Psychrometric State Point

Calculate relative humidity, dew point, and humidity ratio from dry-bulb and wet-bulb temperatures using ASHRAE psychrometric formulations.

Trades & Construction

Standard: ASHRAE Handbook - Fundamentals · Chapter 1

Sling Psychrometer Observations

ASHRAE Fundamentals Chapter 1 · Magnus-Tetens Formulation

Dry-Bulb Temperature (Sensible Air)

°F

Wet-Bulb Temperature (Evaporative Limit)

°F

Standard Atmosphere Baseline: Standard sea level atmospheric pressure (1,013.25 hPa / 29.921 inHg) assumed. For high-altitude design (e.g. Denver at 5,280 ft), actual barometric pressure reduces air density and shifts saturation vapor curves.

Relative Humidity

47.4%

Depression: 13.0°F

Dew Point

53.6 °F

Condensation threshold

Humidity Ratio

61.2 grains / lb

Specific moisture content

Actual Vapor Pressure

14.04 hPa

Saturation: 29.63 hPa

Interactive Worked Example — Psychrometric State Point

Thermodynamic state resolution from sensible and wet-bulb temperatures

ASHRAE Handbook - Fundamentals · Chapter 1

Design Scenario

Resolving moisture content and relative humidity for an airstream measured at 75°F dry-bulb and 62°F wet-bulb at standard sea-level barometric pressure (1,013.25 hPa).

Design Parameters

Dry Bulb75 °F

Wet Bulb62 °F

Depression13.0 °F

Atmospheric Pressure1,013.25 hPa

Mathematical Solution

1Step 1: Compute wet-bulb depression

75 - 62

13.0 °F

2Step 2: Determine saturation vapor pressure at wet-bulb and dry-bulb temperatures

e_s(T_{wb}=16.7°C) = 18.96 hPa; e_s(T_{db}=23.9°C) = 29.63 hPa

e_s(wb) = 18.96 hPa, e_s(db) = 29.63 hPa

3Step 3: Solve actual vapor pressure using psychrometer equation

18.96 - (1013.25 × 0.000673 × 7.2)

14.04 hPa

4Step 4: Calculate relative humidity and dew point

(14.04 ÷ 29.63) × 100

47.4% RH (Dew Point: 53.6 °F)

Engineering Conclusion: At 75°F DB and 62°F WB, the airstream possesses a relative humidity of 47.4% and a dew point of 53.6°F.

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What is Psychrometric Thermodynamics (Air & Vapor)?

Relative humidity reflects moisture saturation relative to dry-bulb temperature. To determine absolute moisture content, HVAC engineers measure dry-bulb temperature (sensible heat) and wet-bulb temperature (evaporative limit) using a sling psychrometer. From these two points, psychrometric properties including relative humidity, dew point, and specific humidity ratio are computed per ASHRAE psychrometric formulations.

Mathematical Foundation

Magnus-Tetens Vapor Constant

e_s = 6.112 \times \exp\left(\frac{17.67 \times T}{T + 243.5}\right)

e_s

= Saturation Vapor Pressure (hPa)

T

= Sensible Dry-Bulb Air Temperature (°C)

6.112

= Baseline saturation pressure multiplier

Psychrometer Differential Pressure

e_a = e_{s,wb} - \left(P_{atm} \times A \times (T_{db} - T_{wb})\right)

e_a

= Actual Vapor Pressure (hPa)

T_{db} - T_{wb}

= Wet-Bulb Depression (°C)

A

= Psychrometric constant: 0.00066 × (1 + 0.00115 × T_wb)

Laws & Principles

Thermodynamic Boundary: The wet-bulb temperature cannot physically exceed the dry-bulb temperature in atmospheric air. When wet-bulb equals dry-bulb, the air is at 100% relative humidity.
Magnus-Tetens Validity: The Magnus formulation provides high accuracy between -30°C and +35°C (-22°F to +95°F) at standard atmospheric pressure.
ASHRAE 55 Humidity Guideline: Indoor relative humidity is typically maintained between 30% and 60% for thermal comfort, indoor air quality, and mold prevention.
Barometric Pressure Impact: Saturation vapor curves shift with altitude; installations above 2,000 feet should adjust for reduced atmospheric pressure.

Step-by-Step Example Walkthrough

" Indoor air quality reading in a commercial facility: 75°F dry bulb (23.9°C) and 62°F wet bulb (16.7°C) at sea-level pressure (1,013.25 hPa). "

1. Calculate Wet-Bulb Depression: 75°F - 62°F = 13°F (7.2°C).
2. Compute Saturation Pressures: e_s(23.9°C) = 29.65 hPa; e_s(16.7°C) = 19.00 hPa.
3. Solve Psychrometer Equation: e_a = 19.00 - (1013.25 × 0.000673 × 7.2) = 14.09 hPa.
4. Calculate Relative Humidity: RH = (14.09 ÷ 29.65) × 100 = 47.5%.

Final Result: At 75°F DB and 62°F WB, relative humidity is 47.5% with a dew point of 53.6°F and humidity ratio of 61.2 grains/lb dry air.

