Psychrometric calculator

How it works

The tool solves the perfect-gas moist-air equations in ASHRAE Handbook—Fundamentals 2001, Chapter 6. ASHRAE says these relations are accurate enough for most air-conditioning work: errors stay under 0.7% for humidity ratio, enthalpy and specific volume of saturated air between −58°F and 122°F at standard pressure.

The inputs:

• Dry bulb in °F, read with a plain sensor out of radiant heat.
• Second reading: wet bulb (°F), relative humidity (%) or dew point (°F). Pick the one your instrument gives you.
• Elevation in feet (optional, blank = sea level).

What it computes, and with which equation:

• Air pressure. Standard atmosphere, Equation 3, which ASHRAE gives as accurate from about −16,000 to 36,000 ft. The tool shows the psia it used.
• Saturation pressure of water vapor. Hyland–Wexler fits, Equation 5 (over ice, below 32°F) and Equation 6 (over liquid water).
• Humidity ratio W, pounds of water vapor per pound of dry air, from Equation 22. The tool shows it in grains per pound (7,000 grains = 1 lb) and in lb/lb.
• Wet bulb. Equation 35 links humidity ratio to dry bulb and thermodynamic wet bulb. The library uses it for wet bulb at or above 32°F only, so a wet bulb input below 32°F is rejected, and a computed wet bulb below 32°F is shown as “below 32°F”.
• Relative humidity is vapor pressure over saturation pressure at the dry bulb (Equation 24).
• Dew point is the temperature whose saturation pressure equals the air’s vapor pressure (Equation 36).
• Enthalpy from Equation 32, reported in Btu/lb of dry air with zero at 0°F dry air. ASHRAE’s SI equation puts zero at 32°F (0°C), so compare the tool’s figure only with charts and tables on a 0°F basis, or compare differences.
• Specific volume from Equation 28, in ft³ per pound of dry air.

A sling or digital psychrometer reads a wet-bulb temperature, which ASHRAE notes is not quite the thermodynamic wet bulb; only small corrections separate them.

Worked example

The tool opens at 80°F dry bulb and 67°F wet bulb at sea level. It returns:

The same wet bulb means more moisture at altitude. The reverse matters with RH probes: 80°F and 51.1% RH gives 67.0°F wet bulb at sea level but 66.2°F at 5,000 ft. If your meter converts RH to wet bulb, check that it knows your elevation.

Standard air is 0.075 lb/ft³ (Brooks, SMACNA), about 13.3 ft³/lb. Return air at 80/67 is 13.85 ft³/lb at sea level and 16.73 ft³/lb at 5,000 ft. The 1.08 factor in the airflow calculator assumes standard air, and that tool corrects it for elevation.

Reading the result

Wet bulb for target superheat. CEC’s fixed-orifice charge table is read by return-air wet bulb and outdoor dry bulb, and the test is only valid with return air at 70°F dry bulb or warmer and 76°F wet bulb or lower (CEC RA3.2). If you only have dry bulb and RH, enter them here, take the wet bulb, and carry it to the target superheat calculator. Then compare with actual superheat.

Coil sensible and latent load. The enthalpy drop across the coil is the total heat removed per pound of dry air; the drop in grains is the moisture removed. Equation 32 splits it. Say you measure 80/67 entering and 58/56 leaving. The tool gives 31.45 and 23.80 Btu/lb, so 7.66 Btu/lb total, and 78.2 down to 63.6 gr/lb. The sensible part is the dry-bulb drop times the moist-air specific heat (1.006 + 1.805 W kJ/kg·K in Equation 32, about 0.245 Btu/lb·°F here): 22 × 0.245 = 5.39 Btu/lb. The other 2.27 Btu/lb is latent, a sensible heat ratio of about 0.70 for those readings. No grains removed means the coil is only cooling, not dehumidifying.

Condensation and dew point. By definition, air reaches saturation when cooled to its dew point at the same moisture content. Any duct, boot, grille or pipe surface below the surrounding air’s dew point will collect water. At 75°F and 50% RH indoors the dew point is 55.1°F. Attic air at 95°F dry bulb and 75°F wet bulb has a 66.8°F dew point, so supply duct whose outer surface is colder than that will sweat unless the insulation and vapor barrier keep the surface warmer.

No result. The tool refuses an RH of 0 or above 100%, a dew point or wet bulb above the dry bulb, and a wet bulb below 32°F.

Common mistakes

• Dry wick or still air on a wet-bulb thermometer. ASHRAE describes the wet bulb as the equilibrium reached when water evaporates from a wetted wick in an airstream; without both, the reading is not a wet bulb.
• Ignoring elevation when converting RH or dew point to wet bulb or grains. In the example above, 5,000 ft moved the wet bulb by 0.8°F, almost a full column in the CEC table.
• Comparing enthalpy across datums. The tool uses 0°F dry air; SI tables use 0°C. Use differences.
• Reading in the wrong place. Take return wet bulb in the return plenum, not at a grille next to an open door.
• Using RH for condensation. RH changes with temperature at the same moisture; dew point does not. Compare surfaces against the dew point.