[meteorite-list] Micromounts and weights - Standards Vary

From: Mark Grossman <markig_at_meteoritecentral.com>
Date: Fri, 1 Jul 2011 20:22:41 -0400
Message-ID: <E8AE214B79B54E3697A8D9439DE7D2CB_at_QED>

Just to clarify a bit - there is a difference between precision and
accuracy.

If the finest division on a scale is 0.1 mg, this is an indication of the
precision of the scale - how repeatable the measurement is. You could weigh
a specimen several times on the scale and get close agreement of the
measurements to the 0.1 mg limit to which the scale reads.

However, this is not tied to the accuracy of the scale, which might be
expressed as a percent for a particular weight range (e.g. 5% for the 0 to
10 gram range).

So you can weigh a specimen several times to the nearest 0.1 mg and get
results that agree very well (good precision), but the results could be
highly inaccurate.

It's similar to using a ruler that was incorrectly manufactured so that it
is actually 13 inches instead of 12 inches. Every time you measure a
length, you come up with 12.0 inches, so the measurements are very precise,
but highly inaccurate, because you really measured 13.0 inches.

So the accuracy has to do with the calibration of the scale, thermal
conditions, etc. Having a scale that reads to the 0.001 mg gives no
guarantee at all that the readings are accurate to 0.001 mg, or for that
matter anywhere close. You need to check the specs of the scale for the
accuracy.

Mark

Mark Grossman
Meteorite Manuscripts


----- Original Message -----
From: "Richard Montgomery" <rickmont at earthlink.net>
To: "Michael Blood" <mlblood at cox.net>; "Met. Mike Bandli"
<fuzzfoot at comcast.net>; "Met. Michael Gilmer" <meteoritemike at gmail.com>;
"Meteorite List" <meteorite-list at meteoritecentral.com>
Sent: Friday, July 01, 2011 7:53 PM
Subject: Re: [meteorite-list] Micromounts and weights - Standards Vary


