Hello list,
we are looking for a solution for measuring the amount of potatoes in a
bin filled with water (analog measuring). All potatoes are below the
water surface.
The water level in the bin will be held at a constant level.
The bin is about 10m3 big
Has anyone a suggestion or has anyone experience?
kind regards

I have no experience, but I do have thoughts. First, what is the
density of potatoes vs. water? IIRC they don't float, but that's all I
can remember.
If they are significantly more dense and you are really keeping level
well controlled (or if you measure it well enough) weigh the bin -- you
should be able to calculate the extra weight due to the spuds.
If they just kind of drift down to the bottom a density-based measure
won't work. But if the water is clear enough, how about an optical
measurement? A minimal solution would be to shoot some laser beams
through the water and look for interruptions -- this would only work if
the water is quite clear. If the water is turgid enough to scatter your
beam, but not too muddy to stop light altogether you could immerse a set
of sensors into the water and illuminate it -- the sensors that are
blocked would indicate the presence of those opaque spuds.

At least you don't write "the number of potatoes"; that would be very hard.
Potatoes sink in fresh water. With the water level held constant, the
weight of the bin and contents will increase linearly in proportion to
the amount of potato. It should be easy to calibrate if you can weigh
the bin with enough accuracy.
Jerry

If you start with no potatoes in the bin and hold the water level
constant then you could measure the amount water you need to remove
from the bin to hold the water level constant as you add potatoes to
the bin. With the volume of water removed from the bin being equal to
the volume of potatoes added to the bin you can then calculate the
weight of the potatoes in the bin.

Umm.. not quite. It's a nice idea though.
Unless the density of potatoes is the same as that of water, what you'll get
is the *volume* of potatoes - not the weight.
Cameron:-)

Maybe be try load cells on the the container?
Weigh the water
Weigh an "average" spud
Subtract the water from the total weight and
divide it by the "average" spud weight.
sQuick..

In the physics class I took we were told weight equals volume times
density. So you don't care if the density of potatoes is not equal to
the density of water. All you need to know is the density of
potatoes.

I think you missed part of the point. From "the volume of water removed
from the bin" ... "you can then /calculate/ the weight of the potatoes
..." (Emphasis added.)
Jerry

Measuring the weight will give a to rough reading we think. The density
of the potatoes is to close to the density of water and the density is
varies.
The reading doesn't have to be that accurate if it is lets say 10 %
accurate it will do.

Measuring the amount of water added and removed to and from the system
is not possible however while the potatoes are fed to the system
together with water. The level in the bin keeps at constant level it
just overflows

The level won't stay constant if the control is overflow. The exit
sluice is essentially a weir, so the level will depend on the flow rate.
Perhaps you can do something with absorption of radiation of a suitable
type. Ultrasound? RF?
Jerry

The following is a ball park estimate of the praticality of using a
load cell to measure the quanity of potatoes in a tank.
A google search indicates the density of potatoes is 1.6 grams/ml. So
when you add a 100 pound, 45.4 kg, bag of potatoes to the tank you
will displace:
Volume = weight / density = 45.4 kg/ 1.6 kg/l = 28.4 l of water. Or
in other words every time you add a 100 pound bag of potatoes to the
tank you add 17 kg, 37.4 pounds to the weight of the tank. You also
said you had a 10 cubic meter tank. When full with water this tank
will weigh 10,000 kg. Hence you will need a load cell with an accuracy
of 17/10,000 or 0.17% to obtain a resolution equal to a 100 pound bag
of potatoes.
If the tank is half full of potatoes then you have increase the weight
of the tank by:
1.6 kg/liter x 5000 liters - 1 kg/liter x 5000 liters = 0.6 kg/liter x
5000 = 3000 kg.
When the tank is half full of potatoes then 10% accuracy is equal to
300 kg. Hence it looks llike what you want to do is practical.
Howard

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