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Old 03-23-2012, 18:32   #1
Taterhead
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RCBS 505 Scale Precision

I was having a discussion recently with a fellow reloader who had recently acquired an electronic powder dispenser and scale. It is not a brand I am familiar with. SmartReloader, I believe.

He also has an RCBS 505 (Ohaus) beam scale. He said that he was glad to have purchased the electronic scale because it revealed to him just "how inaccurate the RCBS scale is." He would throw a charge with the electronic unit and then weigh it on the RCBS. He said that it showed the RCBS scale to be, "all over the place" because it would weigh something different than what the electronic scale read. He was convinced that the beam scale was crap and that he was freaked out to have been using it all of these years.

I explained that he had it exactly backward. The 505 should be used to verify the electronic scale. The variances would show the inconsistencies with the electronic system. Not the other way around. His counterpoint was that the electronic unit zeroed itself frequently so it had to be right on.

Check weights are invaluable for a peace of mind. I have an RCBS set. $30 is money well spent. I forwarded him this photo showing how the 505 reads with various check weights. Dead on from 0.5 grains to 55+ grains and everything between. Very reassuring.

Side note. I cut the top of the box out and use it as a cover when not in use. I like to keep things clean.

Attachment 224004

Last edited by Taterhead; 06-11-2012 at 00:17..
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Old 03-23-2012, 19:38   #2
TDC20
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You are absolutely correct correct, Taterhead. Everything that is manufactured has variables in precision. That includes the mechanical tolerances of the beam scale as well as the electronic properties of a load cell. The difference is that load cells use a microprocessor to convert an electronic signal into a weight. That by itself is not so scary. What is scary is that the software has an algorithm that includes a characterization of how a "generic load cell" turns a physical quantity (mass) into an electronic signal, which is then interpreted by the software algorithm and displayed.

So, back to the variables in precision. The mechanical system is vulnerable to mechanical tolerances, which are well known, but the basics of a beam balance is a very simple linear function. the fine measurements are repetitive, so any errors there would repeat on a smaller scale. However, the algorithm for any given load cell is based on a characterization of many load cells, and then derived from a mean. Load cells can be very accurate, but when cost comes into play, usually there is a compromise. The electronic scale is calibrated using a two or three point check weight system, and then the curve is fit in between, based on a generic load cell function. The beam balance is a function of linear mechanical tolerances. Either one could have imperfections and loss of precision, but the fact that the load cell is a mathematical curve fit to a physical property of a generic mean load cell gives me reason to trust the linear mechanical balance beam as more capable of precise measurements from unit to unit than the load cell.

In any case, you could convince your fellow reloader that the beam balance is the better standard by showing him how the check weights align with both measurement systems.

I've been around a long time and have decades of experience with electronic measurements and systems in general. If I don't trust software, I'm usually right 90% of the time. I'm not saying that an electronic measurement system can't be accurate. On the contrary, a load cell could be calibrated to a very very high degree of precision, but that usually doesn't happen on a production line for a resale value of < $300.

Just my opinion, let the check weights determine the most precise instrument.
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Old 03-23-2012, 21:20   #3
_The_Shadow
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It has been my findings the the RCBS 5-10 Beam Scale has been very accurate for my needs...recently I have compared the Frankford Arsenal digital to it. I set both to zero using the scale pan of the RCBS placed on the Farnkford Arsenal, that way I can transfer the measured powder charge between both scales.

However the digital is limited to even numbered digits in the tenths column, so I have to sample using and even number in the tenths. Check weights are measured properly on both. I do find that the digital will drift (could be the battery needs replacement) and requires re-zeroing after using for sometime.

Digitals are great to find the weights of unknowns like bullets.

I have trusted my life to the RCBS 5-10 since 1979
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Old 03-23-2012, 21:37   #4
Taterhead
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Quote:
Originally Posted by TDC20 View Post
You are absolutely correct correct, Taterhead. Everything that is manufactured has variables in precision. That includes the mechanical tolerances of the beam scale as well as the electronic properties of a load cell. The difference is that load cells use a microprocessor to convert an electronic signal into a weight. That by itself is not so scary. What is scary is that the software has an algorithm that includes a characterization of how a "generic load cell" turns a physical quantity (mass) into an electronic signal, which is then interpreted by the software algorithm and displayed.

So, back to the variables in precision. The mechanical system is vulnerable to mechanical tolerances, which are well known, but the basics of a beam balance is a very simple linear function. the fine measurements are repetitive, so any errors there would repeat on a smaller scale. However, the algorithm for any given load cell is based on a characterization of many load cells, and then derived from a mean. Load cells can be very accurate, but when cost comes into play, usually there is a compromise. The electronic scale is calibrated using a two or three point check weight system, and then the curve is fit in between, based on a generic load cell function. The beam balance is a function of linear mechanical tolerances. Either one could have imperfections and loss of precision, but the fact that the load cell is a mathematical curve fit to a physical property of a generic mean load cell gives me reason to trust the linear mechanical balance beam as more capable of precise measurements from unit to unit than the load cell.

In any case, you could convince your fellow reloader that the beam balance is the better standard by showing him how the check weights align with both measurement systems.

I've been around a long time and have decades of experience with electronic measurements and systems in general. If I don't trust software, I'm usually right 90% of the time. I'm not saying that an electronic measurement system can't be accurate. On the contrary, a load cell could be calibrated to a very very high degree of precision, but that usually doesn't happen on a production line for a resale value of < $300.

Just my opinion, let the check weights determine the most precise instrument.
I read this entire thing, and I now feel a tad nerdier! Ha!!

Seriously though, great post. Great explanations. The last statement is spot on.
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Old 03-24-2012, 07:44   #5
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100% on, guys.

My 505 is perfect... and my check weights verify that often, or when changing charges.
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Old 03-24-2012, 09:14   #6
roundball
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Took my big dual beam Ohaus scales and two 5-10's to the local scale company and had them cleaned and checked for accuracy. All was well and was able to verify with Frankfort electronic scale while reloading. Have found over time Ohaus scales to be is consistent. +1 on re-zeroing electronic scales. What can I expect for $35.00.
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