Reading the Psychrometric Chart: A Field Guide

What the psychrometric chart is and why field techs use it

The psychrometric chart is a single graph that holds every property of moist air at a given pressure. Air is never just dry air. It carries water vapor, and the amount of that vapor changes how the air behaves when a coil heats it, cools it, or wrings water out of it. The chart lets a technician take two simple measurements and read off everything else, without a calculator and without a table.

That is the practical value. A thermostat reads one number, the dry-bulb temperature, and a dry-bulb reading alone tells you almost nothing about comfort or about what a coil is doing. Two rooms at the same 75 degrees can feel completely different because one is dry and one is damp. The chart shows that difference as two separate points, and it shows the moisture as a real, countable quantity.

Field crews reach for the chart to size dehumidification, to confirm a cooling coil is removing the latent load and not just the temperature, to check mixed-air conditions at an economizer, and to settle humidity complaints that a temperature reading cannot explain. Once you can plot a point and read a process line, the chart stops being a classroom exercise and becomes a diagnostic tool you keep in the truck.

The two temperatures: dry-bulb and wet-bulb

Dry-bulb temperature is what an ordinary thermometer reads, and it runs along the bottom of the chart, the horizontal axis. It is the sensible temperature of the air, the number a thermostat acts on. Lines of constant dry-bulb run straight up and down.

Wet-bulb temperature is read from a thermometer whose bulb is wrapped in a wet wick with air moving across it. As the water evaporates it cools the bulb, and how far it cools depends on how dry the surrounding air is. Very dry air drives a lot of evaporation and a low wet-bulb, so the gap between dry-bulb and wet-bulb is wide. Saturated air allows no evaporation, so the two readings match. Lines of constant wet-bulb run diagonally down to the right and meet the curved left edge of the chart.

Those two readings are the field tech's normal way onto the chart. A sling psychrometer or a digital meter gives both at once. Find the dry-bulb on the bottom, follow it up to the diagonal wet-bulb line, and the crossing is the state of that air. From that one point every other property is a short read away, which is why dry-bulb plus wet-bulb is the pairing most often measured in the field.

Dew point and what it tells you

Dew point is the temperature at which the air becomes saturated and water starts to condense out. On the chart it is read by moving straight left from the point, horizontally, to the curved saturation line, then reading the temperature there. Lines of constant dew point are horizontal, because dew point depends only on how much water vapor the air holds, not on its temperature.

Dew point is the most useful single number for sweating and condensation problems. Any surface colder than the air dew point will collect water. That is why a chilled-water pipe drips, why a supply register stains the ceiling in a humid space, and why a cold window fogs. If you know the room dew point and the surface temperature, you know whether that surface will sweat, with no guessing.

It also frames dehumidification. To dry air you must cool some of it below its dew point so water drops out on the coil. The lower you want the room moisture, the lower the dew point you must reach at the coil. Reading dew point straight off the chart turns a vague humidity complaint into a target temperature you can design and measure against.

Relative humidity: the curved lines

Relative humidity is the family of curved lines that sweep across the chart, parallel to the saturation curve at the upper left. Relative humidity compares the water vapor in the air to the most it could hold at that same temperature, expressed as a percent. The outer curved boundary is 100 percent, fully saturated, and the lines step inward at 90, 80, 70 percent and so on down toward the dry bottom right.

Relative humidity is what most people quote, but it is a ratio, not a quantity, and that trips up field work. The same pound of moisture in the air shows a high relative humidity when the air is cool and a low relative humidity when the same air is heated, because warm air can hold more. Heat a room without adding water and the relative humidity falls even though no moisture left. That is why winter air feels dry indoors once it is heated.

On the chart this shows up cleanly. Move right along a horizontal line, which holds moisture constant, and you cross into lower relative humidity curves as the dry-bulb climbs. Comfort standards usually target a relative humidity band, but you size equipment off the moisture quantity and the dew point, not off relative humidity alone.

Humidity ratio: the actual amount of water

Humidity ratio, sometimes called the moisture content or specific humidity, is the real, countable amount of water vapor in the air. It runs up the right side of the chart, the vertical axis, usually in grains of moisture per pound of dry air, or in pounds of water per pound of dry air. Field meters and tables often work in grains, where 7,000 grains equal one pound.

