SHREE SWAMI ATAMANAND SARASWATI INSTITUTE OF

TECHNOLOGY, SURAT

PREPARED BY:BOGHANI KAUSHAL B.(130760109002)

CHARACTERISTICS OF INSTRUMENTATION &STRAIN GAUGE

CHARACTERISTICS OF INSTRUMENTATION SYSTEM

• The performance characteristics of an instrument are mainly divided into two categories:•

i) Static characteristics

ii) Dynamic characteristics

Static characteristics:

The set of criteria defined for the instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, i.e., do not vary with time, is called ‘static characteristics’.

• The various static characteristics are:

• Accuracy:

It is the degree of closeness with which the reading approaches the true value of the quantity to be measured. The accuracy can be expressed in following ways:

Point accuracy:

Such accuracy is specified at only one particular point of scale.It does not give any information about the accuracy at any other Point on the scale.

Accuracy as percentage of scale span:

When an instrument as uniform scale, its accuracy may be expressed in terms of scale range.

Accuracy as percentage of true value:The best way to conceive the idea of accuracy is to specify it interms of the true value of the quantity being measured. Precision: It is the measure of reproducibility i.e., given a fixed value of a quantity, precision is a measure of the degree of agreement within a group of measurements. The precision is composed of two characteristics:

Conformity:Consider a resistor having true value as 2385692 , which is being measured by an ohmmeter. But the reader can read consistently, a value as 2.4 M due to the non availability of proper scale. The error created due to the limitation of the scale reading is a precision error.

Number of significant figures:The precision of the measurement is obtained from the number of significant figures, in which the reading is expressed. The significant figures convey the actual information about the magnitude & the measurement  precision of the quantity. The sensitivity denotes the smallest change in the measured variable to which the instrument responds. It is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity to be measured. Mathematically it is expressed as, Thus, if the calibration curve is liner, as shown, the sensitivity of the instrument is the slope of the calibration curve. If the calibration curve is not linear as shown, then the sensitivity varies with the input. Inverse sensitivity or deflection factor is defined as the reciprocal of sensitivity. Inverse sensitivity or deflection factor = 1/ sensitivity

Reproducibility:It is the degree of closeness with which a given value may be repeatedly measured. It is specified in terms of scale readings over a given period of time.

Repeatability:It is defined as the variation of scale reading & random in nature Drift: Drift may be classified into three categories:

zero drift:If the whole calibration gradually shifts due to slippage, permanent set, or due to undue warming up of electronic tube circuits, zero drift sets in.

span drift or sensitivity driftIf there is proportional change in the indication all along the upward scale, the drifts is called span drift or sensitivity drift.

Zonal drift:In case the drift occurs only a portion of span of an instrument, it is called zonal drift.

Resolution:If the input is slowly increased from some arbitrary input value, it will again be found that output does not change at all until a certain increment is exceeded. This increment is called resolution.

Threshold:If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument.

Stability:It is the ability of an instrument to retain its performance throughout isspecified operating life.

Dynamic characteristics:The set of criteria defined for the instruments, which are changes rapidly with time, is called ‘dynamic characteristics’.

The various static characteristics are:i) Speed of responseii) Measuring lagiii) Fidelityiv) Dynamic error

Speed of response:It is defined as the rapidity with which a measurement system responds to changes in the measured quantity.

Measuring lag:It is the retardation or delay in the response of a measurement system to changes in the measured quantity. The measuring lags are of two types:

Retardation type:In this case the response of the measurement system begins immediately after the change in measured quantity has occurred.

Time delay lag:In this case the response of the measurement system begins after a dead time after the application of the input. Fidelity: It is defined as the degree to which a measurement system indicates changes in the measurand quantity without dynamic error. 

Dynamic error:It is the difference between the true value of the quantity changing with time & the value indicated by the measurement system if no static error is assumed. It is also called measurement error.

