Operational Physics and Limits of Soil Tensiometers:

• Assertion (A) is Correct: Commercial tensiometer vacuum dial gauges are graduated from $0\text{ to }100\text{ centibars}$ ($0\text{ to }1\text{ bar}$ or $0\text{ to }100\text{ kPa}$), but their practical, reliable operational range in the field is restricted to $0 - 80\text{ or }85\text{ centibars}$.
• Reason (R) is Correct: Water inside the sealed tensiometer tube is placed under negative pressure (tension). At approximately $0.85\text{ bar}$ ($85\text{ kPa}$) suction, water reaches its boiling/cavitation vapor pressure threshold at ambient temperature, causing dissolved air to exsolve and water vapor bubbles to nucleate. This cavitation breaks the continuous hydraulic water column between the porous ceramic cup and the vacuum gauge, rendering the instrument inoperative.
• Causal Link: Cavitation and bubble formation at $85\text{ centibars}$ is the exact physical mechanism explaining why the practical operational range terminates at $85\text{ centibars}$.

Hydraulic Significance of NPSHA in Pump Operation:

• Acronym Definition: NPSHA stands for Net Positive Suction Head Available.
• Engineering Definition: It represents the absolute hydraulic head available at the pump suction nozzle above the vapor pressure of the liquid:
  $$\text{NPSHA} = h_{\text{atm}} + h_s - h_f - h_{\text{vp}}$$
  To prevent destructive cavitation, the system's available suction head must strictly exceed the pump's required suction head ($\text{NPSHA} \gt \text{NPSHR}$).

Pump Characteristic Curves and Efficiency Analysis:

• True Operating Facts:
• For standard backward-curved centrifugal pumps, the head-capacity ($H-Q$) curve slopes downward; head decreases as discharge increases.
• Shut-off head occurs at $Q = 0$ when the delivery valve is completely shut.
• The pipe thickness and pressure rating must withstand maximum hydrostatic pressure at shut-off head without bursting.
• Statement 4 is completely False: Pump hydraulic water horsepower is proportional to discharge: $\text{WHP} = \frac{\gamma Q H}{75}$. At shut-off head, because $Q = 0$, useful water power output is zero, and therefore pump efficiency is exactly ZERO ($\eta = 0\%$), not maximum! Maximum efficiency occurs at the Best Efficiency Point (BEP) at intermediate design flow.