Because humidity ratio is a true quantity, it is what you track through a process. When a coil removes water, the humidity ratio drops by a measurable number of grains, and that drop times the airflow is the latent load the coil handled. When a humidifier adds water, the ratio climbs. Relative humidity can rise or fall with temperature alone, but humidity ratio only changes when water actually enters or leaves the air.

Reading it is simple. From your plotted point, move straight right, horizontally, to the vertical scale and read the grains. Two points at the same height on the chart hold the same water, no matter their temperature. That single idea, that horizontal means constant moisture, is the foundation of how every cooling, heating, and mixing line on the chart is read.

Enthalpy: the total heat in the air

Enthalpy is the total heat content of the air, both the sensible heat tied to temperature and the latent heat tied to the water vapor, measured in Btu per pound of dry air. On the chart, enthalpy is read on the diagonal scale set just outside the saturation curve at the upper left, and the lines of constant enthalpy run almost parallel to the wet-bulb lines, sloping down to the right.

Enthalpy matters because total cooling and total heating are enthalpy changes. When you cool and dehumidify air, the drop in enthalpy times the airflow is the total load the coil carried, sensible plus latent together. An economizer decision often comes down to comparing the enthalpy of the outdoor air against the return air, choosing whichever carries less total heat, which is why the control is called an enthalpy economizer.

In the field the near match between enthalpy lines and wet-bulb lines is handy. Wet-bulb temperature tracks total heat closely, so a single wet-bulb reading is a fast stand-in for enthalpy when you are checking whether outdoor air is a help or a burden. For load math, though, read true enthalpy at each point and take the difference.

Specific volume and why air density matters

Specific volume is the space a pound of dry air and its moisture occupy, in cubic feet per pound. It appears as a second set of steep diagonal lines crossing the chart, spaced wider than the wet-bulb lines. Warm, moist air is less dense, so it has a higher specific volume and takes up more room per pound than cool, dry air.

This is the bridge between the chart and the airflow you measure in cubic feet per minute. The standard load formulas assume a standard air density of about 0.075 pounds per cubic foot, which is close to sea-level air at moderate conditions. When the real air is hot, humid, or at altitude, it is lighter than standard, and a given cubic-feet-per-minute reading is carrying fewer pounds, so it moves less heat than the standard formula predicts.

For most comfort work near sea level the standard assumption is close enough. For a rooftop unit in a hot, high city, or for precise commissioning, reading specific volume off the chart and correcting the air density keeps the capacity numbers honest. The chart makes that correction a quick read rather than a separate calculation.

The saturation curve and 100 percent humidity

The curved line that forms the upper left boundary of the chart is the saturation curve, the line of 100 percent relative humidity. Along that curve the air holds all the water vapor it can at each temperature, and dry-bulb, wet-bulb, and dew point all read the same value at any point on it. Air cannot exist to the left of this line, because that would mean holding more water than is physically possible, so the moisture would condense.

The saturation curve is the reference every reading swings off. Dew point is found by sliding left to the curve. Wet-bulb is found along its diagonal lines back to the curve. A cooling coil that is dehumidifying drives the air down and to the left toward this curve, and the colder and wetter the coil surface, the closer to saturation the leaving air gets.

Air that reaches the curve is fog, with free water present. In a duct that shows up as condensation, carryover off the coil, and water blowing downstream. Keeping the leaving air a little to the right of saturation, and trapping and draining the condensate the coil makes, is part of every cooling design, which is covered in the condensate management guide.

How to plot a point from two measured properties

Every reading starts by fixing one point, the state of the air you measured. The most common field pairing is dry-bulb and wet-bulb. Find the dry-bulb value on the bottom axis and draw a line straight up. Find the wet-bulb value on the curved left edge and follow its diagonal line down to the right. Where the vertical dry-bulb line and the diagonal wet-bulb line cross is your point.

Any two independent properties will fix the same point. Dry-bulb and relative humidity work, as do dry-bulb and dew point, or dry-bulb and humidity ratio. You only need two, because the chart is built so that two known properties lock in all the rest. Pick the two you can measure most accurately with the meter you have.

