Silo fill measurements with strain gauges.

Hi all.

I have a project with individual silos (100 t capacity each), and am thinking about using strain gauges for the fill level measurement.

1. It does not have to be super accurate. +/- 5% is plenty good. The silos stand with their own individual legs. Should I measure on one leg, several legs, or all legs ?

2. Should I use a single strain gauge foil on each leg, or should I on each leg arrange 4 foils in a bridge arrangement ?
2a. I can only find strain gauge foils with a single strain gauge. Does there exist foils with 4 strain gauges pre-configured in a bridge connection ?
(I have a lot of experience with load cells, where you always use bridge connections, but I have no experience with strain gauges).

3. I can find strain gauges of steel or aluminium. Why would I chose one over the other ?

4. Any good suppliers in the EU ?

5. Any other useful tips or links to good sites about strain gauges ?

Thanks in advance.

edit:

6. What about the physical wire connections on the strain gauges ? What do people do ?
I am thinking about cutting the bottom of a regular terminal box and place it over the strain gauge(s), and also place a DIN rail for the further connection or even place the converter directly in the box.

I have now found an 'industrial-grade' strain gauge unit:
http://www.mts-systems.de/fileadmin...lter/daisy/datenbaetter_deutsch/DBL_daisy.pdf

Fully heavy duty enclosed and full-bridge. I am going to investigate this.
Still very interested in feedback on point 1.

I have found strain gauges are better at measuring stretch than compression. Why do you not want to just use load cells?

Hi Clay.

A strain gauge can be glued onto an existing beam.
A load cell has to be inserted into the construction and be part of the construction.

True a strain gauge is supposed to be "bolt on" but bear in mind that its accuracy is very hard to predict. Unlike a load cell your actual deflection is going to be unknown until you attach the sensor to the beam. You will have to experiment a good bit to find a good location on your frame to place the strain gauges. Make sure there are no frame cross supports going around your strain gauge as this will really throw your readings.

Also one thing I learned the hard way. Attach you strain gauge with the silo full. Strain gauges are designed to measure stretch not compression.

Also on the very bottom end where the silo is almost empty you may see a deadband where the weight is not enough to cause any measurable compression of the beam.

I dont need the accuracy. I will do a simple full-empty calibration and be done with it.
It will just show 0-100% on the HMI.
I am aware about the installation constraints.

Thanks for the tip regarding gluing the sensor with the silo full.