experimental planning

Experimental Planning

Definition of Terms

Readability indicates closeness with which the scale of the instrument may be read.

Dependent on the scale length, spacing of graduations, size of pointer and parallax effect.

Least count is the smallest difference between two indications that can be detected on theinstrument scale.

Dependent on the scale length, spacing of graduations, size of pointer and parallax effect.

Readability

Least count

Definition of Terms (contd.)

Sensitivity of an instrument is the ratio of the linear movement of the pointer on an analoginstrument to the change in the measured variable causing this motion.

For example, a 1-mV recorder might have a 25 cm scale length. It sensitivity would be 25cm/mV, assuming that the measurement was linear all across the scale.

For a digital instruments readout the term sensitivity does not have the same meaningbecause different scale factors can be applied with a button.

Sensitivity

Sensitivity refers to the ability of a measuring device to detect small differences in a quantity.


Hysteresis

An instrument is said to exhibit hysteresis when there is a difference in reading dependingon whether the value of the measured quantity is approached from above or below.

Hysteresis may be the result of mechanical friction, magnetic effects, elastic deformationor thermal effects.

It is the difference between the indications of measuring instrument when the same valueof the measured quantity is reached by increasing or by decreasing that quantity.


For example a temperature measuring device measures 102 K, 103 K, 99 K, 102 K forknown input 100 K, then the accuracy of the device is 3% would be accurate within 3Kover the entire range of the device.

Accuracy of an instrument indicates the deviation of the reading form a known input.

Accuracy

Precision of an instrument indicates its ability to reproduce a certain reading with a givenaccuracy.

Precision

Precision is a term that describes an instruments degree of freedom from random errors.


Deviation

Deviation of an instrument reading form a known value.

The deviation is called the error.

Error

Uncertainty

In many experimental situations we may not have a known value with which to compareinstrument readings and yet we feel fairly confident that the instrument is within a certainplus or minus range of the true value. In such cases we say that the plus or minus rangeexpresses the uncertainty of the instrument readings.

Calibration

The calibration of all instrument is important, for it affords the opportunity to check theinstrument against a known standard and subsequently to reduce errors in accuracy. Thecalibration of all instrument is important, for it affords the opportunity to check theinstrument against a known standard and subsequently to reduce errors in accuracy.

Calibration procedures involve a comparison of the particular instrument with either(1) a primary standard(2) a secondary standard with a higher accuracy than the instrument to be calibrated, or(3) a known input source.

Standards

In order that investigator in different parts of the country and different parts of the worldmay compare the results of their experiments on a consistence basis, it is necessary toestablish certain standard units of length, weight, time, temperature and electricalquantities.

NIST has the primary responsibility for maintaining these standards in the United States.

The meter and the kilogram are considered fundamental units upon which throughappropriate conversion factors, the English system of length and mass is based.

At one time, the standard meter was defined as the length of a platinum iridium barmaintained at very accurate conditions at the International Bureau of Weight andMeasures in Serves, France.

Standards (contd.)

Similarly kilograms was defined in terms of a platinum iridium mass maintained at thissame bureau.

1 meter = 39.37 inches1 pound-mass = 453.59237 grams

Standard of length and mass are maintained at NIST for calibration purposes.

In 1960 the General Conference on Weight and Measures defined the standards meter interms of the wavelength of the orange-red light of kryptopm-86 lamp.

The standard meter is thus1 meter = 16,50,763.73 wave lengths

Standards (contd.)

In 1982 the definition of the meter was changed to the distance light travels in1/299,792,458ths of a second.

For the measurement, light from a helium neon laser illuminates iodine which fluoresces ata highly stable frequency.

The inch exactly defined as1 inch = 2.54 centimeters

Experiment Planning

The key success in experimental work is to asking continually:

What I am looking for?

Why am I measuring this- does the measurement really answer any of my questions?

What does the measurement tell me?

Experiment Planning (contd.)

Some particular questions which should be asked in the initial phase of experimentplanning are:

1. What primary variables shall be investigated?

2. What control must be exerted on the experiment?

3. What ranges of the primary variables will be necessary to describe the phenomenaunder study?

4. How many data points should be taken in the various ranges of operation to ensure goodsampling of data considering instrument accuracy and other factors?


5. What instrument accuracy is required for each measurement?

6. If a dynamic measurement is involved, what frequency response must the instrumenthave?

7. Are the instruments available commercially or must be constructed especially for theparticular experiment?

8. What safety precautions are necessary if some kind of hazardous operation is involved inthe experiment?


10. What provisions have been made for recording the data?

11. What provisions have been made for either on line or subsequent computer reductionof data?

9. What financial resources are available to perform the experiment, and how do thevarious instrument requirements fit into the proposed budget?

12. If the data reduction is not of a research nature where manipulation and calculationsdepend somewhat on the results of measurements, what provisions are made to havedirect output of a data acquisition system available for the find report?

Generalized experimental procedure

1. Establish the need for the experiment.

2. Establish the optimum budgetary, man power and time requirements, including timesequencing of the project. Modify scope of the experiment to actual budget, manpowerand time schedule which are allowable.

3. Begin detail planning for the experiment: clearly establish objectives of experiments. Ifexperiments are similar to those of previous investigators be sure to make use ofexperience of the previous workers. Never overlook the possibility that the work mayhave been done before and reported in the literature.

Generalized experimental procedure (contd.)

4. Continue planning by performing the following steps:

Establish the primary variables which must be measured

Determine as nearly as possible the accuracy which may be required in the primarymeasurements and the number of such measurements which will be required forproper data analysis.