Once the point is set, read the others by moving along the right kind of line. Straight left to the saturation curve gives dew point. Straight right gives humidity ratio. Follow the curved lines to read relative humidity, and the diagonal scales for wet-bulb and enthalpy. Plot the supply, return, and outdoor points for a system and the relationships between them tell the whole story.

Sensible heating and cooling: moving sideways

When you heat or cool air without adding or removing any water, the humidity ratio does not change, so the point moves straight across the chart, horizontally. This is a sensible-only process. A heating coil, an electric strip, or a dry cooling coil that never reaches the air dew point all move the point sideways with no vertical change.

Move the point to the right and you are heating: dry-bulb rises, the moisture stays put, and because warm air can hold more water, the relative humidity falls. Move the point left and you are cooling, as long as you stop before the saturation curve. Dry-bulb drops, moisture is unchanged, and relative humidity climbs as the air approaches its dew point.

The sensible load of that move is read straight off the temperature change. The common field formula is Q sensible equals 1.08 times CFM times the dry-bulb temperature difference, where 1.08 comes from standard air density and the specific heat of air. A purely horizontal line on the chart is the picture of that formula: temperature changed, moisture did not, only sensible heat moved.

Latent change: moving up and down

Adding or removing water moves the point vertically, changing the humidity ratio. Pure humidification, such as a steam humidifier injecting moisture without much temperature change, drives the point up. Pure moisture removal drives it down. Vertical movement is latent change, the heat tied to the phase change of water rather than to temperature.

In practice few processes are purely vertical, because adding or removing water usually changes temperature too. The value of the vertical axis is in the math. The latent load is read from the change in humidity ratio, using Q latent equals 0.68 times CFM times the change in grains of moisture per pound. That tells you how much of a coil capacity is going to drying the air rather than cooling it.

Tracking the vertical drop across a cooling coil is how you confirm a system is actually dehumidifying. If the leaving-air point sits at nearly the same height as the entering-air point, the coil removed temperature but almost no water, and a humidity complaint will not clear no matter how cold the supply gets. The vertical change is the proof that latent work happened.

The cooling and dehumidification line

A real cooling coil that drops the air below its dew point both cools and dries, so the point moves down and to the left at the same time, a diagonal line toward the lower left. Dry-bulb falls, which is the sensible part, and humidity ratio falls, which is the latent part. That single process line is the most important one a field tech reads, because it is what an air conditioner does on a humid day.

Plot the entering air, usually the mixed air ahead of the coil, and the leaving air just past the coil, and draw the line between them. The horizontal length of that line is the sensible cooling. The vertical drop is the dehumidification. The straight-line distance, read as the change in enthalpy, is the total cooling the coil delivered.

The slope of that line is the story. A steep, mostly vertical line means the coil is doing heavy latent work, good for a muggy space. A shallow, mostly horizontal line means it is mostly lowering temperature and barely drying, which leaves a cold, clammy room. Reading the slope tells you whether the equipment matches the load it actually faces.

Sensible heat ratio and what the coil is built for

The sensible heat ratio, or SHR, is the share of the total cooling that is sensible, the sensible load divided by the total load. On the chart it is the slope of the coil process line. A high SHR near 0.9 is a nearly flat line, a coil doing mostly temperature work. A lower SHR around 0.7 is a steeper line with much more drying.

Every coil and every space has an SHR. A data hall or a dry office runs a high SHR because the load is almost all heat from equipment and people-sensible gains, with little moisture. A packed restaurant or a humid coastal building runs a lower SHR because people, cooking, and infiltration add a heavy latent load. Matching the equipment SHR to the space SHR is the difference between comfort and a chronic humidity callback.

Reading SHR off the chart, by comparing the slope the coil delivers against the slope the space needs, is a fast commissioning check. It connects to the Manual J load calculation, where sensible and latent are split, and to equipment selection, where each unit is rated at a stated SHR for given entering conditions.

Mixing two airstreams: return plus outdoor air

When two airstreams blend, such as return air mixing with outdoor air ahead of a coil, the mixed point lands on the straight line drawn between the two source points. Where it lands depends on the proportions. Twenty percent outdoor air puts the mixed point one fifth of the way from the return point toward the outdoor point, by airflow.

