Thursday, December 8, 2016

Homemade Strain Gauge

I work with a lot of electronic scales, weighing raw materials, trucks and finished product, every one of those scales contains one or more load cells. Up until a few weeks ago, load cells were a mysterious aluminum block that turned static load into a weight on a scale. After a couple hours of Google-Fu I found that the important part of every load cell was a strain gauge, a variable resistor that changed value as it is deformed.

Strain gauges are generally described as a conductor that will be stretched along with a substrate, causing the conductor to become thinner and more resistive. When the substrate is squeezed the conductors are bunched up and become thicker, decreasing the resistance, it sounds simple enough until I figured out how little the resistance actually changes. Using about 250cm of very fine wire I was only able to measure a change of about 8mOhms. Normally 8mOhms would not be a huge problem for measurement but the total resistance of the strain gauge was 50.008 ohms.

To think of it another way, imagine measuring a 50meter long bar and trying to find out if it has grown or shrunk by 8mm BUT the tool you use has to measure the entire length, there aren't many tools that do that kind of accuracy for cheap. Instead of trying to measure the whole value I used a wheatstone bridge to measure only the change in resistance, this is like laying out a known 50meter bar next to the one you want to measure and then checking the difference between the two, this is much easier and can be done with non-specialized tools in both the physical and electrical examples.