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

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 ?

...read more.

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

...read more.

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)

...read more.

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

See related essaysSee related essays

Related AS and A Level Electrical & Thermal Physics essays

  1. Marked by a teacher

    Sensing project

    5 star(s)

    so an increase in resistance means less current if flowing through the circuit therefore the p.d is lower so as the angles are changed at regular intervals the voltage will decrease accordingly in a linear fashion. I also predict that voltage decreases in a linear fashion so I would expect

  2. silicon project

    The positive holes in the P-type silicon get attracted to the negative terminal of the battery. No current flows across the junction because the holes and the electrons are each moving in the wrong direction. If you flip the battery around, the diode conducts electricity just fine.

  1. Investigating how temperature affects the resistance in a wire

    the standard deviation of the individual results at each temperature varies with the temperature. Conclusion/Observations: The results show me that the temperature is inversely proportional to the resistance, which goes against my prediction and against the theory behind this relationship.

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

    Using the multimeter, we measured its resistance during class. Data: Length (� 0.05 x 10-2 m) Sample Resistance (�0.05 x 103 ?) Trial 1 Trial 2 Trial 3 Total Average 1 1 263.0 200.0 160.0 158.1 � 104.9 2 125.5 127.7 117.0 3 135.0 140.0 155.0 2 1 266.0 210.0

  1. AC instrumentation transducers

    The primary advantage of an LVDT over a potentiometer for position sensing is the absence of physical contact between the moving and stationary parts. The core does not contact the wire windings, but slides in and out within a nonconducting tube.

  2. physics sensor coursework

    It is impossible to accurately change the light intensity with just blocking off the light. Instead, a trial-and-error method is adopted, where we continually move the book back and forth until there is the correct light intensity. There is also the problem of maintaining the light intensity at the same

  1. To find which of the circuits, shown below, are most suitable to measure a ...

    * A Power Pack, on which I can vary the Voltage (and therefore current) * A Set of labelled Resistors - quite a large range, from about 50? to 50000?. I decided not to use a rheostat because of its severe limitation - not being able to accurately read the resistance to start with.

  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.

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