• Join over 1.2 million students every month
• Accelerate your learning by 29%
• Unlimited access from just £6.99 per month
Page
1. 1
1
2. 2
2
3. 3
3

# Measuring weight with a strain gauge.

Extracts from this document...

Introduction

Measuring weight with a strain gauge

A strain gauge is a wire which is used to measure strain by the

change in its resistance when it gets either longer and thinner or

shorter and thicker. They are attached to a surface for which the

strain is wanted, and need to be able to move as if they are part of

the surface. Modern strain gauges are etched onto foil because its

thin and flexible, and therefore able to move with the surface.

Gauges are glued onto the test object with superglue so that they

move as if they are a part of the object.

Elastic modulus = stress/strain (When stress is a linear tensile or

compressive stress, the elastic modulus is called Young’s modulus).

A tensile strain will be accompanied by a reduction (and

compressive strain by an increase) in lateral dimensions. The ratio of

the lateral strain to the longitudinal strain is called Poisson’s ratio1.

For most materials the value is between 0.25 and 0.4, and written as

a positive number although the signs of the lateral and longitudinal

strain are always opposite. The gauge factor of a strain gauge (G) =

(?R/R)/(?l/l) where R = resistance and l = length. Since ?l/l is the

strain (e) in the object which the gauge is attached to this can be

written as ?

Middle

R1(1+x)(1+y), and                         Vo = VsRg/(Rg+R’g)-Vs/2 ?

VsGe/4. This method also has drawbacks, though: that an unstrained

specimen of the original material has to be provided and that the

dummy gauge is not necessarily at the same temperature as the

active one. These problems can be solved by mounting the dummy

gauge on the same member as the active one and at right angles to

the direction of strain, so that the gauges are unlikely to have a

measurable difference in temperature. The dummy gauge will be

strained at right angles to its active axis, which will make it slightly

shorter along its active axis, as explained above, which means that

the resistance will decrease by an amount proportional to Poisson’s

ratio (v). If Rg = R1(1+x)(1+y) as before, and R’g = R1(1-vx)(1+y),

then Vo = VsR1(1+x)(1+y)/[R1(1+x)(1+y)+ R1(1-vx)(1+y)]- Vs/2 ?

Vs(1+v)Ge/4.

This method can be used to give a measurement of strain in a

member under tensile stress, but I’m planning to use a cantilever, the

end of which I will be putting weights on. In this situation I will be

able to increase my output readings by putting active gauges on both

sides of the cantilever, because as the cantilever bends the gauge on

Conclusion

results. Even so, the gauges would not stick at all the proper way

round, with the plastic backing stuck directly to the cantilever, so I

had to glue them on with the foil on the metal, and hope that the

superglue would insulate the gauge from the cantilever, which it

seems as though it did.

The choice of what to use as a cantilever is a very important one,

as if it is too rigid it will not bend much and so not give a big enough

reading, but if it is too flexible it will bend too much and the gauges

limit will be exceeded. I think that the metal I used as the cantilever

was too thick, and gave too small a reading, but I was restricted in

my choice by the materials available to me

I think that my technique was a suitable one, as it gave accurate

results without causing major problems or being too complicated:

the most complicated part of the project was assembling the bridge

itself. It is also an adaptable one, with different cantilevers being

used for different weight ranges, and different arrangments of the

bridge available.

Bibliography

Strain gauges supplied by RS (http://rswww.com)

1 Instrumentation units 1 and 2 by the instrumentation course team

(The Open University Press, Walton Hall, Milton Keynes, MK7

6AA)

2 Advanced Design and Tecnology by Eddie Norman, Joyce

Cubitt, Syd Urry, Mike Whittaker (Addison Wesley Longman

Limited, Edinburgh Gate, Harlow, Essex CM20 2JE)

3 RS Electronic Catalogue data sheets (http://rswww.com)

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

## Found what you're looking for?

• Start learning 29% faster today
• 150,000+ documents available
• Just £6.99 a month

Not the one? Search for your essay title...
• Join over 1.2 million students every month
• Accelerate your learning by 29%
• Unlimited access from just £6.99 per month

# Related AS and A Level Electrical & Thermal Physics essays

1. ## Sensing project

5 star(s)

The same fixed resistor is used thought out the experiments so as to ensure the variables remain constant. The resistor that will be used in the circuit will be a 10K? resistor, which has similar resistance to the rotary potentiometer.

2. ## Investigation On The Resistivity Of Apples. Since we are measuring the resistance of an ...

To reduce this we will only cut the apples before measuring its resistance. This might improve the results by making sure that the apples are still fresh and full of moisture when it is being measured. Also while we were conducting our preliminary trial, we discovered that the resistance for one slice of apple varied.

1. ## Sensors Project Report

Since the temperature at any time will be slightly different from one another, so the reading may not be accurate enough. 4. After long time of experiment the potentiometer starts to heat up. When the temperature increase, the resistance of the potentiometer will increase and the potential difference (voltage)

2. ## Experiment to investigate how the resistance of a strain gauge attached to a piece ...

I do not want the wires of the strain gauge to get stretched too much whilst being heated, as this will increase the overall resistance and affect the final results. At high temperatures the wire will expand, and at low temperature they will shrink.

1. ## physics sensor coursework

My final design of circuit: A safety issue to be considered is not to have too high a voltage setting on the power supply. If there is a high voltage of about 10V and there is a low resistance component in the circuit, then the component may heat up and there is the risk of a fire.

2. ## silicon project

Pure silicon crystals can be found as inclusions in gold, or in volcanic exhalations. In America regular grade silicon (99%) costs about \$0.50/g. Silicon 99.9% pure costs about \$50/lb; hyper pure silicon may cost as much as \$100/oz. Miners, stonecutters, and others engaged in work where siliceous dust is breathed

1. ## AC instrumentation transducers

It is differential because of the way the two secondary windings are connected. Being arranged to oppose each other (180o out of phase) means that the output of this device will be the difference between the voltage output of the two secondary windings.

2. ## Testing a rotary potentiometer for use as a depth sensor

varying resistance of the potentiometer using Ohm's Law, V = IR (V=voltage, I=current, R=resistance). I will do this because in the complete sensor circuit (in which an alarm will sound when the level of bath water is sufficient) it will be the altering resistance which will cause the circuit containing the alarm to be complete.

• Over 160,000 pieces
of student written work
• Annotated by
experienced teachers
• Ideas and feedback to