Terms of measuring systems Line standards End standards

Terms of measuring systems:

(i) Sensitivity

(ii) Readability

(iii) Calibration

(iv) Repeatability Sensitivity

Sensitivity may be defined as the rate of displacement of the indicating device of a instrument, with respect to the measured quantity. In other words, sensitivity of an instrument is the ratio of the scale spacing to the scale division value. For example, if on a dial indicator, the scale spacing is 1.0 mm and the scale division value is 0.01 mm, then sensitivity is 100. It is also called as amplification factor or gearing ratio.

If we now consider sensitivity over the full range of instrument reading with respect to measured quantities as shown in Fig., the sensitivity at any value of  y = dy/dx where dx and dy are increments of x and y, taken over the full instrument scale, the sensitivity is the slope of the curve at any value of y.

The sensitivity may be constant or variable along the scale. In the first case we get linear transmission and in the second non-linear transmission and in the second non-linear transmission.

Sensitivity refers to the ability of measuring device to detect small difference in a quantity being measured. High sensitivity instruments may lead to drifts due to thermal or other effects, and indications of lower sensitivity.

Readability

Readability refers to the ease with which the readings of a measuring instrument can be read. It is the susceptibility of a measuring device to have its ​indications converted into meaningful number. Fine and widely spaced graduation lines ordinarily improve the readability. If the graduation lines are very finely spaced, the scale will be more readable by using the microscope, however, with the naked eye the readability will be poor.

To make micrometers more readable they are provided with vernier scale. It can also be improved by using magnifying devices.

Calibration

The calibration of any measuring instrument is necessary to measure the quantity in terms of standard unit. It is the process of framing the scale of the instrument by applying some standardized signals. Calibration is a premeasurement process, generally carried out by manufactures.

It is carried out by making adjustments such that the read out device produces zero output for zero measured input. Similarly, it should display an output equivalent to the known measured input near the full scale input value.

The accuracy of the instrument depends upon the calibration. Constant uses of instruments affect heir accuracy. If the accuracy is to be maintained, the instruments must be checked and recalibrated if necessary. The schedule of such calibration depends upon the severity of use, environmental conditions, accuracy of measurement required etc. as far as possible calibration should be performed under environmental conditions which are vary close to the conditions under which actual measurements are carried out. If the output of a measuring system is linear and repeatable, it can be easily calibrated.

Repeatability

It is the ability of the measuring instrument to repeat the same results for the measurements for the same quantity, when the measurement are carried out

by the same observer. With the same instrument. Under the same conditions. Without any change in location.

Standards:

Line and End Measurements

A length may be measured as the distance between two lines or as he distance between two parallel faces. So, the instruments for direct measurement of linear dimensions fall into two categories

1. Line standards 2. End standards

Line standards. When the length is measured as the distance between centres of two engraved lines, it is called line standard. Both material standards yard ​and metre are line standards. The most common example of line measurement is the rule with divisions shown as lines marked on it.

Characteristics of Line Standard

1. Scales can be accurately engraved but the engraved lines them selves possess thickness and it is not possible to take measurements with high accuracy.

2. A scale is a quick and easy to use over a wide range.

3. The scale markings are not subjected to wear. However, he leading ends are subjected to wear and this may lead to undersize measurements.

4. A scale does not posses a ―built in ― datum. Therefore it is not possible to align the scale with the axis of measurement.

5. Scales are subjected to parallax error.

6. Also, the assistance of magnifying glass or microscope is required if sufficient accuracy is to be achieved.

End standards: When length is expressed as the distance between two flat parallel faces, it is known as ends standard. Examples: Measurement by slip gauges, end bars, ends of micrometer anvils, vernier calipers etc. the end faces are hardened, lapped flat and parallel to a very high degree of accuracy.

Characteristics of End Standards:

1. These standards are highly accurate and used for measurement of close tolerance in precision engineering as well as in standard laboratories, tool rooms, inspection departments etc.

2. They require more time for measurements and measure only one dimension at a time.

3. They are subjected to wear on their measuring faces.

4. Group of slips can be ―wrung‖ together to build up a given size; faulty wringing and careless use may lead to inaccurate results.

5. End standards have built in datum since their measuring faces are flat and parallel and can positively locked on datum surface.

6. They are not subjected to parallax effect as their use depends on feel.

The accuracy of both these standards is affected by temperature change and both are originally calibrated at 20+ or 20- 1/2°C.

It is also necessary to take utmost case in their manufacture to ensure that the change of shape with time, secular change is reduced to negligible.

​line and end standard measurements:

Comparison between line standards and End Standards:

Sr. No. Characteristics - Line standard - End standard

1. Principle

- Length is expressed as the distance between two lines

- Length is expressed as the distance between two flat parallel faces

2. Accuracy

- Limited to is +or-0.2 mm for high accuracy, scales have to be used in conjunction with magnifying glass or microscope.

- Highly accurate for measurement of close tolerances up to + or - 0.001 mm.

3. Ease and time of and easy.

- Measurement is quick and easy. 

- Use of end standard requires skill and is time consuming.

4. Effect of wear

- Scale markings are not subject to wear. However, significant wear may occur on leading ends. Thus it may be difficult to assume zero of scale as datum.

- These are subjected to wear on their measuring surfaces.

5. Alignment 

- Cannot be easily aligned with the axis of measurement.

- Can be easily aligned with the axis of measurement.

6. Manufacture and cost

- Simple to manufacture at low cost.

- Manufacturing process is complex and cost is high

7. Parallax effect

- They are subjected to parallax error.

- They are not subjected to parallax error.

8. Examples

- Scale (yard, metre etc.,)

- Slip gauges, end bars, V. caliper, micrometers etc.