Psychrometric Chart Explained: A Guide to Humidification

A psychrometric chart maps the relationship between air temperature, moisture content and energy. For a facility manager specifying humidification, it helps answer a practical question: how much water must the system add to move incoming air toward the required conditions?

The chart becomes easier to read when you start with two measurements, temperature and relative humidity, then follow the properties needed for your calculation. ASHRAE’s psychrometrics reference explains the relationships behind those readings.

Psychrometric Chart Explained: What the Lines Mean

A standard psychrometric chart displays several air properties on one diagram at a specified pressure. Start with temperature, relative humidity and humidity ratio before moving to the energy-related scales.

The chart’s curved outer boundary represents saturation, or 100% RH. Ohio University’s psychrometric chart guide illustrates the relationships between these lines.

Check the units before reading a value. Humidity ratio may appear as kilograms of water per kilogram of dry air, or grams per kilogram. Confusing those scales introduces a thousandfold error.

How to Read a Psychrometric Chart

To read a psychrometric chart, locate the intersection of two independent properties, then trace the relevant lines through that point. Dry-bulb temperature and RH are a convenient starting pair.

1. Check the chart pressure. Use a chart appropriate to the facility’s barometric pressure or elevation.
2. Find the dry-bulb temperature. Locate the measured temperature on the bottom scale.
3. Move upward to the RH curve. The intersection represents the air condition.
4. Read the humidity ratio. Move horizontally to its scale.
5. Read the dew point. Move horizontally left to the saturation boundary and read the corresponding temperature.

For a humidification assessment, plot both the incoming condition and the required condition. Label them clearly so the moisture difference is easy to identify.

An annotated chart with those two points is more useful in a specification meeting than an unexplained RH target.

Why Relative Humidity Alone Cannot Size a Humidifier

Relative humidity changes when temperature changes, even if no water enters or leaves the air. Humidity ratio tracks moisture on a dry-air mass basis, making it useful for calculating water addition.

During sensible heating, air temperature rises while humidity ratio stays constant. On a conventional chart, that is a horizontal movement to the right. RH falls along the way.

That distinction matters when interpreting winter ventilation air. An outdoor RH reading can look high while the air still needs additional moisture after heating.

It also explains why “increase humidity by 20%” is an incomplete equipment requirement. The starting temperature, target temperature, airflow and pressure must be defined.

For the related condensation question, CleanSPOT’s guide to dew point in facilities explains how the moisture condition relates to cold surfaces.

Worked Example: Calculating the Required Water Addition

A simplified humidification calculation multiplies dry-air mass flow by the increase in humidity ratio. The following example uses assumed conditions to demonstrate the method, rather than describe a CleanSPOT installation.

Assume:

Using standard psychrometric relationships, the rounded humidity ratios are:

• Incoming air: 0.00431 kg water/kg dry air.
• Required final air: 0.00726 kg water/kg dry air.

PsychroLib’s documented calculation functions describe how humidity ratio is obtained from temperature, RH and pressure using ASHRAE-based equations.

The moisture difference is:

Multiply that difference by the dry-air mass flow:

The simplified air stream therefore needs approximately 3.54 kg/h of added water vapour to reach the specified moisture content.

That result is the moisture requirement for the stated assumptions. It is not a complete humidifier selection. The example excludes leakage, internal moisture gains, control margins and incomplete evaporation.

If the available airflow is in m³/h, first convert it to dry-air mass flow using the specific volume at the condition where that airflow is defined. Do not multiply volumetric airflow directly by a humidity ratio in kg/kg.

What Changes When the Humidifier Adds a Mist?

Evaporating liquid water into air adds moisture and produces a cooling effect. Under an idealised adiabatic process, the path runs approximately along a constant-enthalpy line rather than vertically upward at constant temperature. The psychrometric process examples from NC State University illustrate this distinction.

For the worked example, meeting the moisture target does not automatically establish the required final temperature. The complete air-treatment sequence must account for heating and evaporation together.

Ask the equipment supplier to show:

• The air condition entering the humidification section.
• The expected condition after the mist has evaporated.
• The heating requirement needed to reach the final target.
• The installation conditions needed for complete evaporation.

For a central installation, the location of that humidification section matters. CleanSPOT’s explanation of air handling units shows how humidity treatment fits alongside filtration, heating and cooling.

Turning the Calculation Into an Equipment Specification

A useful humidifier enquiry combines the calculated moisture requirement with the facility’s operating conditions. Sending only the floor area leaves important sizing assumptions unresolved.

Prepare the following information before requesting a selection:

CleanSPOT’s psychrometric calculator provides separate room-based and duct/AHU modes for a preliminary sizing recommendation. Its published limitations explicitly exclude building leakage and process moisture loads, which need a separate review before final selection.

For regulated facilities, include the documentation and operating requirements in the enquiry as well. CleanSPOT’s GMP facility requirements guide connects environmental control with qualification, monitoring and maintenance.

Frequently Asked Questions

Plan Your Humidification Around the Actual Air Conditions

Start with the incoming condition, the required condition and the airflow. Use the chart to understand the change, then calculate the moisture requirement before comparing equipment capacities.

Send CleanSPOT those inputs together with your room or AHU layout. The engineering review can then focus on the appropriate direct-room or duct/AHU configuration, the water supply and the operating requirements that the initial calculation does not capture.