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Old 12-03-2013, 02:06 AM   #81
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I have a 1000 lb gauge on my Sherline. An indication of 300 lbs on my scale would be an actual weight of 280 lbs. My scale always indicated on the heavy side.
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Old 12-03-2013, 01:04 PM   #82
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Hi all, I have to apologize for yesterdays post.

The numbers I gave in the post were copied from a hastily created spreadsheet, and were from the wrong column. The mistake didnít dawn on me until 2am, and that was the end of sleeping. The numbers I put in the post were the max deviation from the norm for each weight. The good news, the scale is actually better then I stated in the post.

Just using the average of the runs for nominal, I get the following:
For a 300 lb actual, my scale indicates 302 (on average), +/-25 (mfr spcs)
For a 400 lb actual, my scale indicates 406 (on average), +/-22.5 (mfr spcs)
For a 500 lb actual, my scale indicates 509 (on average), +/-20 (mfr spcs)
For a 600 lb actual, my scale indicates 618 (on average), +/-22.5 (mfr spcs)
For a 700 lb actual, my scale indicates 721 (on average), +/-25 (mfr spcs)
For a 800 lb actual, my scale indicates 820 (on average), +/-27.5 (mfr spcs)
For a 900 lb actual, my scale indicates 923 (on average), +/-30 (mfr spcs)

This isnít what I would expect from a gauge specified at +/-2%fs at the 50% mark and +/-3%fs at the 10% and 90% marks. When I asked the manufacturer of the gauge about this, I didnít get an answer, so it is what it is.

Again, sorry for the bad information.
Tom
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Old 12-03-2013, 01:12 PM   #83
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Looks like around 98-97% accuracy, that is good enough for me
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Old 12-03-2013, 10:57 PM   #84
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I don't know about Northern Tool and Equipment's selection, but for those in Canada - Princess Auto carries a 200 kg (440 lb) capacity electronic scale with a remote (cabled) display and tare feature (so it does what you want, not locking on a reading when it feels like it) for $100. I have one, and will likely try it out with test weighs for repeatability and the trailer for usability... next spring. Sorry, no metrology lab available!
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Old 12-03-2013, 11:00 PM   #85
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Quote:
Originally Posted by cpaharley2008 View Post
Looks like around 98-97% accuracy, that is good enough for me
Although that's not how accuracy should be specified, I get what you mean. A problem is that's an average. Any individual reading could be much further from correct - up to 20 to 30 pounds further off. Do you want to repeat measurements several times to take an average?
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Old 12-03-2013, 11:02 PM   #86
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Sherline said:
Quote:
Originally Posted by TAfraser View Post
Ashcroft specifications for the gauge are 2% at mid-range and 3% at the extremes. This means it could be as much as 30 pounds off at 100 and 900 pounds and be within the acceptable range. They do make a 1% gauge (2% at extremes) but that one is quite a bit more expensive and we never considered the small difference significant when it comes to determining safe trailer loading
I understand their reasoning. Just as some people have changed their gauge to change the range of their Sherline scale, a more accurate gauge could be installed to improve the accuracy. Is it worth it, knowing that some variation is due to the seal imperfections? That's an individual call.
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Old 12-03-2013, 11:13 PM   #87
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My weight on my fairly cheap bathroom scale is quite reproducible. Since the scale does not go high enough to measure my tongue weight from a single point, maybe I should buy a second scale just like it and use both to measure tongue weight. Would just have to support the tongue about mid way between the two scales on a cross piece spanning both scales. Then the total tongue weight would be the sum of the two scale readings. To minimize non centric loading on each scale, I would place a small block of wood between the cross piece and the surface of each scale.
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Old 12-03-2013, 11:26 PM   #88
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"Would just have to support the tongue about mid way between the two scales on a cross piece spanning both scales. Then the total tongue weight would be the sum of the two scale readings."

That would work, but if you think about it, you could do the same with just one scale and multiply by two. (If you supported the tongue exactly midway on the cross piece...)
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Old 12-04-2013, 01:56 AM   #89
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Sherline

Brian, I agree with what youíre saying. I also dislike taking multiple measurements in order to have some type of confidence in that measurement. We call that, ďTesting for SuccessĒ, and its practice is frowned upon. The tendency, human nature, is that more weight is given to the good, or favored returns. Iíve always felt that if I have to take an average of multiple measurements, I should look for a different piece of test equipment. In this case there doesnít seem to be much out there, at least not within an acceptable price range thatís convenient and simple enough for all of us to use.

I selected using the average of 5 measurements for determining the offset value because, when applying the calculated offset to the measurement, the result cancels much of the deviations. This brought all the measurements well within the stated tolerances.

At 300 lbs, the calculated offset is -2 lbs, the max deviation of all 5 measurements was +10; adding the offset to the max deviation leaves us at a +8, worst case. The manufacturers stated tolerance for 300 lbs is +/-25.

The others are as follows:
400 lbs, offset -6 lbs, max deviation +20, leaving us with a worst case of +14 for a tolerance of +/-22.5.
500 lbs, offset -9 lbs, max deviation +20, leaving us with a worst case of +11 for a tolerance of +/-20.
600 lbs, offset -18 lbs, max deviation +25, leaving us with a worst case of +7 for a tolerance of +/-22.5.
700 lbs, offset -21 lbs, max deviation +25, leaving us with a worst case of +4 for a tolerance of +/-25.
800 lbs, offset -20 lbs, max deviation +25, leaving us with a worst case of +5 for a tolerance of +/-27.5.
900 lbs, offset -23 lbs, max deviation +30, leaving us with a worst case of +7 for a tolerance of +/-30.

We could also use the medium of the deviations with the following results:

300 lbs, offset -0, max deviation +10, leaving us with a worst case of +10 for a tolerance of +/-25
400 lbs, offset -10, max deviation +20, leaving us with a worst case of +10 for a tolerance of +/-22.5
500 lbs, offset -12.5, max deviation +20, leaving us with a worst case of +7.5 for a tolerance of +/-20
600 lbs, offset -20, max deviation +25, leaving us with a worst case of +5 for a tolerance of +/-22.5
700 lbs, offset -20, max deviation +25, leaving us with a worst case of +5 for a tolerance of +/-25
800 lbs, offset -20, max deviation +25, leaving us with a worst case of +5 for a tolerance of +/-27.5
900 lbs, offset -25, max deviation +30, leaving us with a worst case of +5 for a tolerance of +/-30

Let me know if you think Iíve lost my marbles. This scale is already cost effective, easy to transport and use, and looks to be robust. Iím just trying to prove this scale is accurate and consistent enough for our use. Without application of the offsets, this gauge would fall outside manufacturers stated tolerance.
Tom
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Old 12-04-2013, 10:56 AM   #90
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Please remember, deviating from manufacturers stated specifications constitutes a qualified certification (if done in a lab by qualified technicians). Using these numbers for all Sherline scales would require more testing on a much larger number of scales. With testing being done on only one scale, these numbers are for my Sherline only. Other Sherlines could test very different from this one, so take this with great assault.
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