Absolute humidity, expressed in grains of moisture per cubic foot, is a key psychrometric parameter used by engineers and meteorologists to assess air quality and comfort. The calculation requires four measurements: atmospheric pressure, dry‑bulb temperature, wet‑bulb temperature, and a conversion to the appropriate units.

Collecting the Required Data

1. Atmospheric Pressure: Use a digital barometer and record the pressure in inches of mercury (inHg). If your instrument reads psi, multiply by 2.036; if it reads Torr or mmHg, multiply by 0.0393; if it reads atm, multiply by 29.92.
2. Dry‑Bulb Temperature: Turn on the digital psychrometer and read the dry‑bulb (air) temperature in degrees Fahrenheit.
3. Wet‑Bulb Temperature: From the same psychrometer, record the wet‑bulb temperature in degrees Fahrenheit.
4. Tools: Ensure you have a digital barometer, a digital psychrometer with separate dry‑bulb and wet‑bulb thermometers, and a graphing calculator for the calculations.

Calculating Absolute Humidity When Wet‑Bulb ≤ 32°F

1. ΔT = Dry‑Bulb – Wet‑Bulb
2. Part1 = ΔT × Atmospheric Pressure
3. Part2 = Wet‑Bulb × 2.336×10⁻⁷
4. Part3 = Part2 + 3.595×10⁻⁴
5. Part4 = Part1 × Part3
6. WetPlus459 = Wet‑Bulb + 459.4
7. Part5 = –4869.38 ÷ WetPlus459
8. Part6 = Part5 + 10.0343
9. Exp1 = 10^(Part6)
10. Exp2 = WetPlus459^(–0.32286)
11. Mul1 = Exp1 × Exp2
12. Diff1 = Mul1 – Part4
13. DryPlus459 = Dry‑Bulb + 459.4
14. Ratio = Diff1 ÷ DryPlus459
15. AbsolutePounds = Ratio × 0.82455
16. GrainsPerCubicFoot = AbsolutePounds × 7000

Calculating Absolute Humidity When Wet‑Bulb > 32°F

1. ΔT = Dry‑Bulb – Wet‑Bulb
2. Part1 = ΔT × Atmospheric Pressure
3. Part2 = Wet‑Bulb × 2.336×10⁻⁷
4. Part3 = Part2 + 3.595×10⁻⁴
5. Part4 = Part1 × Part3
6. WetPlus459 = Wet‑Bulb + 459.4
7. Part5 = –5287.32 ÷ WetPlus459
8. Part6 = Part5 + 23.2801
9. Exp1 = 10^(Part6)
10. Exp2 = WetPlus459^(–4.9283)
11. Mul1 = Exp1 × Exp2
12. Diff1 = Mul1 – Part4
13. DryPlus459 = Dry‑Bulb + 459.4
14. Ratio = Diff1 ÷ DryPlus459
15. AbsolutePounds = Ratio × 0.82455
16. GrainsPerCubicFoot = AbsolutePounds × 7000

Required Equipment

• Digital barometer
• Digital psychrometer (dry‑bulb & wet‑bulb)
• Graphing calculator

Follow these steps to obtain accurate absolute humidity values in grains of moisture, essential for HVAC design, weather forecasting, and environmental monitoring.