Quick Answer: How do you calculate a Psychrometric State Point?

To resolve a Psychrometric State Point, enter measured Dry Bulb and Wet Bulb temperatures. The calculator processes the wet-bulb depression through the Magnus-Tetens empirical formulation to determine relative humidity, dew point, and moisture mass ratio without requiring manual chart reading.

The Magnus-Tetens Vapor Formula

Empirical saturation vapor pressure equation for atmospheric air calculations:

e_s(T) = 6.112 × exp((17.67 × T) / (T + 243.5))

Yields saturation vapor pressure in hectopascals (hPa) for temperature T in degrees Celsius.

Typical Psychrometric State Points

Environment	Dry/Wet Readings	State Point Result
Commercial Office	72°F DB / 59°F WB	46% RH (ASHRAE 55 Comfort Envelope)
Data Center Cold Aisle	68°F DB / 48°F WB	20% RH (Low moisture, electrostatic awareness zone)
Indoor Natatorium (Pool)	85°F DB / 78°F WB	~74% RH (Requires dehumidification control)

Psychrometric Best Practices

Best Practices

✓Verify coil dew point: Ensure cooling coil surface temperatures drop below airstream dew point when latent moisture removal is required.
✓Use distilled water on psychrometer wicks: Mineral buildup from tap water can insulate the wet wick and produce artificially high wet-bulb readings.

Common Mistakes

✗Overlooking altitude effects: At elevated sites (e.g. Denver at 5,280 ft), atmospheric pressure is ~830 hPa, altering vapor saturation limits.
✗Inadequate aspiration: Sling or aspirated psychrometers require 900 ft/min airflow across the wick to achieve steady-state evaporative equilibrium.

Frequently Asked Questions

What is a Wet-Bulb Depression?

It is the difference between dry-bulb and wet-bulb temperatures. A large depression indicates drier air with greater evaporative capacity, while a zero depression indicates 100% relative humidity (saturation).

Why can wet-bulb temperature never exceed dry-bulb temperature?

Evaporative cooling can only lower the wet-bulb thermometer reading to or toward the adiabatic saturation limit. Heat cannot spontaneously transfer from cold water to warm air to drive the wet bulb higher than ambient dry-bulb temperature.

What inputs does the Psychrometric State Point Calculator require?

The calculator requires dry-bulb and wet-bulb temperatures in either Imperial (°F) or Metric (°C) units. It computes relative humidity, dew point, and humidity ratio using standard sea-level barometric pressure.

How does dew point relate to condensation on chilled surfaces?

When any cold surface (such as an uninsulated chilled water pipe or supply air diffuser) drops below the surrounding air dew point temperature, moisture condenses out of the air onto that surface.

Related Balancing Tools

Moist Air Enthalpy Calculator Cooling Tower Approach Calculator Air Changes per Hour (ACH) Calculator Commercial Chiller IPLV Calculator

Calculation Provenance & Validation Record

Method

Magnus-Tetens Saturation Vapor Pressure and Psychrometer Equation

Formula

e_s = 6.112 \exp\left(\frac{17.67 T}{T + 243.5}\right), \quad e_a = e_{wb} - 1013.25 \times 0.00066(1 + 0.00115 T_{wb})(T_{db} - T_{wb})

Assumptions

Standard atmospheric sea-level barometric pressure (1,013.25 hPa / 29.921 inHg / 14.696 psia).
Thermodynamic boundary: wet-bulb temperature cannot exceed dry-bulb temperature.
Calculations use the Magnus-Tetens formulation per ASHRAE Handbook - Fundamentals Chapter 1.
Assumes clean, uncompressed moist air free of particulate contamination or chemical vapors.

References

ASHRAE Handbook — Fundamentals (Chapter 1: Psychrometrics) (2021 Edition) — ASHRAE Handbook - Fundamentals Chapter 1 (Psychrometric Formulation)

Last substantive review: 2026-10-04

Automated test status: 3 golden test vectors passing (PSP-01, PSP-02, PSP-03)

Method, assumptions & governing standards

Calculation Methodology

Trade estimation calculations derived from standard mechanical, electrical, and construction formulas.

Governing Standard 2021/2025 Edition · Chapter 1

Standard:ASHRAE Handbook - Fundamentals

Statutory building, electrical, and mechanical codes vary by jurisdiction. Confirm local municipality amendments before installation.

Key Assumptions & Constraints

Standard atmospheric sea-level barometric pressure (1,013.25 hPa / 29.921 inHg / 14.696 psia).
Thermodynamic boundary: wet-bulb temperature cannot exceed dry-bulb temperature.
Calculations use the Magnus-Tetens formulation per ASHRAE Handbook - Fundamentals Chapter 1.
Assumes clean, uncompressed moist air free of particulate contamination or chemical vapors.

Field Trade Notice: For trade planning and engineering estimates. Final installations must conform to project blueprints, authority having jurisdiction (AHJ) code approvals, and site-specific inspections.