Frequently asked questions

A single graph of the state of moist air. The horizontal axis is dry-bulb temperature and the vertical axis is humidity ratio (grams of water vapor per kilogram of dry air). Lines of relative humidity, enthalpy, wet-bulb temperature and specific volume are drawn on top, so knowing any two values lets you read all the others.

It is used for HVAC design, cooling coil and humidifier selection, building environment control and condensation checks.

(1) Dry-bulb is read on the bottom axis along vertical lines, (2) humidity ratio on the right axis along horizontal lines, (3) relative humidity follows the dark red curves rising to the right (the bold black outer curve is 100 %, saturation), (4) enthalpy runs down to the right in green from the scale outside the saturation curve, (5) wet-bulb is the teal dashed line almost parallel to enthalpy, and (6) specific volume is the steep grey-violet dotted line.

The dew point is where a horizontal line from the point meets the saturation curve. In this calculator, hovering shows all values in a card next to the cursor.

Any two independent values among dry-bulb, relative humidity, wet-bulb, dew point, humidity ratio, enthalpy and specific volume (24 combinations). Pairs that carry the same information, such as humidity ratio and dew point, and states above saturation (over 100 % RH) are rejected with the reason.

Example: 30 °C dry-bulb and 22 °C wet-bulb give 50.0 % RH, 18.4 °C dew point and 13.30 g/kg humidity ratio.

Dry-bulb is the actual air temperature measured by an ordinary thermometer.

Wet-bulb is measured by a thermometer wrapped in a wet wick with air moving over it. Evaporation takes heat away, so it reads lower than dry-bulb. The drier the air, the bigger the difference; at saturation (100 % RH) they are equal. It matters for cooling towers and evaporative cooling.

This calculator gives the thermodynamic wet-bulb temperature from the ASHRAE equations.

Relative humidity (%) is the amount of water vapor in the air as a share of the maximum the air can hold at that temperature. With the same vapor content, RH changes when the temperature changes.

Humidity ratio (g/kg dry air) is the mass of water vapor per kilogram of dry air. It describes vapor content only, regardless of temperature. Strictly, absolute humidity is vapor mass per cubic meter of air (g/m³), a different quantity.

Example: air at 20 °C and 50 % has 7.26 g/kg, at 30 °C and 50 % it has 13.31 g/kg. Same RH, almost twice the water.

Enthalpy (kJ/kg dry air) is the heat content of one kilogram of dry air plus the water vapor it carries: h = 1.006 t + W (2501 + 1.86 t). The first term is the sensible heat of dry air, the second the latent and sensible heat of the vapor.

The heat needed to heat or cool air is mass flow times enthalpy difference. On the chart, lines of constant enthalpy slope down to the right.

The temperature at which air, cooled without changing its moisture content, becomes saturated and starts to condense.

If a wall, window, duct or pipe surface is colder than the dew point, condensation forms. Example: room air at 25 °C and 50 % has a dew point of 13.9 °C, so any surface colder than that will sweat.

On the chart, go horizontally left from the point to the saturation curve.

Specific volume (m³/kg dry air) is the volume taken by one kilogram of dry air together with its vapor. Density (kg/m³) is the mass of one cubic meter of moist air, vapor included.

They are roughly reciprocal, but exactly density = (1 + W) / specific volume, with W in kg/kg. Example: at 25 °C and 50 %, specific volume is 0.858 m³/kg and density 1.177 kg/m³.

Warmer or more humid air is lighter. Specific volume converts volume airflow (CMH) to mass flow, which matters for fans and ducts.

It uses the equations of ASHRAE Handbook Fundamentals 2017, chapter 1: Hyland-Wexler saturation pressure, with wet-bulb and dew point solved iteratively rather than approximated.

The engine is our own and is checked automatically against PsychroLib, an open library that implements the same ASHRAE equations: temperatures agree within 0.001 °C and humidity ratio, enthalpy and specific volume within a relative error of 1e-9. For very dry air with a wet-bulb near 0 °C, the water/ice switch in the ASHRAE equations can make the wet-bulb slightly ambiguous.

At 101.325 kPa: humidity ratio 9.88 g/kg, enthalpy 50.3 kJ/kg, wet-bulb 17.9 °C, dew point 13.9 °C, specific volume 0.858 m³/kg, density 1.177 kg/m³ and vapor pressure 1,585 Pa.

Total = dry-air mass flow × (outlet enthalpy − inlet enthalpy), sensible = mass flow × (1.006 + 1.86 × inlet humidity ratio) × temperature difference, latent = total − sensible, condensate = mass flow × humidity ratio difference.

Example: cooling 33 °C/60 % air to 26 °C/50 % at 3,600 kg/h of dry air gives 29.25 kW total, 7.29 kW sensible, 21.96 kW latent, SHF 0.25 and 31.0 kg/h of condensate. At 10,000 CMH (inlet conditions) it is 90.9 kW and 96.4 kg/h.

In this calculator, connect two points and enter the airflow.

The engine takes pressure as an input (for example about 84.56 kPa at 1,500 m). Changing pressure on screen is in preparation; the current screen uses sea level, 101.325 kPa.

Move the cursor over the chart to see dry-bulb, RH, humidity ratio, enthalpy, wet-bulb and dew point in a card next to it. Click to add a state point and drag to move it.

In the panel, pick two defining values and type numbers to place a point exactly. Connect two points for a process (cooling, heating, humidifying and so on) with kW for a given airflow, or use "Mix" for the mixed state of two airstreams.

On a phone, tap to add a point and long-press to aim and read values.

Image export and a printable report are not available yet; they are in preparation.

For now, use "Copy table" on the property table, point list or process results to move numbers into Excel, and take a screenshot of the chart. A blank chart for plotting by hand is on the PDF download page (https://psychart.kr/en/download/).

Yes. The PDF download page (https://psychart.kr/en/download/) has three free versions, standard color, light (for plotting by hand) and black and white, each in A4 and A3 landscape.

Range is dry bulb -10 to 50 °C and humidity ratio 0 to 30 g/kg, SI units, 101.325 kPa, with fine ticks (1 °C, 0.5 g/kg, 1 kJ/kg) and a sensible heat factor protractor. Print at "Actual size".

Yes, it is free with no sign-up, for study, research or work.

"Copy table" copies a tab-separated table that pastes straight into Excel.

Current Chrome, Edge, Safari and Firefox. Phones and tablets work too: tap to add, long-press to read values, drag points.

A desktop screen is easier for many points and processes. Use the top-right button for dark mode.