A psychrometric chart helps visualize moist-air point properties and HVAC process questions, but a chart reading is not the same thing as a licensed ASHRAE table reproduction, a load calculation, a comfort compliance report, a moisture diagnosis, or commissioning acceptance.
This guide explains the chart relationships and where ToolGrit's psychrometric screen can help organize inputs. It also keeps the source gaps visible: field instruments, station pressure, surface temperatures, envelope details, equipment data, current standards, code/AHJ requirements, and qualified HVAC or building-science review still control real decisions.
Reading the Psychrometric Chart
The psychrometric chart has dry-bulb temperature on the horizontal axis (bottom), humidity ratio on the vertical axis (right side), and a curved saturation line that forms the upper boundary. Every point inside the chart represents a unique state of moist air defined by its temperature and moisture content.
Dry-bulb temperature lines run vertically. This is the temperature measured by a standard thermometer.
Wet-bulb temperature lines run diagonally from upper left to lower right. They follow lines of constant adiabatic saturation, which is why a wet wick in moving air reads this temperature.
Relative humidity curves run roughly parallel to the saturation line. The saturation line itself is 100% RH. RH decreases as you move down and to the right from the saturation line. At any given dry-bulb temperature, moving vertically changes the humidity ratio and RH simultaneously.
Humidity ratio lines are horizontal, read from the right-side scale in grains of moisture per pound of dry air (or lb/lb). This is the absolute measure of moisture content, unlike RH which is relative to temperature.
Enthalpy lines run diagonally, approximately parallel to wet-bulb lines but read from a separate scale along the curved edge. Enthalpy represents the total heat content of the air (sensible + latent) in BTU per pound of dry air.
Psychrometric Calculator
Calculate psychrometric properties from any two known values. Dry bulb, wet bulb, dew point, relative humidity, humidity ratio, enthalpy, and specific volume. ASHRAE standard correlations.
Key Properties and What They Mean
Dew point temperature: Dew point is useful moisture context, but it does not by itself determine condensation safety, mold risk, envelope suitability, or corrective action. Surface temperature, duration, materials, ventilation, vapor control, and moisture sources still matter.
Enthalpy: Enthalpy difference can feed a separate coil or load calculation, but the chart or app does not approve coil capacity. Airflow, density basis, measured entering and leaving states, bypass factor, equipment data, and qualified review still control.
Specific volume: Specific volume helps convert between mass-flow and volume-flow thinking. Actual airflow, density correction, altitude, fan performance, and measurement method still need project-specific review.
Sensible heat ratio (SHR): SHR is equipment and load context, not a dehumidification approval. Use manufacturer performance data and a full design basis before selecting equipment.
The Cooling Coil Process on the Chart
Cooling-coil process lines are useful for review, but they do not replace a coil selection or load calculation. Entering and leaving air conditions, measured CFM, density basis, bypass factor, condensate behavior, airflow distribution, controls, manufacturer data, and commissioning criteria all matter.
Use enthalpy and humidity-ratio differences as prompts to organize questions before formal load, latent-capacity, and equipment review. Do not treat a chart sketch or a ToolGrit point-property output as capacity proof for temperature or moisture load.
Air Mixing and Economizer Cycles
Air-mixing lines help explain how return air, outdoor air, or zone streams interact. Real economizer and ventilation decisions still need measured or design mass flow, damper behavior, sensor placement, controls, enthalpy limits, code ventilation procedures, weather data, and equipment data.
Use the chart to frame the question, then carry the result to a full ventilation, coil, control, and commissioning review. The guide does not approve economizer operation or outdoor-air load assumptions.
Altitude Effects on Psychrometrics
Psychrometric properties depend on atmospheric pressure. A sea-level chart can be misleading at elevation, but altitude mode is still only a standard-atmosphere prompt. Critical field work should use actual station pressure and a documented pressure basis.
Specific volume, humidity ratio, and enthalpy prompts can shift with pressure. Carry the pressure assumption into airflow, coil, economizer, and commissioning review instead of treating an altitude correction as a final design adjustment.
Indoor Air Quality Applications
Psychrometric values can support IAQ and comfort review, but they do not determine ASHRAE 55 compliance, health outcomes, ventilation compliance, mold risk, or dehumidifier sizing by themselves.
Comfort review needs operative temperature, radiant temperature, air speed, clothing, metabolic rate, solar effects, occupancy, and documentation. Moisture review needs surface temperatures, materials, duration, moisture sources, envelope paths, ventilation, and qualified building-science review. Ventilation review needs code procedures, measured airflow, controls, and commissioning criteria.