Dry and Wet Bulb Temperatures

Dry Bulb Temperature

"The dry-bulb temperature isthe temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture.  Dry bulb temperature is simply the temperature of the air as collected from a normal glass column thermometer that has a 'dry bulb'".  (Leadership In Restorative Drying, page 78)

 The thermometer is not being influenced by the effects of water on the bulb.

Wet Bulb Temperature

"The wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor.  Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only.  It is the temperature you feel when your skin is wet and is exposed to moving air."  (Leadership In Restorative Drying, page 79)

In order to understand the properties at work in this measurement we have to understand water at a molecular level.  The first thing is that a water molecule has polarity.  The hydrogen atoms have a slightly positive charge and the oxygen atom has a negative charge.  "Water is in its liquid or solid form when the magnetic bonds keep the molecules together.  Those magnetic forces cause the molecules to bond together with each other when the molecules are moving slow enough.  The hydrogen component of the water molecule bonds with another molecule's oxygen.  This is called 'hydrogen bonding'. (Leadership In Restorative Drying, page 69)

A second thing to understand is that water molecules are moving at an alarming rate.  "'At 72F, most molecules in the environment (nitrogen, oxygen etc) are moving at about 600 miles per hour.  Water vapor in that air is moving at twice that speed - about 1200 miles per hour.'  Clearly it is impossible for 'air to be holding water' when in fact the water molecules are moving much faster than the other air constituents are moving" (Leadership In Restorative Drying, page 69)

 "As this vibrating molecule moves around the environment, the molecules will give and receive some of this energy [emphasis mine] among themselves.  Therefore, if some of these molecules slow down enough and come near the liquid (or other cooler surface with less thermal energy), the water vapor molecule may condense back into the liquid and give some of its thermal energy back to the liquid."  (Leadership In Restorative Drying, page 79 - 80)

This giving of energy back to the liquid causes the cooling effect that would be present on your arm in a breeze. It was found that these two measurements, the dry bulb and the wet bulb, can be used to understand environmental vapor pressure and then to be able to plot it on a chart.

 "[T]he sling psychrometer was the device that was used to begin most environmental consideration.  It worked by passing air over two thermometers; one with a dry bulb and the other with a wet bulb.  Once the dry bulb and wet bulb data points were plotted on the psychrometric chart, all other measures would then be possible.  While very simple in concept, the accuracy of the readings produced with sling psychrometers was not too accurate due to the different speeds at which the users would spin the device...Today we have electronic meters that automate the mechanical process of the sling psychrometer in evaluating the environment." (Leadership In Restorative Drying, page 80)

By taking these measurements we can now plot the data on a psychrometric chart and from that information be able to determine the various properties at work in the environment and their influence on each other.  We are also able to determine how we can influence change within the environment to enhance the drying process such as altering the state or phase in which the water is currently established. This is the at the heart of developing a sound drying strategy.

One of the first peices of data we need to consider in our drying strategy will be vapor pressure.