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Sunday, 3 March 2019

Standards for Measurement

Standards for Measurement:

A standard is a physical representation of a unit of measurement. The term standard is given to a piece of equipment having a known measure of physical quantity. A unit is realized by reference to a material standard or to a natural phenomenon including physical and atomic constants.

Example : The fundamental unit of mass in the metric system (that is SI system) is the kilogramme. It is defined as the mass of the cubic decimetre of water at its temperature of maximum density of 4°C.

The above unit of mass is represented by a material standard. The mass of the International Prototype kilogramme consists of platinum-iridium hollow cylinder. This unit is preserved at the International Bureau of weights and measures at SERVERES, near PARIS. It materially represents the kilogramme.

In a similar manner, standards have been developed for other units of measurement, including fundamental units as well as some derived mechanical and electrical units.

There are different types of standards of measurement. They are classified as follows :

1. International standards.
2. Primary standards
3. Secondary standards
4. Working standards

1. International standards :

These are defined on the basis of international agreement. They represent the units of measurements that are closest to the possible accuracy obtainable with present day Scientific and Technological methods. These standards are checked and evaluated regularly and periodically by absolute measurements in terms of the fundamental units. International Bureau of Weights and Measures, maintains these standards. These are not available to common user for purpose of comparison or calibration.

2. Primary standards :

These are absolute standards of very high accuracy that can be used as the ultimate reference standards. These standards are maintained by National Standards Laboratories in different parts of World. The primary standards are not available outside the National Laboratories. The primary standards representing the fundamental units and some of the derived mechanical and electrical units are independently calibrated by absolute measurements at each of the national laboratories. The results obtained out of these measurements are compared against each other, yielding a world average figure for the primary standard. One of the main functions of primary standards is the verification and calibration of secondary standards.

3. Secondary standards :

These are the basic reference standards used in industrial measurement laboratories. They will be maintained by the particular industry involved and are checked locally against other reference standards in the areas. The responsibility of maintaining and calibration of the secondary standards lies with the industrial laboratory itself. These standards are periodically sent to National standards laboratories for comparison and calibration against the primary standards. The National standards laboratories will certify the values in terms of primary standards.

4. Working standards :

These are the principal tools of a measurement laboratory. They are used to compare and calibrate general laboratory instruments for accuracy and performance.
In the field of electronic measurements, we are connected with electrical and magnetic standards of measurement. The following are the standards :

Standards for Time and Frequency :

We know that the frequency is inversely proportional to time. So when a standard for time is established it also establishes the standard for frequency.

The standard reference for uniform time in the early days was the period of rotation of earth about its axis, with respect to Sun.

It was found that the rotation of earth around sun is irregular. Hence solar time could not represent uniform time scale. Latter mean solar time was taken in to consideration to give more accurate time scale. Taking the mean solar day to be the average of all the apparent days in the year, mean solar second was defined as 1/86,400 of the mean solar day. This also proved inadequate as a fundamental unit of time. The reason is non uniform rotation of earth. Universal time or mean solar time is based on earth's rotation. It is known as UT0. This is prone to periodic, long-term, and irregular variations. Two corrected universal time scales UT1, and UT2, considering the polar motion, seasonal variations of earth have come up. However the accuracy that could be obtained was only a few milliseconds. Further search yielded ephemeris time (ET). This is based on astronomical observations of the motion of the moon about the earth. From the year 1956 the ephemeris second' has been defined by the International Bureau of Weights and Measures as a fraction 1/31, 556, 925, 9747 of the tropical year for 1900 January 0 at 12 h ET, and adopted as the fundamental invariable unit of time.

Use of the ephemeris second has a disadvantage that it can be determined only after several years in arrears and then only after indirectly by the observations of the positions of the sun and the moon.

For Physical measurements we now use atomic standard. The universal second and the ephemeris second, will continue to be in use for navigation, geodetic surveys, and celestial mechanics.

Atomic resonators have been developed that control the frequency of an oscillator. Therefore using frequency conversion atomic clocks could be made. These clocks have great precession and accuracy. The transition between two energy levels E, and E2 of an atom is accompanied by the emission (or absorption) by radiation given by :

E2—E1 = hv

Where h = Planck's constant.

v = a physical constant (which is a frequency) provided that the energy states are not affected by external conditions such as magnetic fields. This physical constant depends only on the internal structure of the atom.

The International Committee of Weights and Measures has now defined the second in terms of frequency of the cesium transition, assigning a value of 9, 192, 631, 770 Hz to the hyperfine transition of the cesium atom unperturbed by external fields.

An atomic clock with a precision exceeding 1 micro-second per day is in operation as a primary frequency standard at NBS (National time service of NBS). An atomic time scale, called NBS - A is maintained with this clock.

