# Level measuring

• posted

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
• posted
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.
• posted
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
• posted
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.
• posted
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:-)
• posted
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..
• posted
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.
• posted
And the density of potatoes is constant? Must be better potatoes than we can get over here! :-)
Cameron:-)
• posted
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
• posted
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.
• posted
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
• posted
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
• posted
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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