Deviation From Actual Relative Humidity (%) 24 21 18 15 12 9 6 3 0 –3 –6 –9 –12 –15 –18 –21 –24 0 Standard Transmitter Mean Deviation Symbol (% RH) (% RH) –2.9 –3.0 10 20 30 1.2 1.1 40 50 60 70 80 90 100 Actual Relative Humidity (%) Figure 18: Iowa Energy Center NBCIP study: best humidity sensor. 24 21 18 15 12 9 6 3 0 –3 –6 –9 –12 –15 –18 –21 –24 0 Deviation From Actual Relative Humidity (%) www.info.hotims.com/30921-5 Standard Transmitter Mean Deviation (% RH) Symbol (% RH) –1.0 2.0 10 20 30 10.2 9.3 40 50 60 70 80 90 100 Actual Relative Humidity (%) Figure 19: Iowa Energy Center NBCIP study: one of the worst humidity sensors. fixed enthalpy logic ensures humid cool air is not selected, the dry-bulb setpoint should be set for the expected return air temperature (e.g., 75°F [24°C]) regardless of climate, not adjusted downward as in Figure 6. A special type of combination high limit switch is what Standard 90.1 refers to as an “electronic enthalpy” high limit. This very clever electronic controller has been used for many years with packaged AC units with electric or electronic controls. It originally used hygroscopic materials such as nylon for humidity sensing. It is now entirely solid state and much more reliable. Its setpoints (“A” through “D”) form a curve on the psychrometric chart (Figure 16). When set to setpoint “A” (a requirement of Standard 90.1 regardless of climate), it mimics a combination of a fixed enthalpy control with a November 2010 www.info.hotims.com/30921-41 ASHRAE Journal 21