Psychrometric chart basics Do the horizontal and uniformly spaced lines on a standard psychrometric chart represent relative humidity?

Introduction / Context:
Interpreting a psychrometric chart correctly is essential for HVAC calculations. Each family of lines—dry-bulb temperature, humidity ratio, relative humidity, wet-bulb temperature, enthalpy, and specific volume—has a distinct orientation. Misreading these lines leads to wrong loads, coil duties, and comfort predictions.

Given Data / Assumptions:

• Standard psychrometric chart at a fixed barometric pressure (often 1.013 bar).
• Axes: horizontal axis for dry-bulb temperature, vertical axis for humidity ratio (kg/kg of dry air).

Concept / Approach:
On the standard chart, horizontal lines correspond to constant humidity ratio (also called specific humidity). Relative humidity lines are curved lines that connect points of equal RH, sweeping from 100% at the saturation curve to lower values inside the chart. Therefore, the statement that “horizontal and uniformly spaced lines indicate relative humidity” is incorrect.

Step-by-Step Solution:

Verification / Alternative check:
Any standard chart legend confirms line families: constant RH curves are not horizontal except at 100% along the saturation boundary, which itself is a curve, not a straight horizontal line.

Why Other Options Are Wrong:

“Correct” contradicts the defined geometry of the chart.Pressure or low RH caveats do not turn the RH curves into horizontal lines; only the scales might shift slightly with altitude.

Common Pitfalls:
Confusing humidity ratio with relative humidity. Humidity ratio is a mass ratio, while RH is a percentage measure of saturation at a given temperature. On the chart they are represented by different line families.

Final Answer:

Incorrect