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Various types of displacement transducer are used tomeasure the deflection of the elastic elements.

Of these, the strain gauge is used most commonly, since

this gives the best measurement accuracy, 0.05%. Measure masses over a very wide range: 0 and 3000

tonnes. The bottom end of this range; typically be 0.15 kg,

The top of the range; 103000 tonnes.

Elastic force transducers based on differentialtransformers (LVDTs) to measure masses up to 25tonnes. Apart from having a lower maximum measuring capability, they

are also inferior to strain gauge-based instruments in terms oftheir 0.2% inaccuracy.

Their major advantage is their longevity and total lack ofmaintenance requirements.

Piezoelectric device. Used to measure masses in the range 0 to 1000tonnes.

Piezoelectric crystals replace the specially designedelastic member normally used in this class ofinstrument, allowing the device to be physically small.

Such devices can only measure dynamicallychanging forces because the output reading resultsfrom an induced electrical charge whose magnitudeleaks away with time.

It is very difficult to design such instruments to beinsensitive to forces applied at an angle to thesensing axis.

Therefore, special precautions have to be taken inapplying these devices.

Although such instruments are relatively cheap, theirlowest inaccuracy is 1% of full-scale reading, andthey also have a high temperature coefficient.

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The electronic balance is a device that contains severalcompression-type load cells.

Commonly, either three or four load cells are used in thebalance, with the output mass measurement beingformed from the sum of the outputs of each cell.

Where appropriate, the upper platform can be replacedby a tank for weighing liquids, powders etc.

Load-cell-based electronic balance.

Pneumatic/hydraulic load cells

Pneumatic load cell.

Hydraulic load cell.

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Two alternative forms of weigh beam.

Weigh beam

Pendulum scale

Various versions: 1 kg to 500 tonnes, with a typical measurementinaccuracy of 0.1%.

One potential source of difficulty: Oscillation of the weigh platform when themass is applied.

Where necessary, in instruments measuring larger masses, dashpots areincorporated into the cam system to damp out such oscillations.

An further possible problem: when measuring large masses, if the mass isnot placed centrally on the platform.

This can be avoided by designing a second platform to hold the mass,which is hung from the first platform by knife edges. This lessens thecriticality of mass placement.

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Torque measurement Measurement of applied torques is of

fundamental importance in all rotating bodies toensure that the design of the rotating elementis adequate to prevent failure under shearstresses.

Torque measurement is also a necessary partof measuring the power transmitted by rotatingshafts.

The three traditional methods of measuringtorque consist ofa) measuring the reaction force in cradled shaft

bearings,

b) the Prony brake method andc) measuring the strain produced in a rotating body

due to an applied torque.

Reaction forces in shaft bearings Any system involving torque

transmission through a shaftcontains both a power source anda power absorber where thepower is dissipated.

The magnitude of the transmittedtorque can be measured bycradling either the power sourceor the power absorber end of theshaft in bearings, and thenmeasuring the reaction force, F,and the arm length L,

The torque is then calculated asthe simple product, FL.

Pendulum scales are very

commonly used for measuringthe reaction force.

Inherent errors in the method arebearing friction and windagetorques.

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Position of torque-measuring strain gauges on shaft.

Optical torque measurement

Two black-and-white striped wheels are mounted at either end of therotating shaft and are in alignment when no torque is applied to theshaft.

Light from a laser diode light source is directed by a pair of optic-fibre cables onto the wheels.

The rotation of the wheels causes pulses of reflected light and theseare transmitted back to a receiver by a second pair of fibre-opticcables.

Under zero torque conditions, the two pulse trains of reflected lightare in phase with each other.

If torque is now applied to the shaft, the reflected light is modulated.

Measurement by the receiver of the phase difference between thereflected pulse trains therefore allows the magnitude of torque in the

shaft to be calculated.

The cost of such instruments is relatively low, and an additionaladvantage in many applications is their small physical size.

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Optical torque measurement.