Question

Which of the following is true in respect of Volume Median Diameter (VMD) and Number Median Diameter (NMD)?

• A. \(\text{VMD} > \text{NMD}\)
• B. \(\text{NMD} > \text{VMD}\)
• C. \(\text{VMD} = \text{NMD}\)
• D. \(\text{VMD} = 2\text{NMD}\)

Hint:

Because $V \propto d^3$, larger droplets dominate volume $\implies \text{VMD} > \text{NMD}$. For a perfectly monodisperse spray, $\text{VMD} = \text{NMD}$.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Agricultural spray droplet size distribution is characterized by median diameter statistics: Number Median Diameter (NMD, 50% count basis) and Volume Median Diameter (VMD or \(D_{v0.5}\), 50% volume basis).
Key Formula or Approach:
\[ \text{Volume of spherical droplet } (V) = \frac{\pi}{6} d^3 \]

Step 2: Detailed Explanation:
Spray droplets discharged by agricultural nozzles are polydisperse (varying droplet sizes).
Because droplet volume is proportional to the cube of the diameter (\(d^3\)), a single large droplet contains the volume equivalent of hundreds or thousands of tiny droplets.
Consequently, the cumulative volume curve is heavily biased toward larger diameters compared to the cumulative droplet count curve.
Therefore, for all real spray droplet distributions, the Volume Median Diameter is always strictly greater than the Number Median Diameter (\(\text{VMD} > \text{NMD}\)).
The ratio \(\text{VMD}/\text{NMD}\) is called the uniformity coefficient.

Step 3: Final Answer:
Thus, \(\text{VMD} > \text{NMD}\) holds true, matching option (A).