How to measure a quantity of grains for an instant coffee maker project?

Main Question or Discussion Point

Hello everyone !

I have to built a coffee machine with a coffee tank (instant coffee) and a sugar tank. I'd like to measure a certain quantity of those grains.

First, I wanted to make a physical model to know the debit of the grains. But I noticed that it depends on many things like the shape of the tank or the diameter of the aperture compared to the average size of a grain. So I abandoned this idea.

I think that it could be possible to know a quantity of grains with a weight sensor. Let's say if we know the average mass of a grain, if we open the tank with a solenoid valve (I don't know if it's quite good for grains) and with the weight sensor when I reach the quantity I want I close the tank.

So that's my idea but I've a few questions. First, Is it the best way to do it ? If you've some ideas tell me them, please. Also are weight sensors damaged with water ( humidity in the air) ? And do you know a website where could I see and buy some references of weight sensors ?

Thanks in advance.
Regards :)
 

Answers and Replies

Tom.G
Science Advisor
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An auger, gravity fed by the bulk granular material. Dispense volume determined by amount of auger rotation. For complete emptying and ease of cleaning, the bottom shape of the container holding the bulk material should be steeper than the natural angle of repose of the material.

Cheers,
Tom
 
An auger, gravity fed by the bulk granular material. Dispense volume determined by amount of auger rotation. For complete emptying and ease of cleaning, the bottom shape of the container holding the bulk material should be steeper than the natural angle of repose of the material.

Cheers,
Tom
Quite interesting. I've seen that point that granular media have a specific angle of repose. For the shape of the tank, according to what you said, do you think that it'll works with a shape of a funnel ? Also, how do you place place the auger ? Vertically or horizontally at the bottom of the tank ?

Also to calculate the number of rotations that I need, tell me if that's correct. My tank's aperture has an area S. So I need to determine the quantity of grains that I can put (flat) on this surface on the auger. Then with the pitch of the auger I could know the number of rotations that I need... Is it correct ?
 
Tom.G
Science Advisor
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do you think that it'll works with a shape of a funnel ?
Sure, as long as the taper is steeper that the angle of repose.

how do you place place the auger ? Vertically or horizontally at the bottom of the tank ?
Horizontal is the way I've always seen them. Though you could maybe figure a way to make vertically work.

Also to calculate the number of rotations that I need, tell me if that's correct. My tank's aperture has an area S. So I need to determine the quantity of grains that I can put (flat) on this surface on the auger. Then with the pitch of the auger I could know the number of rotations that I need... Is it correct ?
So I need to determine the quantity of grains that I can put (flat) on this surface on the auger.
No, not needed. The opening size from the reservoir to the auger allows continuous filling of the auger. You just need to control the auger rotation. One minor drawback, depending on your detailed design, the first dispense after an empty reservoir may be small.

An alternate approach to an auger is a hollow cylinder replacing the auger. Cut a lengthwise slot in the cylinder wall to allow the coffee in. Rotate the cylinder 180 degrees to dump to the destination. You can vary the volume with a piston inside the cylinder. The piston position can be controlled either with a motor or a user knob.

Cheers,
Tom
 
jrmichler
Science Advisor
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There are gadgets on the market that do exactly that. They are used to measure gunpowder by people who reload their own ammunition. They are called powder measures, and should work for any non abrasive free flowing powder, including coffee. Here is one example: https://www.midwayusa.com/product/1016832532?pid=803372. Coffee is a granular solid of similar particle size to gunpowder, so a powder measure should work very well with coffee.

For people with extreme measurement accuracy needs, there are also powder scales, and tricklers. Set the powder measure on the light side, dump onto the powder scale, and feed in the exact amount with the trickler. To find these, search reloading supplies. Cabela's and Midwayusa are both good outfits.
 
DaveC426913
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Also are weight sensors damaged with water ( humidity in the air) ?
Not if it's purely mechanical.

1572569354203.png

A simple apparatus akin to a balance scale can be set to the desired weight.
Heck, you don't even need two arms. One arm and a spring will do it.

When sufficient coffee is in the hopper, it descends and presses a switch (electrical or mechanical), shutting off the flow of granules, and triggering the next step.

Makes no difference what size or mix of granules you have.

K.I.S.S.


[ Edit ] Or what jrmichler said... :sorry:
 
sophiecentaur
Science Advisor
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An auger, gravity fed by the bulk granular material. Dispense volume determined by amount of auger rotation. For complete emptying and ease of cleaning, the bottom shape of the container holding the bulk material should be steeper than the natural angle of repose of the material.
I know of this method by the name of an Archimedes Screw. It is great for volumetric dosing. Dosing by mass is a lot harder and needs some sort of (sensitive) active control like the system hinted at in the above post. Unfortunately the strength of a cup of coffee depends on the mass of added granules. OTOH, when we make a cup of instant coffee, in the regular way, we use a spoon and volume measurement. So give me Archimedes Screw any time. 360° of rotation for one spoonful.
 

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