Measurement variations?

[EuroStyle]

Is it normal for a loose stone to come up with slightly different numbers when placed mulitple times on a Megascope? How about on a digital scale? I have searched, but have not come across any info on this.....


Sean

 

[DiamondExpert]

Yes, it''s true for any instrument - there will always be some variation - it''s the nature of instrumentation - none is perfect, only more or less precise and accurate.

 

[EuroStyle]

OK, so I guess the follow up is how different will it be on the same equipment different times vs. on two different sets of instruments....or does it not matter. In the end, as a consumer and not a technical person, when do you become concerned when a stone weights two different amounts etc...



Sean

 

[Richard Sherwood]

It would be more common for there to be differences on proportional measurements than on carat weight.

Carat weight is usually more uniform between the more accurate (ie more expensive) scales, which measure to the 1000ths of a carat (or more).

What kind of differences are you encountering?

 

[Garry H (Cut Nut)]

Here is a detailed article from GIA in Rap on exactly this topic.

http://www.diamonds.net/news/newsitem.asp?num=11815&type=all&topic=all

a small part:
All measurements have some uncertainty associated with them. Uncertainty means something not ex-actly known; in the case of measurements, it is often described as the error associated with the measurement. Measurement uncertainties can affect the accuracy of a measurement and they are fundamental in determin-ing its precision. Accuracy describes how closely a given measurement matches the “true” measurement. Precision describes how finely the measurement has been made, but not whether the value is accurate. Precision tells us something about the scale of a measurement and whether a second measurement matches the first one, while accuracy relates to the correctness of it. Precision is important because two measurements can only be determined tobe the same or different with respect to some reported precision. A generally accepted precision for a specific measuring tool or process is often referred to as its tolerance [e.g., a ruler marked every 1 millimeter (mm) has a tolerance of ±0.5 mm].

 

[Garry H (Cut Nut)]

And here is my letter to the Editor for next months Rap mag.

Re: Measurement Tolerances By GIA Research
3/31/2005

The GIA Research team raised the issue of Measurement tolerances in the April issue of Rapaport Diamond Magazine. Their account raised some interesting issues for those of us working in the field of diamond cut research. But there are other ‘in-built’ inaccuracies in the way current optical scanners measure diamonds and process the data produced.

For example most readers will have noticed the 3D outputs that modern scanners produce often show 8 little lines joining the table to the kite facets. These represent the use of a mathematical algorithm to ‘average out’ scanning errors that would result in a non-closed object (i.e. the diamond would have holes in it). In effect the scanner can take the data and build a more symmetrical diamond than the one that was scanned. You see this effect more often on round brilliants because the scanners are programmed to recognize these stones. Check out the 3D scans of new cuts or the small ‘chevrons’ on the pavilion, and the crown facets on a princess; you will see missed or broken facets. The problem is that the optical shadow profile method used by scanners is good for reading facet slope angles, but poor at measuring the orientation or azimuth direction that they are facing; the software manipulates imperfect data to make a more perfect model.

Another example is the case of a slightly triangular shaped round brilliant; say one was cut with its table close to the 3 point face or octahedral face of the rough. When measuring this stone with a Leveridge gauge or one of the first generation optical scanners, the stone might have no diameter deviation at all (e.g. 6.00 – 6.00mm) but it could have a radius deviation (e.g. 2.90 to 3.10mm). Only the new Russian OctoNus® scanners report these and other deviations and build optically accurate 3D models of the type that will be required for new optical performance based Lab Cut Grading.

An article that includes these topics can be found here: www.gemology.ru/cut/english/symmetry/6.htm

Sincerely Garry Holloway

Ideal-Scope.com

Melbourne, Australia

 

[JohnQuixote]

Date: 4/13/2005 8

3:58 PM
Author:EuroStyle
Is it normal for a loose stone to come up with slightly different numbers when placed mulitple times on a Megascope? How about on a digital scale? I have searched, but have not come across any info on this.....

Sean

Sean,

Differences may run ± .2 degrees, and sometimes .2% or so with Sarin (same type of machine as Ogi Megascope). Ogi has been reported as having a slightly larger error, but many rely on Ogi and it should not be significantly worse.

When this question has been asked in the past consumers have wanted to know what should be the ''official'' numbers to go by. Typically the grading report (if it has detailed numbers) is considered the authority.

 

[EuroStyle]

Thanks for all the input.....the scale was giving a 1.508 last week....and yesterday a 1.504, 1.503. As for the Megascope, each detailed report had the measurements slightly different in the percentages from the onw ran last week....each time it was run yesterday, in a row, they were all changed a bit from the original and each other...this is on an EC by the way. I trust the person, just seemed odd that a machine that is touted as accurate could be off in something that is completely measurable...same with the scale....



Sean

 

[JohnQuixote]

Date: 4/14/2005 12:283 AM
Author: EuroStyle
Thanks for all the input.....the scale is giving a 1.508 and then a 1.504, 1.503. As for the Megascope, each detailed report had the measurements slightly different in the percentages....each time it was run, in a row, they were all changed a bit...this is on an EC byt the way....

Sean

Sean,

As far as the Federal Trade Commision is concerned, diamonds are only weighed to the tenth.

AGS weighs to the thousandth, but such minute measurements are taking it (pardon the pun) to the "nth" degree...

If you go into a sealed room with no wind & consistent barometric pressure and temp and use a well-maintained, calibrated scale to weigh this dust-free diamond (from another room with a video monitor) then maybe it will be the same all 3 times

 

[denverappraiser]

Date: 4/14/2005 12:37:32 AM
Author: JohnQuixote

If you go into a sealed room with no wind & consistent barometric pressure and temp and use a well-maintained, calibrated scale to weigh this dust-free diamond (from another room with a video monitor) then maybe it will be the same all 3 times

John,

You forgot to mention the need to calibrate the scale for variations in elevation (which increases the distance from the center of the earth) and latitude (there’s a slight error caused the centrifugal force of the earth spinning).

Neil Beaty
GG(GIA) ISA NAJA
Independent Appraisals in Denver

 

[Richard Sherwood]

Date: 4/14/2005 12:23:42 AM
Author: JohnQuixote

Differences may run +-.2 degrees, and sometimes .2% or so with Sarin (same type of machine as Ogi Megascope). Ogi has been reported as having a slightly larger error, but many rely on Ogi and it should not be significantly worse.

OGI reports an accuracy range of +-.2 degrees in angle measurement and .015 mm in distance measurement.