Alcohol is used in manometer, because ..........
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2 MARCH 2017 Evening Shift
Detailed Explanation
Selecting an appropriate manometric fluid depends on the pressure range, required sensitivity, chemical compatibility, and stability under ambient conditions.
1. Physical characteristics of alcohol (ethanol/methanol) as a manometric fluid:
- Low density (\(\approx 790\text{ kg/m}^3\)): Provides a significantly longer column length for a given pressure difference, greatly increasing measurement sensitivity for small gas pressures.
- Sharp, distinct meniscus: Readily wets glass walls with a consistent contact angle, making it ideal for inclined micro-manometers.
- Colored easily: Can be dyed bright red or blue for effortless visual observation.
2. Examination Key Analysis:
In the official SSC JE answer key, option (a) ('its vapour pressure is low') is designated as the answer. Students should note that while pure mercury has an extraordinarily low vapor pressure (0.173 Pa at 20°C) compared to alcohol (approx 5.8 kPa at 20°C), option (a) is the specific answer required by the commission's answer key.
Why “its vapour pressure is low” Is Correct
Option (a) 'its vapour pressure is low' is the designated answer in the official exam key. In practical engineering, alcohol is preferred in inclined manometers for its low density and sharp readable meniscus.
Why the Other Options Are Wrong
- B. it provides suitable meniscus for the inclined tube: Providing a suitable meniscus is a genuine practical advantage, but not the option marked in the official key.
- C. its density is less: Low density is the true physical reason for high sensitivity, but not the option marked in the key.
- D. it provides longer length for a given pressure difference: Providing longer column length is the physical consequence of low density, but not the key answer.
Key Point
Exam key designates (a) 'its vapour pressure is low'; practical design values alcohol for low density and sharp meniscus.
Key Concepts for Students
- Inclined manometers magnify reading by 1/sin θ (typically inclined at 5° to 30° to the horizontal).
- Manometric fluids must not react chemically with the flowing fluid or absorb moisture from the air.
- Mercury is used for high pressure differences; alcohol or light oils are used for micro-pressure measurements.
Practice more: this is question 29 in Measurement of Pressure MCQs – Part 2. Try the whole set and check every answer instantly.