- Provide barometric pressure or switch to altitude.
| Scenario | State 1 | State 2 | Key expected delta |
|---|---|---|---|
| Cooling + dehumidification | 26°C, 55% RH, 101.325 kPa | 18°C, 85% RH, 101.325 kPa | Δh negative, Δw often decreases |
| Heating + humidification | 18°C, 35% RH, 101.325 kPa | 24°C, 45% RH, 101.325 kPa | Δh positive, Δw increases |
| Outdoor-to-indoor mixing | 35°C, 60% RH, 101.325 kPa | 28°C, 50% RH, 101.325 kPa | ΔT drops, ΔRH varies by moisture |
Pws = 0.61078 × exp(17.2694×T / (T + 237.3))
Pw = (RH/100) × Pws
w = 0.621945 × Pw / (P − Pw)
h = 1.006T + w(2501 + 1.86T)
Tdp = 237.3 ln(Pw/0.61078) / (17.2694 − ln(Pw/0.61078))
Twb ≈ f(T, RH)
- Select units matching your instruments and logs.
- Set pressure using barometric value or site altitude.
- Enter State 1 temperature and relative humidity.
- Enter State 2 temperature and relative humidity.
- Press Calculate to view properties and deltas above.
- Export CSV/PDF to attach to commissioning records.
Why psychrometric deltas matter on active sites
Construction commissioning often depends on proving a system change, not just a single reading. Delta tracking compares two air states taken at the same barometric pressure, revealing the net effect of cooling, heating, humidification, dehumidification, or mixing. A dry-bulb change of 6°C can occur with minimal moisture change, while a small temperature change may hide a large latent shift.
Inputs that control accuracy
Use calibrated sensors and stable placement. For field checks, keep temperature within −10°C to 50°C when possible, and record relative humidity between 10% and 90% for best sensor performance. Barometric pressure is commonly near 101.3 kPa at sea level and can drop to about 90 kPa around 1,000 m elevation, affecting humidity ratio and enthalpy.
Interpreting humidity ratio and dew point
Humidity ratio (w) reflects actual moisture content, independent of dry-bulb temperature. A reduction from 0.012 to 0.009 kg/kg indicates meaningful drying, even if relative humidity increases due to cooling. Dew point provides a condensation risk indicator; issues become likely when dew point approaches slab, ceiling, or duct temperatures. Many teams flag dew point above 16–18°C as a practical warning during interior finishing.
Using enthalpy delta for load verification
Enthalpy delta summarizes sensible and latent effects in one value. For quick estimates, a −10 kJ/kg change typically signals strong cooling and moisture removal, while +10 kJ/kg often indicates heating and/or humidification. Combine Δh with airflow measurements to compare trends across test steps and to confirm control sequences. When Δw is near zero but ΔT is large, the process is mostly sensible; when Δw drops, latent removal is occurring.
Recommended field workflow and reporting
Capture State 1 at the return or mixed-air location, then capture State 2 at the supply or downstream location after stabilization. Typical comfort supply air may fall near 12–16°C depending on design. Log time, sensor IDs, and pressure method. Exported CSV supports daily QA logs, and the PDF format is convenient for submittals and records.
1) What is a psychrometric delta?
It is the difference between two measured air states for the same property, such as ΔT, Δw, or Δh. It highlights the net change caused by equipment, processes, or mixing on site.
2) Why does pressure matter in the results?
Humidity ratio and enthalpy depend on the relationship between vapor pressure and total pressure. Using correct barometric pressure improves moisture calculations, especially at higher elevations.
3) Can relative humidity increase while air is drying?
Yes. Cooling can raise relative humidity even when moisture content falls. Check the humidity ratio (w) and dew point to confirm whether the air is actually gaining or losing moisture.
4) Which delta is best for coil performance checks?
Δh is the most compact indicator because it includes sensible and latent effects. Pair it with airflow or mass flow measurements to compare trends across test steps.
5) How should I take State 1 and State 2 readings?
Use consistent probe depth and location, allow the sensor to stabilize, and record both readings within a short interval. Avoid placing sensors near wet coils, direct sunlight, or leaks.
6) Why is wet-bulb shown as an approximation?
The calculator uses a recognized approximation to provide fast field insight. For high-stakes design verification, confirm wet-bulb using a full psychrometric chart method or certified software.