# full bridge strain gage measurement

#### 0001T

Joined Feb 28, 2007
15
Hi,

i am doing a project on wheatstone bridge (full bridge strain gage measurement). How do I convert the data to stress?

I have the following information.

gage factor, G = 2.08, Kt = 0.9/0.2 resistance, Ro = 120ohm
output from bridge is in voltage

eg. at a load of 1300N, i have an output (Vo) of 3.495V (gain of 640 used).

the strain e was calculated to be 0.000525 using the equation (Vo/gain)=Vi*G*e

I managed to find out the changes in R1 to R4 as follows:

R1=R3 = Ro(1+Ge) = 120.131ohm
R2=R4 = Ro(1-Ge) = 119.869ohm

however, how do i go further from here to obtain the stress result?

#### beenthere

Joined Apr 20, 2004
15,819
The change in resistance in the strain gauge should relate to the compressional/tensional change in length. That should give you your stress.

#### recca02

Joined Apr 2, 2007
1,212
stress is proportional to strain
if u know the young's modulus for the material then
stress=young's modulus*strain.

#### 0001T

Joined Feb 28, 2007
15
thanks for the replies.

i understand that the strain gauges will measure both tension and compressive strains. and using stress-strain equation, i can calculate the stress.

question is - is this strain (or stress) value an average strain(or stress) of the four gauge results? or is it an total strain (stress) result?

#### beenthere

Joined Apr 20, 2004
15,819
I should think the process speaks for itself, but the result is total strain - assuming proper placement of the strain gauges.