One of the most frequently asked questions to our customers purchasing HVAC equipment requiring controls is “Do you want Enthalpy or Dry Bulb Sensors?”
I’d like to explain Enthalpy & Dry Bulb Sensors, and their differences, without getting overly technical, or having to explain how to understand the functionality of a psychometric chart.
Unfortunately, I must define some atmospheric elements which should be understood prior to defining the meaning of Enthalpy & Dry Bulb.
Relative Humidity & Dew Point – Despite the name, Relative Humidity is a poor indicator of how humidity is actually felt and perceived. Dew Point is a much more accurate measurement of humidity. Why? Because Relative Humidity describes how much water air can retain, which varies upon temperature, while Dewpoint describes the temperature which the water in the air will evaporate.
Let me further explain… If the outside temperature is 55°F, the quantity of moisture the air can retain (Relative Humidity) is much less than 95°F air can hold. Therefore, a Relative Humidity of 50% at 55°F retains a much smaller amount of water vapor than a Relative Humidity of 50% at 95°F.
Dew Point is reflected as a temperature. In this example, the Dew Point when measuring 50% Relative Humidity, at 55°F is equal to 37°F, and the Dew Point when measuring 50% Relative Humidity, at 95°F is 74°F.
From this example, a Relative Humidity of 50% at 95°F is 74°F. When the dew point temperature of 74°F is reached, the water volume when the air temperature was 95°F hits the 100% Saturation point, 100% Relative Humidity is referred to as Saturation. Meaning air at the current temperature can no longer contain the moisture mixed in the air, and water droplets begin to condense into liquid water (dew). The dew point temperature is always lower than, or equal to the Dry Bulb air temperature.
If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100 percent.
Let’s Talk Comfort!
|What is Comfortable?|
||Perception of Comfort|
|55°F or Less||Dry|
|55°F to 60°F||Comfortable|
|60°F to 64°F||Rather Humid|
|75°F and Greater||Miserable!|
I earlier stated that Relative Humidity does not necessarily reflect how we perceive humidity. 100% Relative Humidity at 55°F is 55°F, and considered to be comfortable according to the chart. However, only 50% Relative Humidity at 95°F is 74°F, quiet uncomfortable. This is why the summer months can be so unbearable, because warmer air has the capacity to contain a higher volume of water vapor than cooler temperatures.
Dry Bulb Temperature
A standard outside mercury thermometer reads Dry Bulb temperature. Therefore, when people refer to the ambient air temperature as reflected by a household thermometer, they are unknowingly referring to the Dry Bulb temperature.
It is called “Dry Bulb” because the air temperature is indicated by a thermometer not affected by the moisture or humidity in the air.
Wet Bulb Temperature (Enthalpy):
Imagine a standard temperature probe in an air duct measuring the temperature of the air (Measuring Dry Bulb Temperature). I rip off a small section of a t-shirt, saturate it with water, and wrap it around the probe. By placing the wet material over the temperature probe, I am simulating 100% Relative Humidity for that temperature – Saturation. As the air passes over the probe, the air will evaporate only the percentage of water equal to the water (humidity) in the air stream. The wet bulb temperature will always be equal to or lower than the actual temperature. Once the wet bulb temperature stabilizes, I can determine the percentage of Relative Humidity.
Compare a Wet Bulb temperature reading to the temperature you feel when your skin is wet and is exposed to moving air. Additionally, perspiration allows the body to experience wet-bulb temperature.
If the air in the duct is low in humidity, moisture from the saturated material wrapped over the temperature probe will be removed more quickly than if the air is saturated with moisture (high humidity).
- Dry Bulb – Temperature of a standard thermometer.
- Wet Bulb – The lowest temperature that can be reached by the evaporation of water only, measuring humidity.
- Dew Point – the temperature to which the ambient air (Dry Bulb) must be cooled to reach 100% relative humidity – Saturation.
Enthalpy & Dry Bulb… Which Should You Use?
There are many strategies which can be employed allowing you to make the best educated decision for each type. The next article will delve into practical HVAC applications using Enthalpy & Dry Bulb sensors.