thunderfvck
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Cheaps scales vs. Pricey scales
Yes! So I found this REALLY cheap scale (0.01 g, 50 g max) on ebay...
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=1...
And I was wondering if the cheapness of this scale would in anyway affect the actual accuracy of the scale. I've seen other scales similar to
this one go for $60, for 50 g max. For 100-200 g max, I've seen them go for $100. And THEN I've seen the biggin ones (the ones normally in
school, not pocket sized!) that have the same accuracy (0.01) for like $250! I imagine that they hold quite a bit, however. So what's the deal, I
may be able to get a big ass one tomorrow for $150 (0.01g) but would it really be worth it? Or am I better off spending $40 on this little piece...?
And one more thing, these calibration weights. Are these for real? Like, can you calibrate the scale using one of these weights for up to 0.01 g? Or
is it only for 50 g flat, or whatever? Because it would seem pretty silly to have a weight that's EXACTLY 50.00 g, what if it gets chipped or
something? Then I guess you're screwed...But are they this accurate in mass?
Thank you!
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Hermes_Trismegistus
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Grandaddy always said...whatever you do....Invest in good tools from the beginning......
Not neccesarilly the most expensive, but GOOD tools.
With mechanical balances, they have had several thousand years to perfect them, with electronic......twenty minutes!
All of them basically boil down to one thing, squeezing piezio crystals and measuring the output of electric current. Perfect on paper, in
reality........UGH!!!
I used to be in a business where scales were needed as a matter of course, all the time, all sorts of places, and accuracy was a matter of profit and
loss, and maybe life or death......and I went through dozens of them trying to find a decent one.
I spent alot of money finding out that the best investment for good portability was those pen shaped pull scales and they were only good for about
three months and only anywhere near accurate in the very middle of their range. AND the only ones worth their salt were made my "gem" AND
that "Gem" pull scales are frequently counterfeited. It doesn't take much experience to notice the difference between them...but
that's another story.
The reason they are giving you a fifty gram test weight on that scale is because of the way scales are calibrated.
Electronic scales are set to be accurate to a certain weight.(it has to do with the internal sensing apparatus
1. if the weight is at the top end of the range, the scale will be more durable, but horribly inaccurate near the bottom end.
That is the scale that you are looking at. It will reliably give you 50g when you drop on the calibration weight, but if you try to weigh out a few
dozen 1/2 gram piles of.......whatever, they will be totally different in size and only by serendipitous accident will they actually BE 1/2 gram.
2. they can calibrate it to accurately weigh out 1 gram and have the published capacity of 50 grams, but the 50 gram piles will be wildly imprecise
and the scale will be extremely sensitive to any abuse at all.
3. they can do it in the middle and get the best(and worst) of both worlds.
Any way you do it, electronic balances are very sensitive to manufacturing defects and just plain go to shit after a while.
So, unless you are willing to shell out the big bucks for a professionaly covered analytical balance (and don't even think about buying one used)
GO MECHANICAL! There is a reason most of the big diamond houses still use beam balances.
There are two models, both of which have been sold for over 50 years with no change but the paint colour. Both are world famous, both made by Ohaus.
The triple beam is good for larger quantities and is best for 1/2 gram to a few pounds.
This one I will show you underneath is probably the best for you, you can easily find several on ebay right this minute, they are literally bullet
proof and will last for well after you are dead and will be just as sensitive as when you bought it!
It is the ohaus Cent-O-Gram (named back in the 50's I think)
The only problem you could ever have with it is that someone borrows it and never returns it, it weighs up to 3/4 of a pound at 5 milligrams
accuracy., that is as accurate as you will ever get without needing to try and protect your balance from the interfering effects of body heat.
(seriously)
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Hermes_Trismegistus
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P.S.
