Author Topic: Homemade milligram/microgram scale  (Read 3172 times)

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Rhodium

  • Guest
Re: Homemade milligram/microgram scale
« Reply #20 on: December 13, 2001, 12:57:00 AM »
That's why I suggested pure silver, as we already know its density, and because it is comparatively expensive, the diameter should be exactly what they say it is.

notfman

  • Guest
Re: Homemade milligram/microgram scale
« Reply #21 on: March 16, 2002, 08:45:00 PM »
Excellent thread. For anybee interested in general science, membership in the society for Ameteur Sciene is available at $35 US per individual or $50 US per family.

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notƒman

¿Qué te parece? so...waddaya think?

ClearLight

  • Guest
Using Nanoliters
« Reply #22 on: March 25, 2002, 08:08:00 AM »
I picked up some calibrated nanoliter pipettes to use for calibration, after I humidified the interior, but I like the gold wire example better, especially if you use a digital micrometer to verify dimensions.

Infinite Radiant Light - THKRA

platcat

  • Guest
galvanometers
« Reply #23 on: July 22, 2002, 12:18:00 AM »
Sources are widespread for those guages.Poke around a auto junkyard for  for real low cost ones.speed shops as well. On a similar subject swip inherited a old
balance type scale that is calabrated in grains.Resolution is supposed to be less than 1/10 grain? can somebee convert that to metric for swip?  :)

"The crux of the biscuit is the apostrophe"FZ
Are we there yet?

terbium

  • Guest
15 grains in a gram.
« Reply #24 on: July 22, 2002, 12:27:00 AM »
So 1/10 grain is 6.67 mg.

platcat

  • Guest
grains to grams
« Reply #25 on: July 22, 2002, 02:08:00 AM »
Thanks, that makes this piece very usable to me.
:)

"The crux of the biscuit is the apostrophe"FZ
Are we there yet?

scarmani

  • Guest
Note
« Reply #26 on: July 30, 2002, 02:32:00 AM »

Re: SWIG might whip 2gether something with a PIC microprocessor... as soon as a decent galvanometer can b loc8d




A version of this scale was constructed using a Pic processor, breadboard, from scratch code, real time digital LCD output, two optosensors (for galv needle overload / underload).  Added features such as "tare" button, more rapid PWM, voltage filters, fast binary-search response to rapidly changing weight.

Cost due to enhancements was $150 plus many hours of time...but, in the end it does work as advertised.  The quality of the galvanometer is very important (and suitable ones are *very* delicate, bee careful  ::) ).  The galv-response is not really linear, so... making the sensitivity relevant, requires mucho calibration => nonlinearity adjustments in the code.

Measurements with final product matched those of a 0.1 mg analytic balance, with about 3x the resolution.  But despite a plexi enclosure, scale was more fickle with the slightest vibration or air currents.  The weighing range (~1 g max, with tiny foil tray) is not large enough to elevate the scale above novelty item status...  tho, that might depend on what one would hypothetically be weighing.


stop, drop & roll

Rhodium

  • Guest
Scarmani: Would you mind writing an article about ...
« Reply #27 on: July 30, 2002, 05:58:00 AM »
Scarmani: Would you mind writing an article about your scale project, with PIC code, pictures and all for my page?

Diafrag

  • Guest
does anyone have an idea how to make 0,1-100g ...
« Reply #28 on: September 30, 2002, 12:20:00 AM »
does anyone have an idea how to make 0,1-100g electronic scales? who knows, how does such scales work?

Rhodium

  • Guest
Like this for example: http://www.scaleman.
« Reply #29 on: September 30, 2002, 12:40:00 AM »
Like this for example:

http://www.balances.com/scientech/forcemotor.html



The reason noone builds their own 0.1-100g scales is that they only cost $50 or something like that...

jimwig

  • Guest
well let me see
« Reply #30 on: October 09, 2002, 03:15:00 AM »
I feel obligated to point out that the microgram scale upper capacity hardly qualifies as anything near useful. And pray why other than extreme inquiry would one want to measure the lung capacity of butterflies.

all together now - breathe in 1.....2....3.... and now out 4.....5....6... and hold it!!!!!
get back down here heathcliff..........and quit mutating..........

quantum

  • Guest
galvanometer's were do they come from
« Reply #31 on: October 14, 2002, 03:04:00 AM »
well after a little searching I found them in old
analog voltmeters and current meters.
dont know if these would be too sensative but there are
heaps of them around the place

e3500 console login: root
bash-2.05#

DxRtUcG

  • Guest
OTC Calibration
« Reply #32 on: October 15, 2002, 08:13:00 PM »
Why not go really OTC, literally. Get some OTC or prescription medications in gellcaps that contain no fillers, and use the net mg. weight of the medicines to calibrate. Prescription antibiotics usually do not contain fillers, so you could easily use the powder from various strength gelcap amoxicillin or the like. ;D

No really, I ...

