When a 30-metre chain is tested for its accuracy against a steel tape standardised at 20°C with an 8 kg pull, its overall length should NOT deviate beyond 30 m _____.
📝 Detailed Explanation: Chain Surveying Concepts & Tolerances
Concept of Chain Surveying
Chain surveying is the simplest method of surveying, in which only linear measurements are made; no angular measurements are taken. The measurements are taken on the field, and supplementary work like plotting and calculations are carried out in the office. The primary tools required for chain surveying include:
- Chain or Tape
- Ranging Rods
- Arrows
- Cross-Staff & Offset Rods
- Plumb Bob
Standard Conditions and Tolerance
To ensure accuracy, measuring equipment must conform to a standard length within a very small margin of error, known as tolerance. These tolerances account for minor imperfections in manufacturing and the wear and tear that occurs with regular use. The standard conditions for testing a chain's accuracy are:
- Temperature: 20°C (because metal expands or contracts with temperature changes).
- Pull (Tension): 8 kg (to ensure the chain is pulled straight and sag is minimized consistently).
Permissible Tolerances for Metric Chains
The maximum tolerance allowable for metric chains when tested under standard conditions is as follows:
| Chain Length | Permissible Tolerance (Error) |
|---|---|
| 5 m chain | ± 3 mm |
| 10 m chain | ± 3 mm |
| 20 m chain | ± 5 mm |
| 30 m chain | ± 8 mm |
As highlighted in the table, for a 30-metre chain, the overall length should not deviate beyond ± 8 mm. This ensures that any measurements taken are within an acceptable range of accuracy.
Additional Information: Types of Chains
Different types of chains have been used historically and for specific purposes:
- Revenue chain: 33 ft long (16 links)
- Gunter's chain: 66 ft long (100 links)
- Engineer's chain: 100 ft long (100 links)
- Metric chain: 20 m (100 links) or 30 m (150 links)