How to use load cells, weighing very small mass - Page 1
Forum section: electronics / projects

author: ptricks
author_rank: Frequent Contributor
timestamp: November 06, 2019, 04:02:48 pm
content: I need to either build or purchase a scale for weighing small amounts 1-50 milligram. The retail scales I see on places like amazon seem to be okay down to about 500 milligrams.I have never used load cells before and am not sure they are the best option or if there is another tech , resistance based, capacitance ?We have a dog that requires medication that is not sold in her weight so we have to cut the pills and the pills shatter no matter how we cut, tried all the pill cutters. The dog is epileptic so if the dosage is off because the fragments we use are not enough she will still have seizures. There is a liquid version of the drug but it is 20 times more costly . The vet is at a loss on how best to do it except if there were a way to reliably measure the dosage of the cut pills, which is why the scale is needed.I know that industry has to have a way but that is probably expensive compared to what I can afford so hoping for alternatives.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: November 06, 2019, 04:17:19 pm
content: https://www.amazon.com/milligram-scale/s?k=milligram+scaleThis one is 0.001g accuracy:https://www.amazon.com/Homgeek-Milligram-Calibration-Tweezers-Weighing/dp/B071GYVQF3

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2019, 04:21:40 pm
content: There are relatively affordable (e.g. $20-50 range) small electronic scales available, made for something like coins. Going down to 1 mg resolution would be tricky though - best chance there would be a used scientific one, maybe an old style mechanical balance the weights.Building a mechanical balance is also possible DIY - so no electronics. Small range weights can be cut from wire reasonable good.For a DIY solution one could consider using the magnetic compensation - so the sample force is compensated by a mechanical force. There are a few instructions around to reuse an old analog meter movement for very tiny masses (e.g. a hair) - so may be 1 mg full scale. For the larger range there are DIY instructions based on a speaker.Another option is using a suitable spring (e.g. a strip if thin metal) and detect the deflection the capacitive way.Measuring with high resolution usually need an draft shield.If the medication can be dissolved in water - this may be an easy way to divide things down to very tiny fractions.

author: ptricks
author_rank: Frequent Contributor
timestamp: November 06, 2019, 04:35:45 pm
content: Quote from: GeorgeOfTheJungle on November 06, 2019, 04:17:19 pmNot accurate at small doses. If you read the reviews the lower the mass the less accurate it gets. The majority seem to be that way, probably all using the same internals.

author: ptricks
author_rank: Frequent Contributor
timestamp: November 06, 2019, 04:39:04 pm
content: Quote from: Kleinstein on November 06, 2019, 04:21:40 pmNot sure how I would use the analog meter as they are so delicate in the movement but I can also see the benefit to using something that easy to move.The speaker is an idea if I could find one that has the right characteristics.Metal and the deflection is another idea.Thanks

author: beanflying
author_rank: Super Contributor
timestamp: November 06, 2019, 04:45:57 pm
content: My experience with smaller cheap electronic ones is they are accurate but dislike really small weights initially so for small measurements start with a cap of 5-10g then add your cut pills in your case for a better result but I regard sub 0.1g with suspicion for accuracy with them.BUTFor a better result a set of calibration weights and a small beam scale will be more accurate/trustworthy IMO. There should plenty them available secondhand on evilbay. Even the cheap Chinese calibration weights and a secondhand beam might be an option. Not shown is the mg sized weights in the box (under the glass in separate sections) but they work well.For small weights the beam is now my go to device for accuracy but certainly not speed.

author: mzzj
author_rank: Super Contributor
timestamp: November 06, 2019, 04:51:48 pm
content: Quote from: ptricks on November 06, 2019, 04:02:48 pm1 milligram is tricky with the cheapos, 50 milligram is probably ok.50mg +-2mg would be off by 4%, probably more than adequate for the purpose.Scales with 0.1mg resolution start around 300usd , scales with 0.01mg resolution start at around 2000 usd and scales with 0.001mg resolution start at around 20000 usd so its important to decide what accuracy you actually need..

author: ejeffrey
author_rank: Super Contributor
timestamp: November 06, 2019, 05:57:43 pm
content: Is 1-50 mg the total mass or the desired uncertanty? At very low readings you might be seeing the effects of an auto-zero / drift compensation feature. Preloading the scale with a few hundred milligrams should prevent that. The 10 mg resolution scales from amazon seem to work really well, but I don't know about the 1 mg resolution scales. At that range you also need to start worrying about shielding from air currents.One alternative: if there is a liquid version available maybe you can make your own by grinding up the pills and dissolving them in water or oil? You would need to find out what solvents work on the particular drug. That would make your measurement job much easier.Finally, I am sure you have already done this but definitely shop around for the medication. Our dog is on keppra, and the difference between Costco with a coupon from goodrx vs. retail price at CVS is about a factor of 5.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 06, 2019, 06:13:41 pm
content: Better just buy a balance and treat the dog. Developing your own load cell based balance will take a lot of time and anyway will cost you way more than just ordering a ready made balance. A small pocket one will cost about $10-30.However, if you want to fiddle with microgram DIY weighting devices (not load cell based):

author: Bud
author_rank: Super Contributor
timestamp: November 06, 2019, 06:28:33 pm
content: Can you dissolve a pill in water and then divide the water to parts so each part would contain an equivalent dose of the medication ?

author: KaneTW
author_rank: Frequent Contributor
timestamp: November 06, 2019, 06:38:45 pm
content: Buy a used analytical balance off ebay. Load cells aren't super useful for low amounts and getting a force compensation cell right is incredibly difficult.I got a 0.1mg/220g balance for 200EUR a while back (with recent calibration, even), which is admittedly an insanely good deal. Much cheaper than a new scale of that type for several grand.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 06, 2019, 08:14:49 pm
content: An old single pan Mettler balance can go pretty cheap but be sure they know how to lock it up and pack it, and that you know how to unlock it.

author: ptricks
author_rank: Frequent Contributor
timestamp: November 07, 2019, 06:30:22 pm
content: The drug is phenobarbital , 24mg is the dosage but pills come in 15 and 30mg. It is one of those drugs where 2mg either way can do nothing or do harm. It isn't soluble in water , pediatric dosages used ethanol but not going that route , just too iffy.I have been able to cut 30mg into quarters so roughly 3 pieces is 22.5mg but if the cut isn't perfect and she gets less than that she seizes and a really sad sight ,she just stops walking, stares, then starts jerking with her mouth clenched open and falls over convulsing, last about 2 minutes , then she opens her eyes and stumbles around, doesn't want to be held or touched for about 5 minutes.I,m going to see if I can get a beam type scale until I can find another solution. Me , the SO, and our dog Charlie really appreciate all the suggestions.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 08, 2019, 04:20:39 am
content: I just weight our dog's phenobarbital. His pills are 64.8 mg dose and each pill weighs approximately 330 mg, so about 4:1 inert ingredients. Assuming the 30 mg pills are similar composition (weigh yours first!), each one is going to be approximately 150 mg, and your target dose is 120 mg gross. So that works in your favor, getting 120 mg +/- 5 is going to be easier than 24 +/- 1.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 08, 2019, 04:04:23 pm
content: Can you go to a compounding pharmacy and have it done (manipulated) in the dosage you need? This is very common here in Brasil. Sometimes is cheaper to us to get drugs this way.