Issue
How can wet bulb temperature be calculated using only a temperature sensor and a humidity sensor?
Product Line
Satchwell BAS & Sigma
Environment
Cause
Wet bulb temperature is a key parameter in HVAC and psychrometric calculations. While the most accurate method requires dew point and barometric pressure, a reasonably accurate estimate can be derived using just temperature and relative humidity.
Resolution
Method 1: Simplified Formula Using Assumed Pressure
This method assumes a constant barometric pressure (typically 1006 mBar).
Formula:
tw = t * (0.45 + 0.006 * Rh * √(p / 1060))
Where:
- tw = Wet bulb temperature (°C)
- t = Dry bulb temperature (°C)
- Rh = Relative humidity (%)
- p = Pressure (mBar), typically set to 1006
Sigma/BAS Object Code:
10 XFlo = 0.000000
20 YFlo = 0.000000
30 XFlo = XFlo + ( SQRT ( 1006.00 / 1060.00 ) )
40 YFlo = YFlo + ( 0.00600000 * POINT 0|50 * XFlo )
50 YFlo = 0.450000 + YFlo
60 YFlo = POINT 0|49 * YFlo
- P0|49 = Temperature sensor (°C)
- P0|50 = Humidity sensor (%)
Method 2: Polynomial Approximation
This method provides a more accurate estimate within the range:
- Dry bulb temperature: 15–40°C
- Relative humidity: 10–90%
- Pressure: 1006 mBar
- Max relative error: ~6%
Formula:
Tw = (A * T) + (B * Rh) + (C * T²) + (D * Rh²) + (E * T * Rh) + F
Constants:
- A = 0.539126
- B = 0.104784
- C = -0.000749356
- D = -0.00107743
- E = 0.00641463
- F = -5.15153
Sigma/BAS Object Code:
10 XFlo = 0.000000
20 XFlo = XFlo + ( 0.539126 * POINT 0|49 )
30 XFlo = XFlo + ( 0.104784 * POINT 0|50 )
40 XFlo = XFlo + ( -0.000749356 * POINT 0|49 * POINT 0|49 )
50 XFlo = XFlo + ( -0.00107743 * POINT 0|50 * POINT 0|50 )
60 XFlo = XFlo + ( 0.00641463 * POINT 0|49 * POINT 0|50 )
70 XFlo = XFlo + -5.15153
80 RETURN VIA TEXT 101 VALUE XFlo
- P0|49 = Temperature sensor (°C)
- P0|50 = Humidity sensor (%)
Notes
- For higher accuracy, consider using actual barometric pressure and dew point if available.
- These formulas are suitable for most building automation applications where approximate values are acceptable.