Most people used coins to verify the accuracy of the other guy's scales used in the trade.
a shiny (canadian) dime is ~2 grams
our penny is 2.4 g
our nickel is 4.6g
our quarter is ~5 g
our loonie's are ~ 7 g
and twoonies had just come out and used to vary in mass too greatly to be trusted, but it has probably gotten better now that they have been making
them for a longer time.
as you might have noticed, a quarter and a dime equal a quarter  (a quarter
ounce), and a loonie also equals a quarter.
those are good ones, other good ones were cigarrettes, the export "A" company was particularly good at consistency in mass and a brand new,
regular size, straight out of a freshly opened pack export "A" green (green death) weighed either .7 or .9 of a gram, I think it was
.9.....but it's been a long time, but it weighed the same as a dime(bag).
John players special were also good because they weighed either 1.3/1.4.....(long time, bad memory)
Also a brand new 20 dollar bill, weighed a certain amount but it must have changed since they changed the bills over.
anyway......good luck!
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thunderfvck
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Most excellent reply, Hermes. You have convinced me to go with the balance. I had no idea these digital scales can crap out on someone over time. Well
I had a slight idea, hence the calibration weights, but I didn't think it'd be a big problem.
And the cent-o-gram is so cheap! Ebay, like $10-15! For the percision these things give it's hard to pass it up.
You mentioned that body temperature is enough to offset the percision of a measurement when trying to narrow it down to 5 mg. This can actually be
done? EVen though the scale goes to 0.01? How does this work?
Thank you for your most excellent reply, you have saved me from a future of inaccurate deaths.
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Hermes_Trismegistus
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10-15 bucks!!!
I think those must be in the beginning of the auction, they are very common,but also very much in demand, I think you'll end up paying a little
more than that because other people will bid for them.
also remeber shipping, smart people disassemble them for shipping (ther are only like 6 parts) but it still weighs like ten pounds!
As to the body heat thing, YES. For mechanical balances, the metal can absorb a little radiated body heat (remember that your body throws off at least
as much as a 100 watt light bulb), on one side of the mechanical beam/tray and that stretches the scale slightly and throws the calculations off
slightly.
At the peak of the art, beam balances were being used to measure thousandths of a milligram in analytical chemistry, the cup and pivot would have to
be made out of ruby, they would use diffraction gratings to measure They would try tomake almost all parts from the same metal so that the rate of
expansion was almost the same and then just wait for the temp to equalize across the whole scale (room temp)
and they'd have to set up a small spy scope from ~15 feet away to look at the final measurement.
That was turn of the century high technology.
It was followed by millikan's electrostatic oil drop scale that allowed the mass of a single molecule/atom/electron to be reliably measured, but
the capacity of this scale was low.........and that's another story.
anyway, the huge number of different parts and materials in an electronic scale makes them even more affected by differences in temperature. (all the
materials have different expansion co-efficients)
also it's just plain difficult to make a perfect piezeo mass sensor in the first place, growing a bunch of molecularly flawless crystals (on an
industrial scale) coating with a flawless sheet of metal on either side, soldering wires to them, and then expecting the flex you measure to neither
change not ever set up an internal/peripheral instability.......Nah!
even the several thousand dollar lab scales only have a limited service life, and the way they get them so accurate it to buy large amounts of the
sensors and test the crap out of them and harvest only the seemingly perfect ones, reselling the rejects to second and third hand aftermarket
manufacturers to put into handhelds.
the other thing those pricey ones do is try to constantly adjust for the natural change in the crystal output, but the ability to cope only goes so
far.
The only reason anybody uses them is convenience.
With the old mechanical scales, a large lab/pharmacy might employ one or more steady handed and skilled boys whose only job was to sit on a stool and
weigh out stuff into little "weighboats".
That's how the weighboat system came about, sort of like a diner, put in your orders and collect them from the end of the counter when the job
was done.
Pharmacies used scales with little tilting "repeater" weighboats, you'd dial up a certain weight, you'd tap the powder in and it
would tell you when you got close on an over/under guage and you'd just tilt it out into a little paper about the size/shape of a rolling paper.
You can still find them around someplaces as novelties and while they still work, they are contaminated within by god knows what and in any case they
usually didn't weigh past 1000 milligrams.
within a little time you will get adept at weighing out, a hint is to use a pin/edge of a card to move the smaller slide weights, and to lift from the
bottom and then slide them along.
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Organikum
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Hermes is basically right.
Get two mechanical beam balances - one up to two or five kilos and one up to a fifty or hundred grams.
