Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : At sea-level, where the atmospheric pressure is at 1 bar a tensiometer can be used to indicate soil suction values upto 1 bar
Reason (R) : Beyond this value, because of low pressure, the water column in the tensiometer would break into vapour or boil away and leave the tensiometer system making it inoperative
In light of the above statements, choose the correct answer from the options given below.
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Remember that tensiometers are highly accurate but limited to wet-range soil moisture measurements (0 to 0.8 bars). Beyond this, cavitation (vapor column break) occurs, requiring the tensiometer to be refilled with de-aired water.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
(A) is true but (R) is false.
(A) is false but (R) is true.
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The Correct Option isD
Solution and Explanation
Step 1: Understanding the Concept:
A tensiometer is an instrument used to measure the matric potential (suction) of soil water 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. Step 2: Detailed Explanation:
Let us analyze both statements:
Assertion (A): While the theoretical maximum suction a tensiometer can measure is 1 bar (corresponding to local atmospheric pressure), in practice, a tensiometer can only measure soil suction up to about0.8 to 0.85 bar.
It cannot reach a full 1 bar suction because microscopic air bubbles dissolved in the water expand under the vacuum, causing the water column to break. Thus, Assertion (A) isfalse.
Reason (R): When soil suction exceeds 0.8 bar, the absolute pressure inside the tensiometer tube drops significantly below atmospheric pressure, approaching the vapor pressure of water.
Under this high vacuum, dissolved air bubbles expand rapidly, and the water begins to boil or vaporize at ambient temperature.
This vapor gap breaks the continuous water column (cavitation), stopping the transfer of suction to the gauge and making the instrument inoperative. Thus, Reason (R) istrue.
Therefore, Assertion (A) is false but Reason (R) is true. Step 2: Final Answer:
The correct option is (D).
