A manometer is an instrument used for measuring the pressure acting on a column of fluid, which consists of a U-shaped tube of liquid in which the difference in pressures acting in the two arms of the tube causes the liquid to reach different heights in the two arms. Which of the following is NOT a limitation of the manometer?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 09/10/2023 (10am-12pm)
Detailed Explanation
Liquid manometers are among the simplest instruments in fluid mechanics. Distinguishing their genuine operational limitations from their inherent mechanical simplicity is essential.
1. Review recognized limitations of manometers:
- Need for levelling: Inclined or U-tube manometers must be carefully levelled to prevent parallax and geometric errors in column height.
- Large and bulky size: Moderate pressures require long tubes, making the apparatus cumbersome compared to compact dial gauges.
- No over-range protection: Pressure surges easily blow manometric fluid out of the tube, contaminating the line or creating hazards.
- Meniscus and capillarity errors: Surface tension creates capillary rise or depression requiring correction.
2. Evaluate construction complexity:
A manometer consists simply of a bent transparent glass or plastic U-tube filled with a suitable liquid. It has no moving gears, springs, or electrical components, making it exceptionally easy and inexpensive to construct.
Therefore, 'Difficulty in construction' is false and is NOT a limitation.
Why “Difficulty in construction” Is Correct
Liquid manometers are celebrated for their extreme simplicity of construction and absence of complex mechanisms. Calling construction 'difficult' contradicts reality, making option (c) the correct answer to what is NOT a limitation.
Why the Other Options Are Wrong
- A. Need for levelling: Need for levelling is a genuine operational limitation to ensure the vertical height reading is accurate.
- B. Large and bulky size: Large and bulky size is a genuine physical limitation because column height grows directly with pressure.
- D. No over-range protection: Lack of over-range protection is a genuine limitation because pressure spikes blow out the liquid.
Key Point
Manometers are simple and inexpensive to fabricate; 'difficulty in construction' is not a limitation.
Key Concepts for Students
- Capillary effects can be minimized by using tube diameters greater than 12 mm.
- Manometers require no electrical power and do not drift out of calibration over time.
- Primary limitations are fragility, bulky height for high pressures, and dynamic lag for fluctuating pressures.
Practice more: this is question 4 in Measurement of Pressure MCQs – Part 1. Try the whole set and check every answer instantly.