> Hi List,
>
> As a non-dealer, but the ocassional passer-on-of-specimens, (normally the
> ocassional SA batches or other noteables), I use an x.xx scale yet have
> always quoted the weight to x.x -(.x) ...with respect to my error
> potential. Quoting at least a -.x (i.e. 68.8gr instead of the scale's
> reading of 68.9 or 68.92) to at least insure satisfaction, I don't expect
> anyone to get PO-ed when it weighs more than quoted. Yet this does invite
> an inaccuracy element with regard to my labels.
>
> I'd love to hear some feedback here, so please chime in.
>
> For the specimens with a stellar provenence chain-of-custody, I still
> check and adjust. Crumbs can fall...
>
> Or, the original seller may have weighed the specimen from 30K feet :>)
>
> -Richard Montgomery
>
>
> ----- Original Message -----
> From: "Michael Blood" <mlblood at cox.net>
> To: "Met. Mike Bandli" <fuzzfoot at comcast.net>; "Met. Michael Gilmer"
> <meteoritemike at gmail.com>; "Meteorite List"
> <meteorite-list at meteoritecentral.com>
> Sent: Friday, July 01, 2011 1:37 PM
> Subject: spam: Re: [meteorite-list] Micromounts and weights - Standards
> Vary
>
>
>> Hi Mike and all,
>> I absolutely agree. I used to use a $500 digital and now
>> Use a $135 digital and I consider them both about as accurate
>> As the other ~ deduct one decimal point for absolute accuracy.
>> (it is likely far closer than that, but one should not proclaim
>> a specific weight, IE .007g (7mg) unless one has a serious
>> balance beam in an air tight setup. A royal pain in the a**
>> And extremely costly.
>> However, for the most part, I always sell micromounts - the
>> Ones less than 10mg, based on VISUAL COMPARISON. That is
>> What I look for for my own collection... If I want something that
>> Is so small, then the size is far more important to me than the mass.
>> BTW, a micromount has traditionally been defined as any
>> Specimen that fits into an old style 1" X 1" square display box.
>> The new, vastly superior membrane boxes are considerably larger
>> And can hold a decent sized macromount equally well as a micromount.
>> Best regards, Michael
>>
>> On 6/30/11 4:52 PM, "Met. Mike Bandli" <fuzzfoot at comcast.net> wrote:
>>
>>> A little perspective on milligrams:
>>>
>>> There are a lot of meteorite mg weights out there that not accurate. We
>>> can
>>> thank these new, cheap Chinese digital scales that promise accuracies of
>>> +/-
>>> 1mg or less, which are a complete joke. I bought one in Tucson to test
>>> it
>>> out against my high-end calibrated machine and it was off by about 10 mg
>>> on
>>> average for pieces 50 to 100 mg and 5 mg on average for pieces 10 to 50
>>> mg.
>>> Anything fewer than 10 mg - forget about it. The calibration weights it
>>> came
>>> with were even more laughable...
>>>
>>> In reality, in order to be able to accurately measure mg, you need a
>>> machine
>>> that has been recently leveled and calibrated in-situ. I have a recently
>>> leveled/calibrated mechanical scale whose tare changes by the hour due
>>> to
>>> changes in the weather. It even picks up the subtle vibration of the
>>> dishwasher downstairs.
>>>
>>> Bottom line - a $100 mg scale isn't going to get you the accuracy needed
>>> to
>>> accurately measure true mg. Since most people can't afford the hundreds
>>> to
>>> thousands it costs for an accurate mg scale, I don't expect most mg
>>> weights
>>> advertised to be truly accurate. They're close...
>>>
>>> Just my 2 mg worth (+/- 1mg)...
>>>
>>> ----------------------------------------------
>>> Mike Bandli
>>> Historic Meteorites
>>> www.HistoricMeteorites.com
>>> and join us on Facebook:
>>> www.facebook.com/Meteorites1
>>> IMCA #5765
>>> -----------------------------------------------
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: meteorite-list-bounces at meteoritecentral.com
>>> [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of
>>> Michael
>>> Gilmer
>>> Sent: Thursday, June 30, 2011 4:17 PM
>>> To: Meteorite List
>>> Subject: [meteorite-list] Micromounts and weights - Standards Vary
>>>
>>> Hi Listees and Micronauts,
>>>
>>> There has been some discussion recently about people buying
>>> micromounts from a vendor on eBay and not getting the weights they
>>> were promised. I thought I would throw out some thoughts on micros,
>>> since those are my bread and butter.
>>>
>>> First, the definition of "micromount" is relative. There is no
>>> set-in-stone size bracket for what defines a micromount. It seems to
>>> me that the general consensus is that micromounts are in the 1g range
>>> for the more common types and sub-gram in weight for the rare types.
>>> Very rare falls or planetaries are commonly sold by the milligram.
>>> Rockhounds tend to equate meteorite micromounts with mineral
>>> thumbnails. But generally speaking, most micromounts on the market
>>> today are in the sub-gram (<1g) range.
>>>
>>> Ideally, a micromount should be visually appealing (such a well
>>> polished, thin part slice with good surface area to weight ratio) and
>>> big enough to identify the lithology of the type/fall, while at the
>>> same time being cheap enough to afford on a limited budget.
>>>
>>> The more preparation that goes into making a given micromount, the
>>> higher the price, generally speaking. At some point, it's not
>>> financially viable to put a lot of cutting and polishing work into
>>> piece of common find that is only worth a buck or two a gram.
>>> Smaller micros are difficult to work with during preparation, for
>>> obvious reasons, so many of the micromounts seen on the market are
>>> unpolished, rough, or broken.
>>>
>>> What motivates a person to collect micromounts varies from person to
>>> person, but the most commonly cited reason for buying micros is to
>>> temporarily fill a void in a type collection. It could be a
>>> petrologic type, a find from a given geographic area, a fall from a
>>> specific date, etc. Often a micromount is a temporary measure until a
>>> nicer specimen can be acquired, or until the needed finances to buy a
>>> larger piece can be saved up. For the very rare types and
>>> planetaries, a micromount might be the best hope for a collector on a
>>> restricted budget.
>>>
>>> There are a couple of schools of thought when it comes to dealing and
>>> selling micromounts - some dealers sell specimens by weight (by
>>> milligram, even for specks) or some dealers offer specimens by the
>>> piece (by eye/photo). For the most part, I am of the latter school
>>> that sells micros by the piece. That means I don't weigh each and
>>> every micromount, unless it is a very rare and valuable meteorite such
>>> as a planetary or historical fall. Each dealer has their own methods
>>> for handling micromounts and we those aren't really relevant to the
>>> discussion at hand.
>>>
>>> When weighing micromounts, one must use an accurate scale that is
>>> sensitive to 1 milligram - the good ones are used by diamond and gem
>>> dealers. There are many brands of these scales which range in quality
>>> and accuracy. When dealing with small specks that weigh a milligram
>>> or two, the readings can vary from unit to unit when weighing the same
>>> specimen. If a buyer pays for and is promised a micro that weighs
>>> 100mg, it better weigh 100mg and not 50mg or 80mg. Sometimes a buyer
>>> gets an added bonus because their personal scale is more accurate than
>>> the seller's scale and a promised 100mg micro might weigh 120mg or
>>> 150mg. If the seller is not sticking to a strict pricing scheme ($/g
>>> or $/mg), then ultimately what matters is if the buyer is happy with
>>> their micromount.
>>>
>>>> From a collector's standpoint, it pays to shop around for micromounts.
>>> Unless it's a very rare meteorite, it's easy to find several dealers
>>> offering similar-sized specimens for widely-varying prices. One must
>>> also pay close attention to the reputation of the seller and the
>>> provenance of rare specimens. Because micros tend to be small (some
>>> are downright tiny), it would be easy for an unscrupulous seller to
>>> misrepresent specimens as something more valuable than what they truly
>>> are. Chances are, if you are reading this mailing list, you are one
>>> of those people who can find a reputable source and who does their
>>> homework before sending payments across oceans on fiber-optic cables.
>>>
>>> My own personal meteorite collection (the pieces I keep in my cabinet
>>> and are not traded on my website) are mostly micromounts and I keep
>>> the majority of them stored in 1.25" gemjars with paper labels inside
>>> the bottom, under the foam. Some people prefer membrane boxes, small
>>> Riker boxes, or other storage and display methods, but that is the
>>> subject of an entire debate of it's own. The most commonly-seen
>>> container on the micromount market is the gemjar, and thus it is a
>>> general rule of thumb that if a specimen will fit into a gemjar, then
>>> that specimen could/should be called a "micromount".
>>>
>>> Best micro-regards,
>>>
>>> MikeG
>>
>>
>> --
>> Obama is not a brown-skinned anti-war socialist.
>> You are thinking of Jesus.
>> --
>> Add two grains of sugar to everything you say
>> And one of salt to everything you hear.
>>
>>
>>
>>
>>
>>
>>
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Received on Fri 01 Jul 2011 08:22:41 PM PDT