Building a scale with a load cell - Physical platform?


#1

I’m building a scale to measure the weight of a keg (~5 gal, 28kg full). The plan is to measure the weight as beer is poured and therefore estimate the remaining. Currently we’re using a flow meter, but if a pour is very foamy, the volume that comes out doesn’t match well to the volume of beer. Plus, we get compounding errors where each erroneous pour adds to the overall error of what’s left in the keg. If we simply weighed it continuously, we’d get a much better measure of remaining mass.

Anyways, I’ve been reading a lot and found a 30kg load cell that should do the trick. However, I have no clue on how to build the physical platform. It looks like the preferred way to do this is to mount a platform (wood?) both above and below in a “Z” configuration so that it torques and deforms the load cell when weight is applied.

Has anyone built anything like that? Can I get something 3d printed? Also, am I better of going with several strain gauges and wiring that to an amp?

Basically, I’m looking for advice on how to build a scale for the 5-30kg range that’s accurate to roughly 30g, which would be ~ 1oz. Should I just buy a digital bathroom scale and take it apart?


#2

Hi @micahwedemeyer

If you don’t want to build something you can just just buy a 30kg industrial scale with RS-232 output like this one:

If you want to build something, I think 30kg is too heavy for 3d printing. I would try to design a platform that was hinged one side and pressed on the load cell on the other side forming a triangular support. The values on the load cell will be 1/2 of the weight buyt you don’t need anything fancy to hold the platform up.


#3

$285…ouch! I’m trying to keep this in the <$75 range, especially since I’d actually need 2, since we have 2 kegs.

Thanks for the tip on the triangle setup.


#4

I’ve been playing with the HX711 board to read load cells.
It works quite well once I slightly tweaked the HX711_ADC library.

I’m using four single load cells from “deprecated” baby scales (4x 5kg 3-wire cells with combinator shield), but you can use higher weight cells or a single bar cell (4-wire) too.
So your bathroom scales idea is pretty much what I have for less weight higher accuracy.


#5

Hi, the changes to the HX711_ADC library you mention - are those incorporated in the current version of the library?
Thanks!


#6

That’s quite some time ago, so I’d say that the library should have that in already.


#7

that’s what I thought. Thank you!