DIFFERENT TYPES OF STRAIN GAUGES

• Types of Strain Gauges

1. Unbonded metal strain gauges2. Bonded metal wire strain gauges3. Bonded metal foil strain gauges4. Vacuum deposited thin metal film strain gauges5. Sputter deposited thin metal strain gauges6. Bonded semiconductor strain gauges7. Diffused metal strain gauges.

Unbounded metal strain gauges

• An unbounded metal strain gauge is shown in Fig.2. This gauge consists of a wire stretchedbetween two points in an insulating medium such as air. The wires are of copper nickel,chrome nickel or nickel iron alloys. The flexure element is connected via a rod to adiaphragm which is used for sensing of pressure. The wires are tensioned to avoid bucklingwhen they experience a compressive force.

• The resistance element is a thin wire of a special alloy that is stretched taut between two flexible supports, which are in turn mounted on a thin metal diaphragm. When a force such as F1 is applied, the diaphragm will flex in a manner that spreads the supports further apart, causing an increased tension in the resistance wire. This tension tends to increase the resistance of the wire in an amount proportional to the applied force. 

• Similarly, if a force such as F2 is applied to the diaphragm, the ends of the supports move closer together, reducing the tension in the taut wire. This action is the same as applying a compression force to the wire. The electrical resistance in this case will reduce in an amount proportional to the applied force 

Bonded metal wire strain gauges

•  A bonded strain gauge is made by cementing a thin wire or foil element to a diaphragm. Flexing the diaphragm deforms the element. causing a change in electrical resistance exactly as in the unbounded strain gauge. 

• Many biomedical strain gauge transducers are of bonded construction because the linear range is adequate and the extra ruggedness is a desirable feature in medical environments. The Statham P-23 series are of the unbounded type strain gauge transducer but are made in a very rugged housing. These are among the most common cardiovascular pressure transducers used in medicine. In addition, changes in temperature can also cause thermal expansion of the wire and thus lead to large changes in the resistance of a strain gauge. Therefore, very sensitive electronic amplifiers with special temperature compensation circuits are typically used in applications involving strain gauge transducers. 

• Most physiological strain gauge transducers use four strain gauge elements connected in a Wheatstone bridge circuit as shown in the figure. Both bonded and unbounded types of transducers are found with an element geometry that places two elements in tension and two elements in compression for any applied force (tension or compression). Such a configuration increases the output of the bridge for any applied force and so increases the sensitivity of the transducer. Strain gauge elements in a Wheatstone bridge circuit Mechanical configuration Using a common diaphragm 

Bonded metal foil strain gauges

• Strain gauge based technology is utilized commonly in the manufacture of pressure sensors. The gauges used in pressure sensors themselves are commonly made from silicon, polysilicon, metal film, thick film, and bonded foil.

• The bonded metal foil strain gauges are formed by rolling out a this foil of the resistive material and then cutting away parts of the foil by a photo etching process to create the required grid pattern.

• Such strain gauges are called as Bonded metal foil strain gauges

Differentiate different transducers

Passive Instruments Active InstrumentsThe output produced is entirely by quantity measured.

The output produced is by the magnitude of some external power input.

The resolution is less and cannot be easily increased.

The resolution is adequate and can be adjusted by adjusting magnitude of external energy input.

They are simple to design. They are complex in design.

They are cheap. They are costly.

e.g.: pressure gauge, glass thermometer, voltmeter.

e.g.: liquid level indicator.

PRIMARY TRANSDUCER SECONDARY TRANSDUCER

In pressure measurement burdon tubes are primary transducer.

LVDT is secondary transducer.

The force is detected by the column in first so it called primary transducer.

Out put of primary transducer converts into useful output signal is known as secondary transducer.

It is mechanical device. It is electrical device.

Example. load cell Example. strain gauge

ANALOG TRANSDUCER DIGITAL TRANSDUCER

It converts input quantity into analog output which is continuous function of time.

It convert i/p quantity into electrical output in form of pulses

example:Strain gaugeLVDTThermocouplethermistor

Example:Optical systemphotocells

Thank you