How to pick a load cell?

     I have been spending some time on a few electronic forums. And some of these guys are pretty feisty on these sites. I read a long thread about two guys arguing over the specs of a load cell. So I thought I would give my two cents here! If you have time leave a comment letting me know what you think is important. Or even if you just have a question.  

Accuracy:

    The certainty of a load cell is important, no doubt about it. But this is just more then just taken a number and saying that our load cell meets this spec. When it comes to transducers this number can be misrepresented, but not untrue. Be sure to ask at what the percentage of the full-scale value is the accuracy spec at.

Example: ABC LoadCells gives an accuracy spec of (+/-0.02% of F.S.) But XYZ LoadCells states that they have a similar product that has an accuracy of (+/-.004%) but not at full-scale they don’t say. That spec maybe only good for 25% of F.S. value and that does you no good if your using a 100 load cell at anything more then 25 lbs.

May seem simple but its often something over looked when shopping online for your transducer. Read all the specs on the cell and the fine print.

Linearity/Hysteresis:

    Linearity and hysteresis are two different things that effect accuracy. I have heard of calibration houses saying they don’t spec hysteresis on their calibration records. They don’t feel as though its true value. Most manufactures will still state both on their data sheets. The Linearity spec would be the error of the load cell as it sees a force being applied. Hysteresis is the error the load cells has as load is being removed.

    Again these specs may not be important for you needs. If you have a 100 lbs load cell. You may only use that cell to full-scale like for a grain hopper system. You only need to know when that hopper reaches full weight.

    Others though might want to see the entire loading process. For example: A load cell could be used to measure the force in a shock absorber. Bounding and rebounding the load cell would need to be highly accurate from zero to full-scale and back.

Repeatability:

    Repeatability is simple. It’s how often does the cell reads the same value. But there are different standard and methods for testing repeatability. This is a very important number also. Good companies make sure to test all the variables. By variables I mean reasons for why your load cell would not repeat.

    Things like wither the environment its in is hot or cold? Or if it’s being submerged deep under water, how does that pressure affect it? How the transducer is mounted, some Industry tests require that the cell is to be calibrated on 3 separate axis so you can assure repeatability in tension and compression in off axis loads.

Some history on the strain gauge.

So load cells are your thing. At least that’s what you’re telling me by even getting this far. Ok, well lets first find out what a load cell is.  A load cell is a type of transducer or high precision electronic sensor that can be designed for any type of force measurement. 

Most industrial load cells use strain gauges. What is a strain gauge you ask? Well, it’s a thin, I mean thinner then foil, wire that when placed and secured on a surface of an object measures the strain of that said object. Here a little back story about the strain gage.

In 1843 there was this guy Sir Charles Wheatstone he was an English researcher. Using some old technology called a strain gauge. Cause a strain gage cause was 10 years old at the time… Two Americans had already laid claim to that invention. The original idea as thought up by Arthur Claude Ruge. Was to take a piece of cigarette paper and glue a thin wire to it leaving leads on the opposite ends. Then glue the now finished strain gauge to a water tanks outer metal wall. As the tank is filled its metal wall will stretch, causing the strain gauge’s resistance to change. Therefore allowing Sir Charles Wheatstone to be able to design his namesake The Wheatstone Bridge. We will discuss that in another post. It’s just the name for the type of circuit that allows for the accurate voltage measurement of the strain gauge.

So with all that you can get an idea of how a load cell works. The theory on how a load cell works is very simple but that’s about where simplicity stops. It’s in the design aspect where things get convoluted.  Load cells are used in many applications. Medical, military, industrial, agricultural, aerospace, and many more.