This is one of the cleanest, most useful constructions on the chart. Plot the return air, plot the outdoor air, connect them, and step along that line by the outdoor-air fraction to find the mixed air the coil actually sees. That mixed point, not the return point, is the entering condition for the coil process line, so getting it right is what makes the rest of the analysis honest.

It also explains economizer behavior. On a mild, dry day the outdoor point sits low and to the left, so blending in more of it pulls the mixed point toward free cooling. On a hot, muggy day the outdoor point sits high and to the right, and pulling in more of it adds load, which is why the economizer should close. The mixing line makes that trade visible.

Evaporative cooling: following the wet-bulb line

Evaporative cooling adds water to the air while cooling it, and because the energy to evaporate that water comes from the air itself, the process follows a line of constant wet-bulb, sloping up and to the left. Dry-bulb drops, humidity ratio rises, and the total heat, the enthalpy, stays nearly the same. That is why evaporative coolers and the wet media in cooling towers and some data-center systems work so well in dry climates and so poorly in humid ones.

On the chart, start at the outdoor point and slide up the wet-bulb line toward saturation. In dry air the starting point is far to the right of the saturation curve, so there is a long way to travel and a large temperature drop is available. In humid air the point already sits close to the curve, so there is little room to move and little cooling to be had.

Reading the wet-bulb line shows the limit. The air can never cool below its wet-bulb temperature by evaporation alone, and real equipment reaches only part of the way, set by its effectiveness. The gap between the entering dry-bulb and the wet-bulb is the most cooling the process could ever deliver.

Apparatus dew point and coil bypass factor

Extend the coil process line down and to the left until it hits the saturation curve, and the temperature at that crossing is the apparatus dew point, the effective average temperature of the coil surface. It is the condition the air would reach if every bit of it touched the coil. Real air does not, so the leaving point stops short of the apparatus dew point.

The reason it stops short is the bypass factor, the share of air that slips through the coil without fully contacting the cold surface. A coil with more rows, tighter fins, and slower face velocity has a low bypass factor and pushes the leaving air close to the apparatus dew point, giving colder, drier supply. A shallow coil at high face velocity has a high bypass factor and leaves the air warmer and wetter.

This construction explains why two coils at the same chilled-water temperature deliver different supply conditions. Reading the apparatus dew point and the bypass factor off the chart connects the leaving-air target to the physical coil, and it is why dehumidification designs call for deeper coils and lower face velocity rather than just colder water.

Using the chart to troubleshoot a complaint

Most field use of the chart is troubleshooting. A space is comfortable on temperature but feels sticky, or it overcools and never dries, or a surface sweats. Take three sets of readings, dry-bulb and wet-bulb each: the return air, the supply air at the register, and the outdoor air. Plot all three and the picture usually points straight at the cause.

If the supply point sits at nearly the same height as the return point, the coil is not dehumidifying, and the fix is in airflow, coil selection, or fan speed, not in lowering the setpoint. If the mixed condition is far wetter than the return, too much humid outdoor air is coming in. If a surface is sweating, compare its temperature to the room dew point read off the chart, and the answer is whether to insulate, to seal, or to dry the air.

The chart keeps the diagnosis honest because it separates temperature from moisture, which a thermostat cannot do. A reading is logged in a tool like FieldOS against the unit and the space so the next visit starts from data, not from a fresh round of guessing, and so a recurring complaint shows its pattern across visits.

Tight-humidity spaces and the data center

Some spaces care about humidity as much as temperature. A data hall, a museum, a clean space, and a printing plant all run inside a humidity band, and the chart is how that band is drawn and held. The control target is usually stated as a dew point or a humidity ratio range rather than relative humidity, because the equipment loads add heat that swings relative humidity around even when the moisture is steady.

Data centers in particular have moved toward wider allowable envelopes so they can run warmer and drier and lean on free cooling, and those envelopes are drawn as boxes on the psychrometric chart bounded by dew point and humidity lines. Reading where the room condition sits inside or outside that box is the daily check. The dedicated outdoor air, free cooling, and humidity control guides build on these same chart constructions.

