The relationship between resistance and strain is:
GF= ΔR/R = ΔR/R
ΔL/L ε
GF= Gauge Factor, ε= is the strain, R is the resistance and L= length.
Fair Test
Making this a fair test will involve taking into consideration many factors as temperature will affect the results because Strain gauge manufacturers attempt to minimize sensitivity to temperature by processing the gauge material to compensate for the thermal expansion of the specimen material for which the gauge is intended for. While compensated gauges reduce the thermal sensitivity, they do not totally remove it, but a low P.D does not crate a significant increase in temperature.
There could be random errors, noise that could affect the results but to prevent this from happening I will repeat the experiment several times.
I will also keep the P.D the same throughout the experiment, as this will also affect the results.
Safety
Correctly connect all the components and check the wires before hand for any cuts or naked wires. Placing everything effectively that it is not cramped or near the edge as this can lead to falling or causing accidents.
Apparatus
Strain Gauge
Wires
Voltmeter
Power pack
G-clamp
Weights (going up in 10 grams) and holder
Results
Metal _ Graph
Wood
Average Results.
Metal
Wood
Analysis
Looking at the average graphs you can see that the material wood is much stiffer than steel as the results show. Looking at all the metal results we can see that the graphs are all very similar as this suggests that my results are accurate. Similarly the graphs for wood are same as the results show and equally the experiment was a success as there were no anomalous results that affected the graphs.
From the results the graph for average P.D for steel, there is a steep start but as you see near the end it starts curve, I think if I continued to add more weights on it would have levelled off. From the graph I can say that the strain gauge is not a linear device as I can see a curve being formed.
As we know that the strain gauge itself is made by a metal and this suggests that it follows the Rule of Ohms Law V=IR. From this equation we know that we can calculate the resistance or conductance.
Looking at the results From the average P.D of wood being put under strain we can say that in comparison with the results of steel that wood is more stiffer then steel, like we would expect because if you try to break a wooden stick it is not flexible and breaks if you apply a lot of force. The graph is not as steep as the graph for steel but it is more like a straight line but not exactly as the last few results aging show the graph curving slightly and if continued to add weights I think the graph would have levelled off.
Looking at the Graph For steel we can see the sensitivity of the strain gauge as the steepness indicates, this is shown as the P.D increases shapely. These results are satisfying and basically state that as strain increase so does the P.D, until the sensor becomes insensitive as it is stretched out fully.
Evaluation
I think I can safely say that there were no anomalous results as I did repeat the experiment six times, that indicates there were no fluctuations, random errors, noise that occurred as all the results fitted together well.
I still feel there can be improvements in measuring the P.D using a more accurate or voltmeter that reads to 3 decimal figures. Looking back at the weight I notice that the mass of the weights used were only measured to 0.5 of a gram that also could be improved by using a weighing scale that accurately reads the weight to 2 decimal places at least.
Looking at the Graphs I think I need to continue to add weight to show the graph levelling off, this would show the curve better. Also continuing to add load on to the beam until the P.D would not change indicating the certain range for that particular Strain gauge.
I also could take this further as I did not measure the response time of the sensor but observing the change on the voltmeter I can say that it did not take more than a second to get a reading, however a suitable response time was shown by this particular strain gauge
.
Looking back at the Graphs, I can see that the sensitivity of the sensor is seen and to measure this further. Using a smaller scale of weights added at the start, where the graph is steep will show in more detail the sensitivity of the sensor.
To summarise the stain Gauge I can say that it is very sensitive to small changes and this is useful in many industries. The strain Gauge is insensitive to large changes but in industry you do need to detect large changes. To overcome this problem we can use the calibrated curves produced by the results to produce look up tables that indicate suitable strain gauges for certain purposes. Knowing that a certain strain gauge can detect from a range, I could test and evaluate different sensors produced by different companies to measure strain and evaluate the differences.
Bibliography
Advancing Physics Students Text Book
Advancing Physics C-D-ROM
Web Pages.
www.slopeindicator.com/support/strain-gauge/ faq-strain-gauge.html
Diagram
Circuit Diagram
Method
First set up the apparatus as shown in the diagram
It is very important that you check that strain gauge is firmly onto the test specimen. This ensures the strain accurately transfers from the test specimen through the adhesive and strain gauge
Attach the weight holder to a string and carefully tie a knot.
Take the reading of the voltmeter.
Start adding weights on
Carefully record the results
Trial Results.
Improvements To Method.
Make sure that the weight holder does not move as the strain gauge sensor is very sensitive and this could affect the results.
Tightly clamp the beam so it in not touching the bench when there is some load on it and also measure the amount of the beam is over the bench as this will vary the results.