Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : A tensiometer vacuum gauge has divisions in the range of 0 to 100 centibars although in practice the range of operation of a tensiometer is 0 to 85 centibars.
Reason (R) : At the upper limit of 85 centibars, the column of water in the tube will form water vapour bubbles and the instrument will cease to function.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are not correct and (R) is not the correct explanation of (A).
• B. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

Remember that tensiometers are only useful in wet-to-moderate soil moisture ranges (0 to 80 centibars). They are unsuitable for dry soils due to water column cavitation at high tensions.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
A tensiometer is an instrument used to measure soil moisture tension (matric potential) directly in the field.
It consists of a water-filled tube with a porous ceramic cup at the tip and a vacuum gauge at the top.
Detailed Explanation:
Let us analyze both statements:
- Assertion (A) analysis: Tensiometer vacuum gauges are calibrated from 0 to 100 centibars (cb). However, the practical operating limit of a tensiometer is restricted to 0 to 80-85 cb (or 0.8 to 0.85 bar). It cannot measure high soil moisture tensions in dry soils. Thus, Assertion (A) is correct.
- Reason (R) analysis: At tensions around 85 cb, the high negative pressure causes the water column in the tensiometer tube to undergo cavitation. Air and water vapor bubbles form within the tube, breaking the continuous water column. This prevents the transfer of tension to the vacuum gauge, causing the instrument to fail. Thus, Reason (R) is correct.
- Evaluating the connection: Since the formation of vapor bubbles (R) explains the physical mechanism why the practical operating range of a tensiometer is limited to 85 cb (A), (R) is the correct explanation of (A).

Step 2: Final Answer:
Both (A) and (R) are correct, and (R) is the correct explanation of (A). This corresponds to Option (B).