Standards and Calibration

STANDARDS AND CALIBRATION

Standard

The standard is the physical representation of the unit, standard is applied to an instrument having a known measure of physical quantity. This instrument is used for obtaining the physical properties of other instruments by comparison methods.

Unit is represented by a material standard or by reference to a natural phenomenon including physical and atomic constants.

E.g., units of length in SI unit is Metre (m). It is defined as the Length equal to 1,650, 763. 73 wavelengths in vacuum of radiation corresponding to the transition between the Level 2₁₀ and 5_d5 (orange red line) of krpton 86 atom (excited at the triple of nitrogen, 63.15 Kelvin).

According to their functions and applications standards are classified into four types, they are:

1. International Standards:

It is maintained at the International Bureau of weights and measures. These standards represents the units which are closest to the possible accuracy attainable with present technology. These standards are checked and evaluated regularly against absolute measurements in terms of fundamental units. These standards are not available to ordinary user for their Instruments calibration. One of the main reason for using absolute unit is that it do not vary appreciably with time.

2. Primary Standards:

Primary standards are absolute standards of high accuracy and it is used as reference standards. These standards are maintained by national standards laboratories in different parts of the world.

These standards represents all fundamental, supplementary and derived units calibrated by absolute measurements at each national laboratories. These standards are not available outside the national laboratories. Its main function is the verification and calibration of secondary standards.

3. Secondary Standards:

These are the reference standard used in industrial measurement laboratories. These standards are checked Locally against reference standards available in that area. These instruments are sent periodically to the national standard laboratories for calibration and comparison against primary standard. They sent back the instrument. with a certificate mentioning that the instrument is calibrated against primary standards.

4. Working Standards:

Working standards are available in the measurement laboratory. These are used for the calibration of general Laboratory instruments.

Standard of Mass, Length and Time

Mass:

The unit of mass is kilogramme, A kilogramme is equal to the mass of the international prototype (a cylinder of platinum iridium alloy) of mass. The primary standard of mass has an accuracy of part in 10⁸ and verified against the international standard. The secondary standard of mass has on accuracy of 1 ppm and working standard has an accuracy of 5 ppm.

Length:

International standard of Length is given by optical standard. The orange red radiation emitted by krypton - 86 discharge lamp when excited forms the basic unit of Length. This can be measured to an accuracy of 1ppm.

Metre was refined in terms of time, it is the distance travelled by light in vacuum in a time interval 1/299792458 of second. Working standard of length are wide in industries, these are usually precision Gauge blocks madeup of steel. These blocks have parallel surface and the distance between two surfaces is specified. This gives the accurate measurement of about 1ppm.

Time:

The unit of time is second, It is defined as the duration of 9, 192, 631, 770 periods of radiation corresponding to the transition between the two hyperfine Levels of the ground state of the cesium 133 atom.

Other Standard Quantities

Current:

International Ampere is the current which when passed through a solution of silver nitrate, causes deposition of silver at the rate of 1.118 mg/sec. these International ampere is superseded by absolute ampere, in 1948.

Absolute ampere:

It involves the measurement of force because its dimension resembles the dimension of force dimension of (current)²= [MLT^-2]

The force may be exerted by the current in a solenoid (as in tangent galvanometer) or by the current flows through the two different parts of the circuit connected in series. (current balance)

Rayleigh's current balance:

It consists of two fixed coil and a small moving coil. These three coils are connected in series. Moving coil is suspend from the arm of a sensitive balance If a current flows through these coils, force will be exerted on the moving coil and is  given by F = I² dm/dx newton

Were, dM/dx is the rate of change of mutual inductance between the fixed and

moving coils.

In SI unit absolute Ampere is the fundamental unit of current.

Voltage:

Standard volt was given by electrochemical cell (saturated standard cell). But it has the drawback of temperature dependence.

In 1962, Josephson developed a new standard for voltage, it is the voltage developed across a thin film function which is cooled to nearly absolute zero and irradiated with microwave. The voltage developed is related to irradiating frequency by.

v = hf /2e

Where v is the voltage developed

h is the plancks constant

ƒ is the irradiating microwave frequency and e is the electron volt.

Resistance:

The standard resistor is made up of some alloy like manganin, having high electrical resistivity and low temperature coefficient.

Absolute measurement of resistance is done by measuring the velocity or by measuring the Length and time. It is done by Lorenz method.

In this method, current flows through a solenoid producing flux lines, a disc is placed inside the solenoid and tend to rotate. Due to the flow of current through the resistor whose value is to be measured gives a voltage drop. Disc also produces some amount of voltage due to its rotation in the flux path. The Galvanometer reads zero when these two voltages are equal. It is given by

MIN = IR

R = Mn

Where R is the resistance,

M is the mutual inductance,

n is the speed of the disc in rps.

Luminous intensity:

The primary standard of luminous intensity is the black body or planking radiator at the temperature of 2042k (solidification of platinum occur at this temperature). Black body is called the full radiator.

Now candela is 1/60 of Luminous intensity per square centimeter of full radiator. Secondary standards are obtained from special tungsten filament lamps.

Calibration

Calibration is defined as the process of determination of the correct value of each scale reading on a meter or other measuring instrument by measurement or comparison with a standard.

In Laboratory, the calibration of instruments are it done by comparing the reading of the instrument with the sub-standards at several points along the scale. These sub - standards are periodically checked against some standards held permanently in national laboratories. This type of calibration hold good for mechanical measurements.

In process measuring Instruments calibration is done by feeding the known magnitudes of the basic input quantity to the instrument and observes the systems behavior.

Static Calibration

Characteristics of a transducer like linearity, Accuracy, resolution, precision, range and span, etc., are obtained in one form or other through static calibration. A transducer may respond to a number of inputs. Some Inputs are measurable and determinable and some Inputs may be unmeasurable. The measurable input is the desired quantity and others are called Interfering or modifying Inputs.

Kept all the inputs except the desired quantity as constant at some particular value, Now vary the desired quantity over some range in steps, Note down the outputs, plot the graph or tabulate the readings showing input-output relationship. This is called static calibration. This is valid under the stated constant conditions of all other inputs.