Psychrometric chart online: moist air properties and processes

Set the air state by any two parameters (dry-bulb t, relative humidity φ, humidity ratio d, enthalpy i, wet-bulb or dew-point temperature) and get the rest, plotted on the psychrometric (i-d) chart. Add a process: heating, cooling with dehumidification, mixing of two streams or adiabatic humidification, and get the end state, power and moisture. Click the chart to place a point.

What the chart shows

The psychrometric chart (i-d chart in Russian practice, Ramzin chart) relates the properties of moist air at constant pressure: dry-bulb temperature t, relative humidity φ, humidity ratio d (g of moisture per kg of dry air), enthalpy i (kJ per kg of dry air), dew point and wet-bulb temperature. An air state is a point on the chart; any two independent properties fix it. The Russian chart is oblique: lines of constant d are vertical, lines of constant i slope down to the right, and the 0 °C isotherm is horizontal, which uses the plot area fully.

Formulas

Saturation pressure pws(t) from the Hyland–Wexler equation (ASHRAE Handbook — Fundamentals, 2017, ch. 1) or the Magnus formula: 6.112·exp(17.62·t/(243.12 + t)) hPa over water.
Vapor partial pressure pv = φ·pws.
Humidity ratio d = 0.621945·pv / (B − pv), B being the barometric pressure.
Enthalpy i = 1.006·t + d·(2501 + 1.86·t) kJ/kg of dry air.
Dew point is the temperature where pws equals pv.
Wet bulb from the ASHRAE equation linking d, t and the adiabatic saturation temperature.
Moist air density ρ = (B − pv)/(Rd·T) + pv/(Rv·T).

Processes

Heating goes up along d = const and φ falls. Cooling goes down the same vertical; once the dew point is reached, the air follows the saturation line and is dehumidified (condensate). Power Q = G·(i₂ − i₁).
Mixing of two streams: d and i of the mixture are averages weighted by the dry air flows; the mixture point lies on the line between the stream points. If it falls above the saturation line, fog forms.
Adiabatic humidification (sprays, wetted media) follows i = const: humidity ratio rises, temperature falls, the limit is saturation at the wet-bulb temperature.

Pressure and altitude

A chart is valid for one barometric pressure. At altitude the pressure is lower and the humidity ratio at the same φ is higher, so B is an input here (about 89,900 Pa at 1000 m).

Sources

ASHRAE Handbook — Fundamentals, chapter Psychrometrics; Sonntag D., Advancements in the field of hygrometry, 1990 (Magnus formula); Ramzin L. K., moist air diagram; Russian code SP 60.13330 (heating, ventilation and air conditioning).

FAQ

How do I find the dew point from temperature and humidity?

Choose t and φ and enter the values: the calculator finds the temperature where saturation pressure equals the vapor partial pressure. At 20 °C and 50 % the dew point is about 9.3 °C. On the chart it is where the vertical d = const meets the saturation line.

How do I get relative humidity from a psychrometer reading?

Choose t and wet-bulb temperature and enter the dry and wet readings. For example 25 and 18 °C give about 52 % RH. The ASHRAE equation is used for pressure B, so enter your actual pressure if it differs strongly from 101,325 Pa.

How do I calculate heater or cooler power from the chart?

Q = G·(i₂ − i₁), where G is the dry air flow in kg/s and i the enthalpy in kJ/kg. Choose the Heating / cooling process, enter the final temperature and the flow in m³/h: the calculator converts the flow to mass and gives the power in kW. With dehumidification the condensate enthalpy is included.

How do I calculate the mixing of two air streams?

Humidity ratio and enthalpy of the mixture are averages weighted by the dry air flows. Choose the Mixing process and enter both streams and their flows: the calculator returns t, φ, d, i of the mixture and warns if fog would form.

ASHRAE or Magnus: what is the difference?

The Hyland–Wexler (ASHRAE) equation is more accurate over −100 to 200 °C. The Magnus formula is simpler and within 0.5 % of tables for −20 to 50 °C, up to 1 % below zero. Use ASHRAE for coursework.