# Selecting the correct strain gauge and metal thickness for custom load cell

I'm working on project and after a good amount of research I've ended up with a final design that looks roughly like this:

The design is inspired by load cells used in bathroom scales, but I found them to be too clunky for my application so I have to make a custom set of load cells. Each T-shaped arm will have a strain gauge on top of the section that gets narrower where hopefully most of the strain will be. I've kept the 8 load cells on one piece of metal to gain rigidity. On top of this plate I will mount four 80x20mm bars of wood So I can measure four different forces at the same time and also the where the force is being applied on the bar.

My main obstacle is to calculate the optimal thickness of the metal, width of the t-shaped arm and find the right strain gauge so I can measure weights up to 100kg per load cell without breaking. Lets say I use steel or stainless steel for this. Should I use a strain gauge with 1000ohm?

Updated:

This whole part will be bolted down flat on a thick wooden surface and will be used to measure weight distribution on my fingers when I do climbing related excercises like pullups. The part is mounted completely horizontal.

Mechanical designer here - how do you get an inside cut on a sharp angle like that? What tool makes perfect corners?

You said, "On top of this plate I will mount four... bars of wood." So, if the bars bolt to the top, how does this bolt to anything else?

You said, "I've kept the 8 load cells on one piece of metal to gain rigidity," and, "...I can measure four different forces at the same time." You get rigidity by interactions between the individual members - this means one "load cell" will skew the others because they're all coupled together.

You start out asking about the metal thickness and end up asking if you should use a 1$\mbox{ k}\Omega$ strain gauge. All of this leads me to ask (no offense intended) - do you understand how a strain gauge works?

The strain gauge measures strain - elongation/deformation in the material. The change in resistance of the strain gauge is proportional to the amount of deformation in the material.

So, you need to determine how much force you are expecting to experience. Then, via part geometry, determine how much stress will exist in the part at a particular region. Then, via part material, determine how much strain you expect at the region.

Then, use the gauge factor with the expected strain and strain gauge resistance to determine the anticipated range of resistances. Setup your Wheatstone bridge to control the expected voltage swing based on the expected resistance swing and/or use an op-amp to buffer/scale the output voltage. Read the output with an analog to digital converter that has a resolution capable of meeting your project needs.

The important thing to note about the strain gauge is that it only measures the strain in the member to which it is attached. If you want to measure all of the applied force, then you need to make sure that all of the applied force gets transmitted through the part to which the gauge is attached, and that it is all applied in a direction that the gauge is capable of reading.

How accurate are you trying to be? Where will this be setup? Have you taken temperature effects into account? As metal changes temperature it expands and contracts - this will register as a "phantom" stress. Is the member to which the strain gauge attached experiencing pure tension/compression, or pure bending, or a combination?

Right now you don't provide any information about how this will be used beyond being attached to wood and subjected to a 100kg load. Can you provide a drawing of where you're planning to attach the load cell? Of how the wood attaches? Of how the assembly mounts to a structure? Is the load distributed or a point load? Cantilevered or centered?

Without more information about the nature of the load and how your assembly goes together it's not really possible to comment on how the part should be sized. I will say, though, that you would want the strain gauge in an area of uniform stress, which means that you should consider an area of uniform cross-section, which doesn't really appear to exist anywhere in your drawing.

• I've added some detail above and updated the design a bit so the strain gauge will be attached to a more uniform area. regarding the strain gauge, I know I'm only interested in strain one direction so maybe i can use something like this, but if using 8 of these or 16 for compression strain as well I still have the problem with temperature changes. Cheap bathroom scale load cells uses 3 wires, where one wire is for strain and one a control wire to compensate for temperature differences. Can this be done or should I use full-bridge 4-wire strain gauges instead? – user1171426 Nov 3 '16 at 18:12
• @user1171426 "Using... for compression strain..." you're not going to be reading compression here; the 'fingers' of your design are all going to be in bending/shear. Further, the bending is liable to be eccentric/unequal unless your fingers always contact the exact center of the wooden pieces that bolt on top. I think you should check out how strain gauges are oriented. All strain gauges are 2-wire devices because they're all essentially just fancy resistors. Strain gauges that have more than 2 wires just have more than 1 strain gauge built in. – Chuck Nov 3 '16 at 18:46