digital refractometer

[mus]

I have found on ebay the following digital refractometer :
Digital refractometer on ebay


What do you think about this kind of refractometers ? Are they accurate ?
Have you already tested such refractometers ?

 

[Richard M.]

Date: 8/21/2008 11:50:06 AM
Author:mus
I have found on ebay the following digital refractometer :

Digital refractometer on ebay



What do you think about this kind of refractometers ? Are they accurate ?

Have you already tested such refractometers ?

I was intrigued by your question so I asked a man who''s a genuine expert in such things. He hasn''t tested this machine but has worked with many similar to it. I own a similar machine and have found it frustrating, almost impossible, to use.

First, he tells me it''s not really a refractometer even though the digital readings give that impression. It''s a reflectivity meter and the readings depend on the quality of the polish on the surface being measured, and absolute cleanliness. It does not directly measure refraction (light bending) but rather infers it from reflectivity data.

It''s my opinion you''d be much better off spending your cash on a genuine name-brand refractometer. Some of the less expensive ones are not reliable I''m told. Be sure to invest in a model that offers you the capability of sharp sodium line readings as well as a polarizing filter for measuring birefringence. IMO a reliable refractometer and microscope are a gemologist''s most important tools, apart from gemological knowledge. Choose wisely.

 

[Proteus]

I fully support Richard''s opinion of cheap refractometers - you get what you pay for. I''ve used one of the inexpensive "portable" type... they work, but give a slightly fuzzy reading at best, and are annoying to try and focus or adjust. They just don''t have as good of a viewing angle range, or clarity of reading as a good-quality unit will. The polarizing filter that this unit came with is not good, either. It''s pretty hard to get an accurate display of the birefringence with it. They come with a built-in sodium filter, which is sort of nice, but not any particular bonus in this day and age. I''d avoid them. If you need a very inexpensive sodium light, I''d suggest using an amber LED as a light source - the spectrum is pretty close.

 

[Pandora II]

Could you suggest a few brands to consider - I have put off buying one so far as I wasn''t sure which to get.

I have Gem-A''s monochromatic light for use with refractometers (I picked it up for $5 on ebay by sheer chance), so I don''t necessarily need one with a built-in light.

Portable isn''t necessary either.

 

[Richard M.]

Pandora,

I''ve used a GIA Duplex 2 for many years very satisfactorily although it''s quite difficult to get precise readings, especially for determining optic character, without using a loupe. It''s an older model but I think newer ones are very similar. Full-time gemologists I know who take readings all day long strongly recommend the $529 Gem-A model here: Gem-A

Others favor the Kruss, which is a bit more expensive. Many folks find used Duplex 2''s on Ebay and if they''re not really trashed the glass hemicylinders can usually be refurbished for under $100 here in the U.S. But determining whether the glass merely needs repolishing or is chipped and unrepairable is where the danger usually lies. I think new Duplex 2''s are very overpriced.

I''m interested to hear recommendations from others.

Richard M.

 

[oldmancoyote]

As an amateur - as opposed to a pro - who has bought a couple of months ago exactly the same instrument the OP posted, I can offer the following observations:

1. Tested on a dozen or so different gem types, it has produced consistent results that are precise to two decimal places (1.xx or 2.xx) and consistent with published RI ranges for the gem tested

2. It is quite sensitive to planarity of the surface, but I haven't found it very sensitive to cleanliness (a quick wipe with a shammy is enough). However, testing unpolished stones is impossible (even natural crystal facets aren't good enough, unless they are very smooth)

3. It will happily (?) test mounted stones that are large enough (say 5mm diameter or larger)

4. It has a wide range - diamond and CZ are "on scale" and differentiated (2.42 vs. 2.17) instead of reading simply "off scale" as on many analogue instruments

5. It's very fast and not fiddly, requiring no liquids or special prepping - once the stone is stable

6. Cannot be used with polarizing or other filters

Is it of sufficient quality for a professional? Probably not, except as a portable tool. For an amateur testing polished gems, I think it's pretty good.

Just my 2¢

 

[Richard M.]

Hi OMV,

That''s good information, coming from an actual user. I''m curious about what you mean when you say the instrument is "quite sensitive to planarity of the surface" and that it''s very fast "once the stone is stable."

Does the first comment refer to the quality of the polish or flatness of the (faceted?) surface? Can you use it on curved surfaces like cabochons? About stability, do you mean the steadiness of the stone on the platform?

Having a machine that would read over the normal 1.81 limit would be very handy.

Richard M.

 

[oldmancoyote]

Hi Richard,

I have tried to use it on cabochons, but I never managed to get a good reading; unfortunately I don''t have unset cabs, so I never tried it on the flatter surface. On natural crystals, if they have a good termination it''s not a problem, and even on the side of a long, thin topaz crystal it''s OK; however I could not manage to get a reading off a 3.13ct rough diamond which is nicely octahedral but has quite a few indentations. So I think it''s really the flatness of the surface rather than the polish or transparency.

In terms of stability, I don''t know if you managed to look at the eBay pictures - if not I''ll post one - but it has a black round plastic platform with a hole through which the IR beam is projected. The shape of the platform is such that getting a largish stone to lie flat and steady over the hole can be a bit tricky. Once that is done, the actual testing takes loess than one second. I guess a bit of blu-tack strategically positioned to steady the stone may help, but I haven''t tried it.

The tool seems also to be sensitive to its own "flatness"/steadyness, and does not perform well when it''s hand held; however just putting it on a table or shelf is enough to fix it.

Hope this helps

OMC

 

[Pandora II]

Date: 8/22/2008 9:55:40 PM
Author: Richard M.
Pandora,

I've used a GIA Duplex 2 for many years very satisfactorily although it's quite difficult to get precise readings, especially for determining optic character, without using a loupe. It's an older model but I think newer ones are very similar. Full-time gemologists I know who take readings all day long strongly recommend the $529 Gem-A model here: Gem-A

Others favor the Kruss, which is a bit more expensive. Many folks find used Duplex 2's on Ebay and if they're not really trashed the glass hemicylinders can usually be refurbished for under $100 here in the U.S. But determining whether the glass merely needs repolishing or is chipped and unrepairable is where the danger usually lies. I think new Duplex 2's are very overpriced.

I'm interested to hear recommendations from others.

Richard M.

I guess I'll be saving up for the Gem-A version. It will also go with my bargainous lamp!

Makes sense since I'm up the road from them.