the actual weight can be anywhere from 0.1 under the indicated weight to 0.1 over the indicated weight.

Quote:
I had always thought that +/- .1gr meant the scale arm may read a tad under or over but it's still giving the correct weight . Meaning If you zeroed the scale at 5.0 grains and your charge weighs 4.9gr it could still be 5.0gr or 4.9gr but its not likely 4.8gr . Thinking it is the scale arm that is off rather then the weight it self .
I don't even understand what this says. If you are using a beam balance scale "set" (I decline to call it "zeroed" since 5.0 is not zero) to 5.0 grains, and that scale has a mechanical precision (tolerance) of +/- 0.1 grain, than anything you put in the pan that shows the arm at the index mark could be 4.9, 5.0, or 5.1 -- or any smaller increment between 4.9 and 5.1. It won't be 4.8, or 4.85, because that's outside the scale's degree of precision (tolerance).

But the variance is inherent to that machine. If that machine is off by -0.05 grains, then it's probably off by -0.05 grains for every load you weigh. It won't be off by -0.05 for one load and +0.07 grains for the next.

But you may have a {Brand} beam scale that's off by -.05 grains (which by definition is not defective since it's within the manufacturer's published tolerances), and your shooting buddy across town may have the same make and model of scale and his is consistently off by +0.1. His is also not defective, since it is also within the manufacturer's tolerances.

If you need better accuracy than this, that's where you need to get a couple of check weights and calibrate your particular scale so that it reports the weight of checkweights exactly.
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Old March 25, 2020, 01:21 PM   #3
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MG: in your case the charge could be 4.8-5.0 grains. At least that is what the +/- 0.1 grain means to me.
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Old March 25, 2020, 01:31 PM   #4
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That's a really good question. To really understand it, you might want to study up on the topic of statistics, especially the idea of Standard Deviation and Normal Distribution Curves.

looking over
https://www.nist.gov/system/files/do...H44-03-all.pdf

Especially section T gives some help and also leaves me wondering about some things so I will make some assumptions too.

My little Lyman beam powder scale measures up to 510 grains.

+/- .1 grain means if I ...
measure 500 grains, my result must be between 499.9 and 500.1 grains
measure 0.1 grain my result must be between 0 and 0.2 grains.

Think about that... in the first case it has to measure within .02 percent and in the second case within 50%.

So the scale must be ACCURATE- it must read close to the actual value.
It must also be PRECISE- it must read close to the same value every time.

In shooting: accurate guns hit the bullseye, precise guns have a tight group.

The scale should do that in all reasonable temperatures and humidities and air pressures, and every time you measure you should get the same result.

For working up a load, it seems to me that a precise scale that is inaccurate by .25 grain or so would be fine, because every time I measured a load I am working up (working UP.. from less powder!), I will get the same load as last week or last year, providing I use the same scale.

What you point out is true.. it can be a worry... as I hand load .380acp and typical loads are measured in micro mouse farts.

The other thing to consider is that some people measure powder by volume and for many applications, that level on inaccuracy and imprecision is completely serviceable.

Long story short- if you put a 3 grain test weight on your scale you should get between 2.9 and 3.1 grains with NO measurement outside that range.

If you measured 10 time you could get:
2.9, 2.9, 2.9, 2.9, 2.9, 2.9, ... and that would be really good Precision.

you could get:
2.9, 3.1, 3.1, 3.0, 2.9, 3.0, 2.9... and that would pass the specs, but not as precise

A new penny should weigh 38.5809 grains... well, maybe not that exactly, depending on wear.. but see if you get about 38.6 grains. (some sort of accuracy test)

Now take something light.. maybe a primer or a paper clip.. and weigh it. Don't touch it (finger oil) or get it dirty...Write the answer down.. weigh it 10 times and record the answer. Is it always the same weight? Weigh your penny again. Is it the same weight? Convince yourself of your Precision.
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Old March 25, 2020, 01:32 PM   #5
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Beam scale indices will point to somewhere about on the zero point for a given weight measured depending upon the friction between the pivot's knife edge and the V block bottom it rests in. Some wear on that knife edge will cause a spread in indicated weight. The +/- (more/less) number indicates the weight tolerance above and below actual weight being measured. It often increases over time due to wear.