For these spaces the chart is also the design record. The allowable and recommended regions, the economizer changeover, and the humidification and dehumidification setpoints are all points and lines on one chart, so a commissioning agent can confirm at a glance that the sequence keeps the room inside its box across the year.

Altitude and why the standard chart shifts

A psychrometric chart is drawn for one barometric pressure, almost always standard sea-level pressure. Air pressure falls with elevation, and at altitude the air is thinner, so its properties shift. A sea-level chart used in a high city will read humidity ratio and enthalpy off by a meaningful amount, and the standard 1.08, 0.68, and 4.5 load constants are wrong because they assume sea-level density.

The honest fix is to use a chart drawn for the local pressure, or software that takes elevation as an input, for any high-altitude job. The wet-bulb depression behaves differently too, so evaporative cooling that looks marginal on a sea-level chart can be stronger at altitude because the thinner, drier air evaporates more water.

For sea-level and near-sea-level work the standard chart is fine and the standard constants hold. The point is to know which chart you are holding. Confirm the pressure or elevation it was drawn for, and correct the air density through specific volume when the job sits high enough to matter, which keeps the capacity numbers from drifting.

What to document

A psychrometric reading is only useful later if it is written down with the conditions it was taken under. Record the location of each reading, both temperatures, and the property you read off, so the next technician can replot the point rather than start over.

Common mistakes

• Trusting relative humidity as a quantity. It is a ratio that moves with temperature, so heating dry air lowers it without removing any water. Track humidity ratio or dew point for real moisture.
• Plotting the coil off the return air instead of the mixed air. With outdoor air in the system, the coil sees the mixed point, so the process line must start there.
• Calling a coil good because the supply is cold. A cold supply at nearly the same humidity ratio as the return means temperature dropped but the air was not dried, and a humidity complaint will remain.
• Using sea-level load constants at altitude. The 1.08, 0.68, and 4.5 factors assume standard density and read high in thin air; correct for elevation through specific volume.
• Ignoring dew point on sweating problems. Any surface below the room dew point collects water, so compare the surface temperature to the dew point before blaming the equipment.
• Measuring wet-bulb with a dry or still wick. The wick must be wet and the air must move across it, or the wet-bulb reads too high and every property plotted from it is wrong.

Field checklist

Standards and references

The psychrometric chart and the property relationships behind it come from the ASHRAE Handbook Fundamentals, which publishes the standard sea-level chart, the equations, and charts for other pressures. ASHRAE Standard 55 sets the thermal comfort zones that are often drawn as a region on the chart, and the comfort and humidity targets for occupied spaces trace back to it.

The standard air-side load constants, near 1.08 for sensible, 0.68 for latent, and 4.5 for total enthalpy, assume standard air density of about 0.075 pounds per cubic foot and should be corrected for altitude and for hot or humid air using specific volume from the chart. Equipment is rated by the manufacturer at stated entering conditions and a stated sensible heat ratio, and those ratings are the governing numbers for selection.

Treat the values here as the field method and the standard references as the authority. Confirm the chart pressure, follow the manufacturer data for equipment performance, and use the local design conditions and the adopted code for any work that must be signed off.

Units, terms, and conversions

Dry-bulb temperature

The temperature an ordinary thermometer reads, the sensible temperature of the air, plotted on the horizontal axis

Wet-bulb temperature

The temperature read by a wetted, ventilated bulb; the gap below dry-bulb shows how dry the air is

Dew point

The temperature at which air saturates and water begins to condense; depends only on moisture content

Relative humidity

The water vapor present compared to the maximum at that temperature, in percent; a ratio, not a quantity

Humidity ratio

The actual mass of water vapor per pound of dry air, in grains or pounds; 7,000 grains equal one pound

Enthalpy

Total heat content of the air, sensible plus latent, in Btu per pound of dry air

Specific volume

Cubic feet occupied per pound of dry air; the inverse of density, used to correct for altitude

Sensible heat ratio (SHR)

Sensible cooling divided by total cooling; the slope of the coil process line

Apparatus dew point

The effective coil surface temperature where the extended process line meets saturation

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