Line and end standards plan: Introduction Line and end measurements

Table 1. Comparison of line and end standards


Transfer from Line Standard to End Standard

We know that primary standards are basically line standards and that end standards are practical workshop standards. Line standards are highly inconvenient for general measurement purposes and are usually used to calibrate end standards, provided that the length of primary line standard is accurately known. There is a probability of the existence of a very small error in the primary standard, which may not be of serious concern. It is important to accurately determine the error in the primary standard so that the lengths of the other line standards can be precisely evaluated when they are compared with it. From the aforementioned discussions it is clear that when measurements are made using end standards, the distance is measured between the working faces of the measuring instrument, which are flat and mutually parallel. A composite line standard is used to transfer a line standard to an end standard. Figure 1 shows a primary line standard of a basic length of 1m whose length is accurately known. A line standard having a basic length of more than 1m is shown in Fig. 2. This line standard consists of a central length bar that has a basic length of 950mm. Two end blocks of 50 mm each are wrung on either end of the central bar. Each end block contains an engraved line at the centre. The composite line standard whose length is to be determined is compared with the primary line standard, and length L is obtained as using the following formula:

L = L1 + b + c
The four different ways in which the two end blocks can be arranged using all possible combinations and then compared with the primary line standard are
L = L1 + b + c
L = L1 + b + d
L = L1 + a + c
L = L1 + a + d
Summation of these four measurements gives
4L = 4L1 + 2a + 2b + 2c + 2d


Figure 1.Basic length of primary line standard