If a +/- .1 grain spec is given, a 50 grain weight will indicate 49.9 to 50.1 grains weight. Something 500 grains will indicate 499.90 to 500.10 grains.

Poor quality beam scales can show a greater spread with heaviest weights because of the greater friction between knife edge and V blocks. That edge can sometimes be sharpened to reduce the spread weighing something several times.

After zeroing a scale with a +/- .1gr tolerance and the charge reads 4.9 gr., the actual charge could weigh between 4.8 and 5.0 gr.

Last edited by Bart B.; March 25, 2020 at 02:43 PM.
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Old March 25, 2020, 02:16 PM   #6
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I use the term zeroed at 5gr when using a beam scale because I use check weights so as much as can be the case , the scale was zeroed or checked to be accurate at 5gr or what ever the intended weight will be . Digital scales can absolutely be zeroed at a specific weight . I do it all the time when weighing someing that needs to be put in a basket or the like . I zero the scale with the basket on it so when the basket is off the scale it shows a zero of - what ever the weight of the basket .

Should i expect any difference in accuracy in the scale if I’m trying to weigh 25gr and i use a 25gr check weight or a 500gr check weight to zero the scale ?
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Last edited by Metal god; March 25, 2020 at 02:22 PM.
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Old March 25, 2020, 02:37 PM   #7
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Use whatever method you want to zero the scale. I prefer zeroing at zero to avoid the 3M syndrome of Multiple Mental Math cycles converting scale reading to the object's actual weight.

Scale accuracy should be equal across all methods. The more steps involved increases the odds of error.

Fewer than 1 of 1000 reloaders can distinguish a 1/2 percent spread in charge weights their rifle reloads have while shooting them in accuracy events.

Last edited by Bart B.; March 25, 2020 at 03:42 PM.
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Old March 25, 2020, 03:01 PM   #8
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A scale that is accurate to .1 grain is pretty typical.

Quote:
That's a big deal , especially for those weighing there charges in .3gr increments or less in rifle loads
That is why I never worry about increments less than .5 gr. If I look at a loading manual with a starting load of 42.1 and a max of 45.7 I'll start at either 42.5 or 43 gr then work up in .5 gr increments until I get to 45.5.

In rare cases I might even try 46 gr. That depends on the velocities I'm getting along the way compared to what I'm expecting. I've never had any trouble finding a good load that way. Changing powder charges by .3 gr or less has never made enough difference in either velocity or accuracy to justify trying that many different loads.
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Old March 25, 2020, 04:02 PM   #9
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When you weigh something with a scale, you have two variations to think about: all measuring devices have Gage Error, and all parts being weighed have variation from part to part. Both are calculated together. What you're getting is the amount of variation in the measuring process and it's being described as to what is acceptable. The +/- .1 means that for the 5.0 you're trying to achieve you can accept results that are showing +/- .01 which is 4.9-5.1
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Old March 25, 2020, 04:24 PM   #10
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Because of this + or - .1 of a grain variance of most scales, is why i could never understand why people recommend doing OCW tests in .2 increments, you really don't know if you are increasing your ladder loads up by that much, when your scale is that imperfect.
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Old March 25, 2020, 04:34 PM   #11
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MG,

The math of tolerances match with their names. If it is ±0.1 grains, it means whatever weight the scale says you have, add 0.1 grains (the plus) to it and the result will be the upper limit of possible measurement error, and then take the weight the scale says you have and subtract 0.1 grains (the minus) and that result will be the lower limit of possible measurement error.

When you look at SAAMI drawings, you will find the linear tolerances are unilateral, meaning they only go in one direction. For chambers, linear dimensions have only a plus tolerance, meaning the value given is a critical minimum and nothing in the chamber can be any smaller than that, while for cartridges linear dimensions have only a minus tolerance, meaning the value given is a critical maximum above which you cannot make them any larger. This system assures cartridges can chamber.
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Old March 25, 2020, 04:52 PM   #12
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Quote:
Originally Posted by lugerstew View Post
Because of this + or - .1 of a grain variance of most scales, is why i could never understand why people recommend doing OCW tests in .2 increments,
I think the OCW theory is flawed in its claim the muzzle enlargements of .0001 inch upon bullet exit hurts accuracy. I've shot match grade barrels cleaned from the muzzle that's enlargement from steel cleaning rod wear is over .0010 inch that still tested about 2/3rds MOA at 600 yards.

