This application note explains how to use a strain logger written in C#. It reads up to 3 bridges and writes the result to a text file that can be read by a spreadsheet program.
When an object is stretched due to an external force and the length of the object increases from L to L+ΔL, the ratio ΔL/L is called strain.
ε = ΔL/L
As the ratio of deformation is often very small, it is often represented in a units of 10-6 or μstrain
A strain gauge can be used to measure the strain of this object. The most common type of strain gauge consists of a flexible backing which supports a metallic foil pattern etched onto the backing. As the object is deformed, the foil pattern is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge circuit, can be used to calculate the exact amount of deformation.
The gauge factor of a strain gauge relates strain to change in electrical resistance. The gauge factor GF is defined by the formula
where RG is the resistance of the undeformed gauge, ΔR is the change in resistance caused by strain, and ε is strain. ΔR is typically in the region of 0.1%. To measure such small resistance changes, a Wheatstone bridge is employed. There are 3 configurations are used - quarter bridge, half bridge or full bridge. The strain adaptor allows for all these three configurations by plugging in the respective bridge completion resistors and then wiring up the strain gauges via the screw terminals. Excitation voltage of 2.5V is provided. Up to six input channels are available.
The quarter bridge equation is given
where VOFFSET is the offset voltage measured when there is no load and RL is the lead resistance.
The half bridge (bending) equation is
The full bridge (bending) equation is
The strain logger is written in Visual C# 2005. You can use the program as is or modify to suit your specific needs. To modify the program you may use the free Visual C# Express.
The program allows for
To run the program