Get simple ones which are supported on prisms with a simple damping mechanism.
Important: Get a NEW or newly calibrated set of weights for both - boxed in a case and treat the weights with care. Every weight should be present
twice at least - this way you can always easily check the accuracy of the weights and the balance.
The small weights have to be handled with tweezers - those come with the set usually but better are tweezers made from ceramics for static sensitive
electronics - not so expensive but VERY useful as the small weights tend to cling to metal by statics - nervy.
One beam layered on a prism, two scale pans/hooks - perfect and cheap.
Those cheap electronic "dealer style" pocket scales arnt worth shit.
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thunderfvck
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You actually have to calibrate a mechanical scale? I thought it wasn't necessary...This may seem stupid, but I didn't know!
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Organikum
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You dont know what kind of scale I am talking about it seems - doesnt matter, here we go, a picture tells more than thousand words (tenthousand for
Hermes  )
For those you will need weights - thats understood.
This an example of the Ohaus principle, which is easier, but can hardly be as accurate as the above:
Here you see the principle of the Ohaus with moveable weight:
Why I prefer the non-Ohaus scales? Easy: Because I usually weigh stuff in a container or on a plate or in a bag or just like to put a piece of
plasticfoil beyond the stuff I weigh - for protecting the plate and easier moving to the rection flask, not having to clean something but just
dispense the foil - I am lazy....
So I either put the same piece of foil on the other plate for tara or I put the container on one side and equal it out with some table salt before
filling and weighing it. Fast. Easy. Lazy. Thats it.
Hope this clears it up.
[Edited on 19-4-2004 by Organikum]
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Hermes_Trismegistus
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| Quote: | Originally posted by thunderfvck
You actually have to calibrate a mechanical scale? I thought it wasn't necessary...This may seem stupid, but I didn't know!
|
Not a stupid question at all, all weights, measures of length, volume......etc, need to be calibrated initially, and if you have extreme need for
accuracy, they need to be double checked on a regular basis.
With the "stationary" weights that are used to calibrate double tray scales, thin layers of oxide can build up on the outside and increase
thier weight.
They are normally made of brass to minimize this effect as well as minimizing the effect of friction wearing away the bottom.
With the ohaus scales, the sliding weights are initially calibrated, as well as the length of beam required to mass the sample.
So, over time (a LONG time) the beam can wear away underneath the friction of the weights being slid back and forth and the bottoms of the wieghts
themselves.
For this reason, if you needed pinpoint accuracy those double tray scales can be more accurate over many years.
However they pay for the greater precision with increased sensitivity. It is related to two things, inertial mass, and length of beam.
Ohaus scales are very "lazy" (which means they are resistant to the pendulum effect and to the effect of minor drafts to some extent)
Those more sensitive ones can take a LONG time to stop jiggling and be-bopping around and require a draft shield.
When doing multiple or repeated measurements, that can be a HUMUNGO frustration and make you want to go back to pocket electronics.
The ohaus scales utilize the paramagnetic dampening effect to make them so "lazy". There is an aluminum "blade" on the end behind
the pointer, you wouldn't really notice it in normal use, neither would you normally know the function but it was a great leap forward in its
time as a design concept.
The aluminum (non-magnetic) blade is sandwiched between two masses of iron and the induced feild makes the scale want to stop moving (but not really
care where) so it settles down faster.
This is one reason why the minumum capacity can be only 1/200th of a gram (5 milligram) and the precision is ~0.5 milligram on as much as half a pound
or more.
I agree with Orgi, that the others might be better for hotshot measurements, but as a workhorse, you couldn't beat the ohaus with a stick!
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thunderfvck
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Thanks guys, I appreciate your replies.
I do think I will be sticking with the ohaus though. The bid closed at $42 
However, I wasn't the winning bidder. Some poor sap probably had his max bid at 50 or something, mine was at 40. I hate when that happens.
Anyways, there's another one going...So I'll be getting that one hopefully.
All that aside though, am I making a good decision? I am not terribly 100% concerned about the accuracy, but I don't want it to be as bad as it
seems to be with the cheapo electronic scales...How much error would you say is associated with an ohaus? And with the other non-ohaus?