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Old March 25, 2020, 05:31 PM   #13
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Quote:
Originally Posted by Metal god
I use the term zeroed at 5gr when using a beam scale because I use check weights so as much as can be the case , the scale was zeroed or checked to be accurate at 5gr or what ever the intended weight will be . Digital scales can absolutely be zeroed at a specific weight . I do it all the time when weighing someing that needs to be put in a basket or the like . I zero the scale with the basket on it so when the basket is off the scale it shows a zero of - what ever the weight of the basket .
I still think you're using the term "zero" differently than my understanding of it.

When I "zero" a beam balance scale, I put the empty pan on the hanging tray and adjust the body of the scale so the pointer is at the zero index on the dial. That's "zeroed." The only way I can "zero" a scale for 5.0 grains is if I were to somehow have a certified check weight of 5.0 grains. I don't -- and I've never heard of such a check weight being available.

So I zero for zero weight. If I had a check weight, I could then put that in the pan and see if the scale reports the weight correctly. If so, I'm probably safe in assuming that the scale is accurate between zero and whatever that check weight is. Beyond that would be a guess.

Quote:
Originally Posted by Metal god
Should i expect any difference in accuracy in the scale if I’m trying to weigh 25gr and i use a 25gr check weight or a 500gr check weight to zero the scale ?
It depends on the scale. Some scales are more accurate (or more precise) toward the center of the scale, others claim their accuracy/precision extends for the full range. I would use the check weight that's closest to the weight of the charge you want to measure.

If you zero the scale with a 25-grain check weight and the scale then doesn't read zero with an empty pan in place, you have to decide whether you can safely assume that the deviation is linear, or random.
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Old March 25, 2020, 05:50 PM   #14
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It's a combination of all of the above. If you accurately adjust your powder measure, accurately zero your scale, accurately read the scale, etc., etc., your charges will vary by up to two tenths.
That's why, when developing loads, you increase in .2 grain steps.
It's also why I measure ten charges before deciding my load is where I want it; one charge appears to weigh, say, five grains, but ten charges weigh 49 grains?
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Old March 25, 2020, 11:30 PM   #15
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Thanks guys , this helped clear up some things for me and I hope it will for others in the future when they read this thread .

MG
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Old March 26, 2020, 12:14 AM   #16
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Powder varies lot to lot. If you buy powder 1 lb at a time,the powder performance will likely vary as much as the scale does.

Some folks hav a set of certified test weights,and they calibrate their scales.

Most of us don't. I don't expect my old Ohaus 1010 to weigh the same as your new Redding or Lee or Lyman. I don't expect my scale to match yours,but I expect my scales to repeat.

I don't use your charge weights. I work up to mine. What I need is to e able to weigh the same 46.0 gr charge 6 months from now. If its 0.3 gr off what you get,thats OK.

As a machinist,some measrements were made with a 6 in scale. Some with calipers. If I needed to be a little closer,I'd use my micrometer. Icheck my mics with gauge blocks

But if you want to get serious you get out the gauge blocks, wring together a stack that is the desired dimension,calibrated and traceable down to a few millionths of an inch,and set a comparatr device,such as a Swede gauge to the gauge block stack. The comparator will measure how much your part varies from the gauge block stack.

If you want to trely know you are weighing 46.4 gr of powder,You select 46.4 gr of test weights and zero your scale.

IMO,its good to have at least a couple of reference weghts (If not purchased test weights.)

You can weigh a couple different size washers and buzz engrave the results n the washer. Five years later,the washers ought to weigh the same,or your powder charges are varying.
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Old March 26, 2020, 09:21 AM   #17
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Quote:
Originally Posted by HiBC View Post
Powder varies lot to lot. If you buy powder 1 lb at a time,the powder performance will likely vary as much as the scale does.
Lake City arsenal said the IMR powders used in 30-06 and 7.62 ammo typically has a 2 grain or more spread in charge weights across all bulk powder lots to meet pressure, velocity and accuracy specs.

DuPont said their commercial lots of those same IMR powders were blends of production lots to have about a 1 grain spread in their tests.

As I remember from phone conversations with them around 1970.

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Old March 26, 2020, 11:54 AM   #18
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Zero is a mathematical function. All measurements have an error associated with them.
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