Using strain gage to determine hopper level

[brianle]

I am wondering if a strain gage could be used to determine the level of material in a hopper shaped like an inverted pyramid that is made of plate steel. The hopper is about 12 feet wide by 12 feet long at the top and is about 8 feet deep. The weight of material in the hopper when full is about 34,000 lbs. The hopper is welded to support steel at the top. There is a discharge piping system connected to the bottom of the hopper. Could a strain gage mounted on the sloping side of the hopper near the top be used to give an indication of the amount of material in the hopper? I presume an empty hopper would have some baseline strain, and a full hopper would have a higher strain. Could the stain gage output be used the determine whether the hopper is full or empty, or something in between?

 

[MikeHalloran]

Not very accurately.

It would be best to provide some lateral location, e.g. via horizontal flat bars or links, and support the entire hopper on load cells.

Yes, load cells are just arrays of strain gages installed on bars of controlled geometry. ... but the gages are installed under controlled conditions and sealed, so the load cells are at least close to interchangeable, and durable. Getting a good bond and a durable seal on a strain gage applied to existing equipment is, well, for all practical purposes, impossible. Save yourself a bunch of aggravation and don't even think about mounting strain gages yourself.





Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

would s/gauging one of the supports be more accurate ? s/gauging a panel would be affected by the load in the hopper and deformation of the panel. i'd also get readings at intermediate points (not just empty and full) to see how linear the relationship is.

 

[TenPenny]

It used to be a standard way to measure, but the load cells would be mounted on the support legs.

I know that Ramsey used to be one of the big players in the business, also Milltronics.

 

[IceStationZebra]

You could gage the legs, but the accuracy won't be that great. The reason is that the installation will be the most accurate when measuring a large variation in strain, but because the legs have a large safety factor the accuracy will be diminished. A rough guess would be +/- 3-5% with reasonable instrumentation.

Of lesser concern, you would have to calibrate what is full vs. empty. And if the material absorbs water it will throw your calibration off. Another issue is that the instrumentation to read load cells is very sensitive to improper grounding, lightning, etc. A cheap power supply will cause all kinds of problems.

If you can afford to buy commercial load cells, do that. Like Mike said it will save you a tremendous amount of aggravation. I install strain gages as part of my job, and I can say it takes a little experience. You will waste time/money trying to do it yourself.

Another option if you just want a rough idea just google "grain bin level indicator". There are some very simple items that don't even require power.

ISZ

 

[Ron]

Yes. It can be done. Put one strain gage on each of the four sloped sides, then average the results. The strain gage will exhibit zero strain when installed to an already constructed empty hopper. Any strain developed by the load would then be picked up as tensile strain, assuming the discharge piping provides no load support. You'll have to calibrate the gage system by measuring strains at various loading depths, then compare that to calculated weight-volume relationships based on the average density of the material being placed in the hopper.

 

[PeterCharles]

For maximum accuracy you make your hopper with four support points at the hip line and support your hopper on a steel structure. Then you put load cells between the hopper support points and the structure and monitor the the loads in each load cell. To my knowledge this has been done in bulk materials handling for the last forty years.

 

[rb1957]

seriously, could you make the hopper out of perspex (sp?, = clear plastic) so you could see the level ? i know s/gauges Can work, but my experience with them is that thye tens to be finicky little things.