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unionised
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I never checked,bt I thought those cheap electronic scale were based on strain gauges rather than piezo transducers.
The balance I use in the lab at work is electronic and measures to 0.01mg over a rance of 210g. They may only have been making electronic balances
recently, but they have got quite good at it.
(and the microbalance reads to 100 ng; its's tricky to get calibration weights for it)
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Hermes_Trismegistus
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ESNIPE !
http://www.esnipe.com/
I told you those scales are in demand didn't I? I knew the bidding would be fierce!
People grow to love them and they have a legendary reputation for long term durability.
my 120 dollar electronic balance would start out being about a 0.1 gram high or low, and over time slowly build up to about a 0.25g high/low and then
get to about 1/3g high/low before REALLY starting to get wickedly wacked out, I would try to replace it at about the .25 inaccuracy mark (two months
of intensive use and transportation and temperature extremes)
My current quadruple beam is at least 15 yrs old, has changed hands several times, and gotten banged around and dropped from high up on various
occasions.
I had to adjust it once by putting a tiny drop of lead under the sample platform (another benefit of those simple mechanical scales is the ease with
which you can repair and adjust), and now (although It's hard for me to be confident in my ability to test it) I am quite confident massing to
two milligrams-0.002 grams with proper protection from drafts.
Better luck next time!
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thunderfvck
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Yep! Well I'm actually bidding on another one. I'm using SnipeRight, they have their own server that places the bids. It has a free 20 day
trail and is the best sniper I've seen (I've only tried two ). But
the fucker outbid me, so...ANyways, this one claims to be "new", though it won't be as new as the one I just lost! That was pretty much
brand spanking new, I wanted that one like the cherry of a fresh virgin. But oh well, I'll settle for this one. It's at 12.50 now, no
one's made a bid probably because there's no picture and the guy has a 0 feedback score. I'll take my chances. What have I got to lose
besides a few bucks, anyway. I had also just bought a book on ebay for something like $12, retail price is somehting like $100. Man, I LOVE ebay.
How are you able to measure in mg quantities? Like, the scale measures to 10 mg, how do you break the barrier?
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Hermes_Trismegistus
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| Quote: | Originally posted by unionised
I never checked,bt I thought those cheap electronic scale were based on strain gauges rather than piezo transducers. |
I guess it depends which ones you use, I haven't really kept up on the state of the art and so I guess some use the wheatstone bridge strain
gauges and some use a push/pull piezo system. I wonder which is more accurate?
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Hermes_Trismegistus
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| Quote: | Originally posted by thunderfvck
How are you able to measure in mg quantities? Like, the scale measures to 10 mg, how do you break the barrier? |
Same way you measure between graduations on a thermometer or pipette (or trying to figure out exactly what the hell your girl is REALLY trying to say)
Read between the lines, remember that there is a full 6 inch beam representing a single gram, so the 100 grad. lines are a fair
distance apart, but when I measure really small masses, I always add something reasonaby heavy like a couple loonies to the tray before zeroing it
out.
In this way I have determined the wieght of the styrofoam packing peanuts I got in my last shipment to be suprisingly reliably 125 mg +/- 2 milligrams
in mass.
EDIT....I also use a small hand magnifying glass to estimate, estimating is much easier when a 1/8th inch space looks as wide as your thumb
[Edited on 19-4-2004 by Hermes_Trismegistus]
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thunderfvck
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haha
Yes, of course! This would seem more obvious to me if only I had my scale. Or maybe if I did a little thinking before asking questions...!
But adding the loonies, and "zeroing out"...? I know that by zeroing out you're talking about the pin going inbetween the two line
things, to the zero. But why do you add loonies?
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Organikum
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Most electronic scales use an capacitor made of two plates which are fixed in a special shaped Al-block so they are kept in parallell when under load.
I have never seen piezo transducers or strain gauges used.
This principle is also used in very expensive lab-scales whereby there is a lot of shielding, temperature and humidity compensation and
self-calibration added so the real weighing mechanism is hard to detect. Therefor many ppl believe the scales work on what the heck ever principles.
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