Rhodium

  • Guest
That is not accurate.
« Reply #33 on: October 15, 2002, 08:32:00 PM »
That is not accurate. Even if there were no fillers, the amount of active ingredients in medications are allowed to fluctuate 1-10% depending on what it is.

You will get a much more accurate reading by weighing a large piece (a square foot or two) of aluminum foil, measure the dimensions accurately, and then calculate how big a piece you need to cut out to get a 250mg weight, a 10mg weight etc.

12345x

  • Guest
how about
« Reply #34 on: November 11, 2002, 01:17:00 PM »
get inert element that will desolve in alcohol
but will remain after evaporation

desolve 1 gm in 10,l flask
mix
draw 1 cc and evaporate in pan for 100 microgram
 
or dilute 1cc with 1 100ml
mix
draw 1 cc and evaporate for 1 micro gram

AgNO3

  • Guest
It is possible to use medicine.
« Reply #35 on: November 20, 2002, 01:15:00 AM »
It is possible to use medicine. I know that the gelcaps can vary by less than .1% of the total.

Rhodium

  • Guest
What gelcaps, and how do you know it?
« Reply #36 on: November 20, 2002, 07:17:00 AM »
What gelcaps, and how do you know it? Have you weighed them yourself on a 0.1mg scale, or just trust the seller?

hongito

  • Guest
Made it and impoved it!
« Reply #37 on: June 23, 2003, 09:13:00 AM »
I built this scale and it works. I improved the electronic circuit a bit so you can calibrate it to read the exact mg value on the display without converting it!

A picture of the new circuit:



I just added an operational amplifier, with the gain resistor you can adjust the amplification of the output signal to bring it to the correct level, like 3.04V for 30.4mg!

ning

  • Guest
I thought about that too....
« Reply #38 on: July 23, 2003, 04:00:00 AM »
And had some ideas. First, I don't think a microcontroller is the best way to go. Reason is, resolution * time are fixed: for PWM, to get more resolution, you need to chop the pulse period into more divisions, which forces you to lower the pulse frequency. At some point it will be too low. At least it will make the settling time of the feedback loop way too long. Most microcontrollers don't have a D/A at all, and those that do, it isn't likely to be better than 12 bits. I have seen 14, but it's rare.

That schematic is a nice beginning, but I think the current mirror is a good idea, for the stated reason. If the resistance of the coil changes in the current design, the calibration will go to hell. My only complaint about a current mirror is that the one shown probably doesn't have a high enough output resistance to support a 16 or 24 bit range, plus it will have base current error (due to it being made out of bipolar transistors). I would recommend an op-amp current pump (schematic later, I promise), degenerated by a high-precision, temperature compensated resistor. Such things can be bought or found. Maybe I can break out my Pease and see how an easy one might be constructed. It all depends how far you want to go. And anyway, why not go all out and close the feedback loop in analog?  I am certain it will be better and more accurate than any digital implementation. I've been thinking about how to do this for a while. Probably the easiest way would bee to mount a small LED on the scale end (too heavy?) and have two photodiodes, one above, one below. Then make a voltage divider reference, and put them as the inputs to an op-amp. Maybe one op-amp could do the transimpedance and current driving, I would use 2. That would be a dual package, still cheap...and then...use a chopper and feed it to your computer's sound card or something. *heh* That would be awfully interesting way of doing DAQ. This is probably too confusing...I will try to figure a way of drawing a schematic for everybody...sorry, electronics can be hard in words...


And here is one last idea for you: If you buy some wire, say  very pure metal, or metal of known composition (nichrome), you should be able to calculate its resistivity (ohms per cm^3). Then, if you measure the resistance of a precisely measured, looooong piece (wire, for example), you should be able to calculate very accurately its diameter, and thus, its mass. This, without a micrometer. So a little math can substitute for a little expensive equipment. Just make sure that wire is uniform...

Hope this helps

StraightEdge

  • Guest
why are lab balances so pricey?
« Reply #39 on: August 07, 2003, 03:23:00 AM »
Why, if this homemade scale is as good as its rumored to be, is an average lab balance so expensive? It would seem that the manufacturers of these things would constantly be undercutting each other until a fair price was reached, say $100. Is it because they are typically sold to universities and corporations, who have much bigger pockets than most individuals? Or is there another reason i am not seeing?