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Join With us. Today Updates. August 8. July June Duggal Free Download June Charles H. Roth, Larry April April 6. Popular Files. Grewal Book Free Download April Bansal Book Free October Active and passive instruments 4. Automatic and manually operated instruments 5. Contacting and non contacting instruments 6. Absolute and secondary instruments 7. Intelligent instruments. Performance of instruments: All instrumentation systems are characterized by the system characteristics or system response.
It consists of two basic characteristics such as static and dynamic. If the instrument is required to measure a condition not varying with time characteristics are called static while for a time varying process variable measurement, the dynamic characteristics are more important.
Static response: The static characteristics of an instrument are considered for instruments which are used to measure an unvarying process conditions. Dynamic response: The behaviors of an instrument under such time varying input — output conditions called dynamic response of an instrument. The instrument analysis of such dynamic response is called dynamic analysis of the measurement system.
Transient Terms in Measurement: Sensitivity: Sensitivity of the instrument is defined as the ratio of the magnitude of the output signal to the magnitude of the input signal. It has wide range of the units which dependent up on the instrument or measuring system. Readability: Readability is a word which is frequently used in the analog measurement. The readability is depends on the both the instruments and observer. The susceptibility of a measuring instrument to having its indications converted to a meaningful number.
It implies the ease with which observations can be made accurately. Accuracy: Accuracy may be defined as the ability of instruments to respond to a true value of a measured variable under the reference conditions. It refers to how closely the measured value agrees with the true value. Precision: Precision is defined as the degrees of exactness for which an instrument is designed or intended to perform. It refers to repeatability or consistency of measurement when the instruments are carried out under identical conditions at a short interval of time.
It can also defined as the ability of the instruments to reproduce a group of the instruments as the same measured quantity under the same conditions. Correction: Correction is defined as a value which is added algebraically to the uncorrected result of the measurement to compensate to an assumed systematic error. It is the process of determining the values of the quantity being measured corresponding to a pre-established arbitrary scale.
It is the measurement of measuring instrument. Interchangeability: A part which can be substituted for the component manufactured to the small shape and dimensions is known a interchangeable part.
The operation of substituting the part for similar manufactured components of the shape and dimensions is known as interchangeability. Constant of a measuring instrument: The factor by which the indication of the instrument shall be multiplied to obtain the result of measurement.
Nominal value of a physical measure: The value of the quantity reproduced by the physical measure and is indicated on that measure. Conventional true value of a physical measure: The value of the quantity reproduced by the physical measure, determined by a measurement carried out with the help of measuring instruments, which show a total error which is practically negligible.
Standard: It is the physical embodiment of a unit. Types of Errors: A Error of Measurement: 1 Systematic error: It is the error which during several measurements, made under the same conditions, of the same value of a certain quantity, remains constant in absolute value and sign or varies in a predictable way in accordance with a specified law when the conditions change.
The causes of these errors may be known or unknown. The errors may be constant or variable. Systematic errors are regularly repetitive in nature. Random errors are non-consistent.
Random errors are normally of limited time duration. B Instrumental error: 1 Error of a physical measure: It is the difference between the nominal value and the conventional true value reproduced by the physical measure.
C Error of observation: 1 Reading error: It is the error of observation resulting from incorrect reading of the indication of a measuring instrument by the observer. D Based on nature of errors: 1 Systematic error: already discussed 2 Random error: already discussed 3 Illegitimate error: As the name implies, it should not exist.
These include mistakes and blunders, computational errors and chaotic errors. Chaotic errors are random errors but unlike the latter, they create chaos in the final results. E Based on control: 1 Controllable errors: The sources of error are known and it is possible to have a control on these sources.
These can be calibration errors, environmental errors and errors due to non- similarity of condition while calibrating and measuring. Calibration errors: These are caused due to variation in the calibrated scale from its normal value. The actual length of standards such as slip gauges will vary from the nominal value by a small amount. This will cause an error of constant magnitude. Instruments are calibrated at these conditions.
If there is any variation in the ambient condition, errors may creep into final results. Of the three, temperature effect is most considerable. Stylus pressure errors: Though the pressure involved during measurement is generally small, this is sufficient enough to cause appreciable deformation of both the stylus and the work piece. This will cause an error in the measurement.
Avoidable errors: These errors may occur due to parallax in the reading of measuring instruments. This occurs when the scale and pointer are separated relative to one another.