Ampere (current) : 

The definition of ampere has been given already under SI units. The absolute value of ampere was measured in early days using current balance. It measured the force between the two parallel conductors. As this measurement was found inadequate to establish the standard, other method was investigated. By International agreement the value of International ampere was based on the electrolytic deposition of silver from a silver nitrate solution. International ampere was then defined as that current which deposits silver at the rate of 1,118 mg/s from a standard silver nitrate solution. Exact measurement of the deposited silver was difficult. Also discrepancies were observed between measurements made independently by the various national standards laboratories. In the year 1948 International ampere was suppressed by the Absolute ampere.

The absolute ampere is now the fundamental unit of electric current in the SI and is universally accepted by international agreement.

Ohm's law gives the relation between voltage, current and resistance. When any of the two quantities are specified the third one is automatically set. Two types of material standards from a combination that conveniently serves to keep the ampere with high precision over very long periods of time. They are the standard resistance and standard cell.

Standard resistance:

The absolute value of the ohm in SI system is defined in terms of the fundamental units of length, mass and time. The absolute measurement of the ohm is done by the International Bureau of Weights and Measures in Serves and also by National Standards Laboratories.

The standard resister is a coil of wire of some alloy like Manganin which has a high electrical resistivity and a low temperature coefficient of resistance. This coil will be mounted in a specially made container. This is to prevent changes in unit due to moisture conditions. With a set of four or five I ohm resistances of this type, the unit of resistance can be represented with a precision of a few parts in 107 over several years.

Standard for voltage:

The primary voltage standard selected is the normal or saturated, Weston cell. It has a positive electrode of mercury and a negative electrode at cadmium amalgam (10% cd). The electrolyte is a solution of cadmium sulphate. These components are placed in a H shaped glass container. The voltage of the weston saturated cell 20°C is 1.01858 (absolute). It maintains the voltage for periods as long as 10 to 20 years when carefully treated. The drift in voltage is 1 micro volt per year.

Standards for capacitance:

Capacitance standards are constructed from interleaved metal plates with air as the dielectric material. Accurate measurement of capacitance can be made with maxwell dc commutated bridge. In this bridge capacitance is computed from resistance bridge arms and the frequency of the dc commutation. As both resistance and frequency can be determined very accurately, capacitance can be measured with great accuracy.

Standard capacitors mentioned above will have their capacitance calculated from the area of the plates, distance between the plates by accurately knowing those dimensions. Such standard capacitors can be used to calibrate the secondary and working standards. Silver mica capacitors are used as working standards due to their excellent stability, small temperature coefficient, and low dissipation factor. Decade capacitances used as standards for working have only give accuracy better than 1 %. When accuracy is important fixed standards are only used.

Inductance standards:

The primary inductance standard is derived from the ohm and the farad. Large geometrically constructed inductors are used in the determination of the absolute value of ohm. Campbell standard of mutual inductance was selected as the primary standard for both mutual and self inductance. Fixed and variable working standards of inductances are available. The range of inductance working standards ranges from 100 micro henry to 10 H. At a notified frequency they give an accuracy of 0.1% for fixed values and for variable ones this is of the order of 2.5%.

Magnetic flux standard:

The hibbert magnetic standard is used as the standard flux source independent of external exciting current.

Having seen the units, standards and their definitions, let us now see to the needs of measurements. Measurement needs a complex set up and not a simple tool. This set up required is called instrumentation. In electronic measurements we are going to study about the various measuring instruments.

We have seen the units, definitions of fundamental units, some definitions, errors and standards of measurements. The knowledge on the above will be helpful in understanding the electronic measurements and measuring instruments.

Measurements need often a complex set up and not a simple tool. This complex set up is instrumentation. Measuring systems are of two types. They are analog and digital. The present day measurements and instrumentation is more of digital system. However the analog instruments still have their place in the field. Digital measuring instruments have certain advantages over the analog ones. They do have their disadvantages. These aspects will be delt with in the topics ahead appropriately.

For an Electronics Engineer the measurements of major interest are, resistance, capacitance, inductance: voltage, current, power and frequency (or time). Though the electrical engineers also are interested in these measurements, there exists a major difference. The power or electrical engineers measure these quantities in larger magnitudes, and at a single frequency, where as the electronics engineers have to cover a wide frequency spectrum for all the above parameters. In addition we deal with very low levels of milli, micro and pico units. All these measurements pose problems of loading, reaction and frequency, wave form effects.

The scope of electronics, electronic measurements and instrumentation is so vast that there is no field so far investigated where electronic instrumentation does not find its place.
The current trend is to have more portable, more accurate test instruments using digital integrated circuits.

The world class technology presents currently two in one test instruments, three in one test instruments.

For precision laboratory work computerized instruments are available that not only give indication of the quantity measured but also can keep a hard copy of the results or records.
To cite a simple example, multi trace oscilloscopes, with storage and recording facilities are in market.

The old analog instruments have totally become obsolete with the present day new generation of Electronic Measuring Instruments, that provide error free accurate, measurement. Further these new generation instruments are user friendly, averting wrong results by application faults of user. They automatically adjust the range, polarity and prevent damage of instruments. They prompt the user of his mistake.

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