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Author Topic: Weight sensor sensitive to grains? (15.42 grain = 1 gram)  (Read 3219 times)
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I am looking for a weight sensor that can sense grains within 1/100 of a grain.  I'm measuring gun powder, and it needs to be accurate.  All the ones I'm finding are sensitive within a gram or two, but that's 15 to 30 times on grain, which is 100 times my tolerance.  So 1500 to 3000 times my tolerance is not acceptable. I know they make them, because they make digital scales to weight the powder, but I can't find a data sheet on any of them to find the type of load cell/sensor they used.  Any ideas?
« Last Edit: June 02, 2012, 10:28:54 pm by hscriber » Logged

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So you want to measure mass in increments of a few hundred micrograms.  What range of mass do you want to measure?
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Grains.  1 gram = 15.42 grains.  subject is backwards, sorry.
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I've seen digital scales that will measure up to 100 gm's... I am thinking that zeroing the cell out at that resolution might be a tough task though. My thought was to start with a load cell in the approximate range and change the gain of the scaling amplifier/buffer that drives the A/D conversion done in a commercial scale. Since, however you are getting close to the edge of that kind of approach, have you looked for load cells yet that are capable of 10 - 20 gms max? and built your own interface... The real trick I should think would be the thermal drift. A Load Cell is a wheatstone bridge type of affair (measures the imbalance or 4 resistors connected in a ring structure) sometimes requires temperature compensation for accurate stable operation though. Nice thing about it is that if you use either ac or dc excitation modifying the response for temp comp shouldn't be more than a summing amplifier added to the A/D input of the measurement device (readout). IMO

Doc
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Here's a company that sells high-precision (100 microgram, 10 microgram) weigh cells:   http://www.sartorius.com/en/products/laboratory/oem-weigh-cells/basic-oem-weigh-cells/

They don't list a price but they sell a desktop scale with the same accuracy for about $2000.
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How about something like this :
http://www.ebay.co.uk/itm/New-Mini-Electronic-Pocket-Digital-Gold-Jewellery-Weighing-Scale-0-01g-100-Gram-/160814070504?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item257143fae8#ht_2918wt_1182
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That's the one I was referring to.. And for load cells, there are available cheaper ones... There's one in that scale...

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Thanks, that looks promising.  Any other ideas?
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As a fellow reloader who has looked into digital scales and wonder why they are so much?  I have thought about buying load cells and trying to implement a digital scale using Arduino or some other MCU to read the cell and display the results.  Load cells are available through Mouser, DigiKey and others, but buying them one at a time will kill your budget.  Once you have the load cells you are going to need to get an instrumentation amplifier chip or make one up using two high quality Op Amps.  Depending upon the dynamic range required you will be probably need better resolution on the analog to digital convertor than the one on Arduino so you will have to do the analysis to see if you need a 12-,14-,16- bit ADC or higher resolution.  One channel of the ACD can probably be used to read a high-accuracy temperature sensor so you can make adjustments to calibration based upon temperature.

Once you have the pieces together that you paid through the nose through as you bought them in low quantities you then have to do the PCB design to get a low drift scale.  Some designers of digital scales say low drift and load cell cannot be used in the same sentence.

This is how I solved the problem I bought a tuned balance beam scale from a guy called Scot Parker, search 6mmBR.com for his contact info if you desire.  I can see needle movement on the scale from one granulae of powder and more importantly it is repeatable.  I have my scales on a piece of 4 inch thick 2 foot x 3 foot piece of granite to help isolate it from ambient vibrations.  I have reloaded for over 35 years and have taken a lot of time over the years thinking about these things.

Maybe this helps, maybe it doesn't but this is how I do it and a little background on load cells.

wade
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