The two common practices to minimise this error are: i Reduce the separation between the scale and pointer to minimum. These avoidable errors occur also due to non-alignment of work piece centers, improper location of measuring instruments, etc. Angular measurements: - Sine bar, angle dekor, Optical bevel protractor Comparator: It is a precision instrument employed to compare the dimension of a given component with a working standard generally slip gauges.
It does not measure the actual dimension but indicates how much it differs from the basic dimension working standard. Slip gauges are rectangular blocks of high grade steel or tungsten carbide with less co-efficient of thermal expansion. These blocks are highly hardened more than HV through out to ensure maximum resistance to wear and are then stabilised by heating and cooling successively in stages so that the hardening stresses are removed.
After hardening, they are subjected to lapping to a high degree of finish, flatness and accuracy. The cross sections of these gauges are 9 x 30 mm for sizes up to 10 mm and 9 x 35 mm for larger sizes. The dimension height is marked on one of the measuring faces of gauge blocks. Wringing of Slip gauges: The slip gauges are wrung together by hand through a combined sliding and twisting motion. The air gap between the gauge faces is expelled out and the adhesion is caused partly by molecular attraction and partly by atmospheric pressure.
The gap between the two wrung slip gauges is only of the order of 0. Selection of Slip gauges for required dimension: Always start with the last decimal place and deduct this from the required dimension. Select the next smallest figure in the same way, find the remainder and continue this until the required dimension is completed.
Minimum number of slip gauges should be selected to build up the given dimension. Roller gauges: Cylindrical rollers with their lengths equal to their diameters may be used as gauges, secondary to block gauges slip gauges. These are produced to fine tolerances. Limit gauges: These are inspection tools for rigid design, without a scale, which serve to check the dimensions of manufactured parts.
They can only be used for determining as to whether the inspection parts are made within the specified limits. These gauges are made up of suitable wear resisting steel and are normally hardened to not less than HV and suitably stabilized and ground and lapped. Gauge Design: Every gauge is a copy of the part which mates with the part for which the gauge is designed.
For example, a bush is made which is to mate with a shaft; in this case, the shaft is the mating part. The bush is check by a plug gauge which in so far as the form of its surface and its size is concerned, is a copy of the mating part shaft. Types of Limit Gauges: The various types of limit gauges used for gauging internal diameters of holes are: 1 Full form cylindrical plug gauge: The gauging surface is in the form of an external cylinder.
Generally a small circumferential groove is cut near the leading end of the gauge and the remaining short cylindrical surface is slightly reduced in order to act as a pilot. The various types of limit gauges used for gauging external diameters of shaft are: 1 Full form cylindrical ring gauge: The gauging surface is in the form of an internal cylinder and whose wall is thick enough to avoid deformation under normal conditions of use.
The surfaces constituting the working size may both be flat or both cylindrical also. The gauges for internal taper are marked with a ring on the gauge planes another ring to indicate the minimum depth of internal taper.
For testing the external taper of the tanged end shank, the ring gauge is inserted, as far as it goes with light pressure. At the extreme position, no part of the tang under test should extend beyond the surfaces A, B and C. The shank surfaces may however, lie flush with these surfaces.
Autocollimator: It is an optical instrument used for the measurement of small angular differences. It is essentially an infinity telescope and a collimator combined into one instrument. Principle of Autocollimator: If a point source of light O is placed at the principal focus of a collimating lens, it will be projected as a parallel beam of light. If this parallel beam now strikes a plane reflector which is normal to the optical axis, it will be reflected back along its own path and refocused at the source O.
The important points about this collimation of a beam of light are: a The distance between the reflector and the lens has no effect on the separation x between source and image. Thus, for a wide range of readings, the minimum distance between lens and reflector is essential.
Angle Dekor: In this system, an illuminated scale is set in the focal plane of the collimating lens outside the field of view of a microscope eyepiece. It is then projected as a parallel beam and strikes a plane reflector below the instrument. It is reflected, and refocused by the lens so that its image is in the field of view of the eyepiece. The image falls, not across a simple datum line, but across a similar fixed scale at right angles to the illuminated image.
This feature enables angular errors in two planes to be dealt with or more important, to ensure that the reading on a setting master and on the work is the same in one plane, the error being read in the other. Thus, induced compound angle errors are avoided. The setup consists of a lapped flat and reflective base above which the optical details are mounted in a tube on an adjustable bracket.
In use, a master, either a sine bar or a group of combination angle gauges is set up on the base plate and the instrument is adjusted until a reading on both sides is obtained.
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