Set up data reduction calculations before conduction the experiments to be surethat adequate data are being collected to meet the objective of the experiment.

Analyze the possible errors in the anticipated results before the experiments areconducted so that modification in accuracy requirements on the variousmeasurements may be changed if necessary.

Generalized experimental procedure (contd.)

5. Select instrumentation for the various measurement to match the anticipated accuracyrequirements.

6. Modify the instrumental apparatus and /or procedure in accordance with the findings initem 5.

7. Collect the bulk of experimental data and analyze the result.

8. Organize discus and publish the findings and results of the experiments, being sure toinclude information pertaining to all items 1 to 7, above.

Primary Stages of Experimental Planning

Establish need for

experiment

Establishtime andfinanciallimitations

Estimatescope ofaccompanyinganalyticalwork

Carefullyreviewpreviouswork inthe field

Establishgeneralfeasibilitywithinoriginalbudgetand timelimitations

Possible modify scope of analytical work

Possible modify original notion of need.

If not feasible require modificationof budget and/or time schedule ofdiscontinue effort

Intermediate Stages of Experimental PlanningBegin preliminary analytical work in order to

Determine variable rangesand accuracies required

Specify instrumentsrequired

Modify in accordancewith budget limitations

Purchase instruments

Arrange fordesign andconstructionequipment

Proceed with analytical work

Compare preliminary data

with theories available

If comparison notfavorable possiblymodify experimentand /or analysis

Collect a fewpreliminary datapoints

Analyzeuncertaintyin data

Final Stage of Planning

Collect bulk of data

Continue analysis including possible

computer runs

Match theories withexperiment, correlatedata, create new theoriesto explain data etc.

Correlate with need for experiment

Discuss and publish results of

experiments

Static Characteristic of Instrument

Accuracy of measurement is one consideration in the choice of instrument for a particularapplication. Other parameters such as sensitivity, linearity and the reaction to ambienttemperature changes are further considerations. These attributes collectively known as thestatic characteristics of the instruments.

Accuracy/ inaccuracy (measurement uncertainty)

Accuracy of an instrument is a measure of how close the output reading of the instrumentis to the correct value.

In practice, it is more usual the inaccuracy figure rather than the accuracy figure for aninstrument.

Inaccuracy is the extent to which a reading might be wrong and it is often quoted as apercentage of the full scale reading of an instrument.

Static Characteristic of Instrument (contd.)

Precision/ repeatability/ reproducibility

Precision term that describes an instruments degree of freedom form random errors.

High precision does not imply anything about measurement accuracy.

A high precision may have a low accuracy.

Low accuracy measurement form a high precision instruments are normally caused by abias in the measurements, which is removable by recalibration.

The term repeatability and reproducibility mean approximately the same but are applies indifferent contexts as given below.


Repeatability describes closeness of output reading when the same input is appliedrepetitively over a short period of time, with the same measurement conditions, sameinstrument, and observer, same location and same conditions of use maintainedthroughout.

Reproducibility describes the closeness of output reading for the same input when thereare changes in the methods of measurement, observer, measuring instrument, location,condition, condition of use and time of measurement.


Tolerance

Tolerance is a term that is closely related to accuracy and defines the maximum error thatis to be expected in some value.

Accuracy of some instrument is some times quoted as a tolerance figure. When usedcorrectly tolerance describes the maximum deviation of a manufactured component formsome specified value.

Range or span

The range or span of an instrument defines the minimum and maximum values of aquantity that the instrument is designed to measure.


Linearity

It is normally desirable that the output reading of an instrument is linearly proportional tothe quantity being measured.

Sensitivity of measurement

The sensitivity of measurement is a measure of the change in instrument output thatoccurs when the quantity being measured changes by a given amount.

For example a pressure of 2 bar produces a deflection of 10 degrees in a pressuretransducer, the sensitivity of the instrument is 5 degree/bar.


Threshold

The minimum value of input signal that is required to make a change or start from zero.

Resolution

The minimum value of the input signal is required to cause an appreciable change in theoutput known as resolution.


Sensitivity to disturbance

All calibrations and specifications of an instrument are only valid under controlledconditions of temperature, pressure etc.

These standard ambient conditions are usually defined in the instrument specification.

As variation occur in the ambient temperature, certain static instruments characteristicschange and the sensitivity to disturbance is measure of the magnitude of this change.

Such environmental changes affect instruments in two main way, known as the zero driftand sensitivity drift.


Zero drift or bias describes the effect where the zero reading of an instrument is modifiedby a change in ambient conditions.

Zero drift or bias

Example of bathroom scale.


Sensitivity drift defines the amount by which an instruments sensitivity of measurementvaries as ambient conditions change.

Sensitivity drift

Give example ofSpring balance.


Hysteresis effects.

All the energy put into the stressed component when loaded is not recovered uponunloading.

So the output of measurement partially depends on input called Hysteresis.

It is defined as the magnitude of error caused in the output for a given value of input,when this value is approached from opposite direction i.e. from ascending order and thendescending order.

This is caused by backlash, elastic deformation but is mainly caused due to frictionaleffects.


Hysteresis effects are best eliminated by taking observation in both the direction i.e. Inascending and then descending order values of input and then taking the arithmetic mean.

Hysteresis is most commonly found in instruments that contain springs, such as the passivepressure gauge and the Prony brake.

Dead space.

Dead space is defined as the range of different input values over which there is no changein output value.

experimental planning

Documents

particular instrument

hysteresisan instrument

known value

scale length

theinstrument scale

known standard

measuring device

measured quantity