Engineering
Dew Point temperature formula (Magnus equation) & condensation risk check
Estimate dew point over liquid water from air temperature and relative humidity using a Magnus approximation.
Estimate dew point rather than promise exact condensation behavior
The Magnus approximation with constants 17.625 and 243.04 gives about 16.70°C for 25°C air at 60% relative humidity. It is an empirical approximation, not an exact measurement or a universal accuracy guarantee.
The coefficients here represent saturation over liquid water. Frost-point calculations require an appropriate ice relation. Sensor uncertainty, pressure assumptions and local surface conditions matter when interpreting a condensation comparison.
The example accepts air temperatures from -40 to 50°C and RH in (0,1]. These input bounds scope the exercise; they do not guarantee accuracy for every resulting dew point. A surface below the estimate suggests risk, while a surface above it does not establish overall equipment safety.
| Air Temp (°C) | Relative Humidity (B2) | Magnus Dew Point (°C) | 15 °C Chilled Pipe Status |
|---|---|---|---|
| 25.0 °C | 60% (0.60) | 16.70 °C | Condensation Risk! (16.70 >= 15) |
| 25.0 °C | 40% (0.40) | 10.47 °C | Above estimated dew point (10.47 < 15) |
| 25.0 °C | 100% (1.00) | 25.00 °C (Exact T!) | Saturated Air |
Estimated Magnus Dew Point (°C) and surface condensation flag
Enter Relative Humidity B2 as a decimal fraction in (0, 1] (e.g., 0.60 for 60% RH).
=IF(OR(COUNT(A2:B2)<>2,A2< -40,A2>50,B2<=0,B2>1),"Invalid temperature or RH",LET(g, LN(B2) + (17.625 * A2) / (243.04 + A2), ROUND((243.04 * g) / (17.625 - g), 2)))Automated cold-surface condensation alert against Surface Temp C2
Flags "Condensation Risk" whenever surface temperature C2 drops at or below the ambient Dew Point.
=IF(OR(COUNT(A2:B2)<>2,A2< -40,A2>50,B2<=0,B2>1,NOT(ISNUMBER(C2))),"Invalid temperature or RH",LET(g, LN(B2) + (17.625 * A2) / (243.04 + A2), dp, (243.04 * g) / (17.625 - g), IF(C2 <= dp, "Condensation Risk", "Above estimated dew point")))Try the example
Use the original values or change them to test your understanding.