Engineering Metrology & Measurement

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.

These  gauges  are  otherwise  called  as  Gauge  blocks  or  Block  gauges  and  are  universally accepted as end standards of length in industry.  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 800 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 0.00635  m which is negligible. 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.

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). Uses of Comparator:   For calibrating the working gauges   Used as working gauges   Used as final inspection gauges Essential characteristics of a good Comparator:   Robust design and construction   Linear characteristics of scale   High magnification   Quick in results   Versatility   Minimum wear of contact point   Free from back lash   Quick insertion of work piece   Provision for compensation from temperature effects   Provision for means to prevent damage during use. Classification of comparators: 1)  Mechanical comparator a)        Dial indicator b)        Johansson „Mikrokator‟ comparator c)        Sigma comparator d)        Reed type mechanical comparator 2)  Optical comparator: a)        Zeiss Ultra optimeter b)        Zeiss optotest comparator 3)  Mechanical – Optical comparator 4)  Electrical comparator 5)  Fluid displacement comparator 6)  Pneumatic comparator a)        Back pressure comparator b)        Flow – velocity Pneumatic comparator In addition, the comparators used in standards room for calibration of gauges are: 7)  Brookes Level comparator 8)  Eden-Rolt „Millionth‟ Comparator

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…

1) Reading error:  It is the error of observation resulting from incorrect reading of the indication of a measuring instrument by the observer. 2) Parallax error:  It is the reading error which is produced, when, with the index at a certain distance from the surface of scale, the reading is not made in the direction of observation…

1)  Error of a physical measure:    It is the difference between the nominal value and the conventional true value reproduced by the physical measure. 2)  Error of a measuring mechanism:   It is the difference between the value indicated by the measuring mechanism and the conventional true value of the measured quantity. 3)  Zero error: It is the indication…

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…

A broad classification of the instruments based on the application mode of operation, manner of energy conversion and the nature of energy conversion and the nature of output signal is given below 1. Deflection and null type instruments 2. Analog and digital instruments 3. Active and passive instruments 4. Automatic and manually operated instruments 5….

1. Line & End Standards:  In the Line standard, the length is the distance between the centers of engraved lines whereas in End standard, it is the distance between the end faces of the standard. Example: for Line standard is Measuring Scale, for End standard is Block gauge.  Primary, Secondary, Tertiary & Working Standards: Primary standard:   It…

1) For Frequency: Hertz (1 Hz = 1 cycle per second) 2) For Force: Newton (1 N = 1 kg-m/s2) 3) For Energy: Joule (1 J = 1 N-m) 4) For Power: Watt (1 W = 1 J/s)