--- title: "Construction of the psychrometric chart" subject: "Applied Thermodynamics" chapter: 4 key: "semester_3/applied_thermodynamics:psychrometric-chart-description" source: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/psychrometric-chart-description site: BCT Engineering --- # Construction of the psychrometric chart Applied Thermodynamics · Chapter 4: Air-Conditioning - Asked in: year not given, 5 marks - Video: [Psychrometric Chart - How to draw](https://www.youtube.com/watch?v=FLPyRJg0F0I) — R. Paul Singh - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/psychrometric-chart-description - Whole chapter: https://www.bctengineer.com/md/semester_3/applied_thermodynamics/chapter-4 ## Question What is a psychrometric chart? Explain its construction by describing the various lines drawn on it, and show on it the position of DBT, WBT, DPT and RH for an air state. ## Short answer A psychrometric chart graphs moist air properties at constant pressure. It has vertical DBT lines, horizontal specific humidity lines, curved RH lines, inclined WBT and enthalpy lines, and a saturation curve. For any state, DBT is read vertically downward, WBT along the inclined line to saturation, DPT horizontally to saturation, and RH from the RH curves. ## Answer A **psychrometric chart** is a graph of the properties of moist air at a constant total pressure (normally 101.325 kPa). From any two independent properties, all the others can be read. #### Axes - **Horizontal axis:** dry bulb temperature (DBT). - **Vertical axis (right side):** specific humidity $\omega$ (or vapour pressure). #### Lines on the chart 1. **Constant DBT lines:** vertical. 2. **Constant specific humidity lines:** horizontal. The dew point is read where the horizontal through the state meets the saturation curve. 3. **Saturation curve (100 % RH):** the upper curved boundary; DBT, WBT and DPT are equal on it. 4. **Constant RH curves:** curves below the saturation curve (10 %, 20 %, ...). 5. **Constant WBT lines:** inclined straight lines running down to the right, ending on the saturation curve. 6. **Constant enthalpy lines:** nearly parallel to the WBT lines; read from a scale left of the saturation curve. 7. **Constant specific volume lines:** steeper inclined lines (m³/kg dry air). 8. **Sensible heat factor (SHF) scale and protractor** at the upper left for process lines. ```text w ^ .-' saturation | .--'' (100 % RH) | DPT <--------- P * <- w = constant | .-'/ | | .-' / | <- DBT (vertical) | .-' WBT line +------------------------------> DBT ``` **Reading a state P:** vertical down gives DBT; horizontal left to the saturation curve gives DPT and horizontal right gives $\omega$; the inclined line through P reaching the saturation curve gives WBT; the curve through P gives RH; the enthalpy and volume lines nearest to P give $h$ and $v$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are the axes of a psychrometric chart? A. DBT horizontal, specific humidity vertical B. WBT horizontal, enthalpy vertical C. RH horizontal, DBT vertical D. Enthalpy horizontal, specific humidity vertical Answer: A. DBT lines are vertical and omega lines horizontal, so DBT is on the x-axis and specific humidity on the y-axis. (Key point: Axes of the chart) 2. How is the dew point of a state P read from the chart? A. Move vertically down to the DBT axis B. Move horizontally (constant omega) to the saturation curve C. Follow the inclined WBT line to the saturation curve D. Follow the constant RH curve to 100% Answer: B. Cooling at constant omega is horizontal, and condensation starts where that line meets the 100% RH curve. (Key point: Reading dew point) 3. What is true on the saturation (100% RH) curve? A. DBT, WBT and DPT are all equal B. DPT equals zero C. WBT is above DBT D. omega is zero Answer: A. At saturation the air cannot evaporate more water, so all three temperatures coincide. (Key point: Saturation curve) 4. Constant WBT lines are drawn as: A. Vertical lines B. Horizontal lines C. Inclined straight lines sloping down to the right, close to constant-enthalpy lines D. Curves below the saturation curve Answer: C. WBT lines slope downward to the right and nearly parallel the enthalpy lines. Vertical lines are DBT and horizontal ones are omega. (Key point: WBT and enthalpy lines) ## Related in Applied Thermodynamics - [Basic psychrometric processes](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Abasic-psychrometric-processes) — Applied Thermodynamics, chapter 4 - [Definitions of psychrometric terms](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Apsychrometric-terms-definitions) — Applied Thermodynamics, chapter 4 - [Supply air quantity for a conditioned room](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Aroom-supply-air-numerical) — Applied Thermodynamics, chapter 4 - [Properties of moist air numerical](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Apsychrometric-properties-numerical) — Applied Thermodynamics, chapter 4 ## Related in other subjects - [Force and position on inclined door](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Adoor-force-and-position) — Fluid Mechanics, chapter 3 - [Bar chart vs CPM/PERT as planning tool](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aplanning-tool-choice) — Construction Management, chapter 2, asked 3× - [TCTO numerical with time-cost chart (five activities)](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atcto-2079-jestha) — Construction Management, chapter 2 - [Planning, scheduling; linked bar chart](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Aplanning-scheduling-linked-bar-chart) — Project Engineering, chapter 3