Why should you care about psychometrics? What are they anyway?
Because we (well some of us) like to try to get our water temps below the ambient temperature. And we also like to avoid condensation. There seems to be a common misconception that any temperature below ambient temperature can result in condensation. This is not true. Condensation will not happen on an object unless it drops below the dew point. The dew point varies based on local temperature and humidity levels. Psychometrics is the $5 word for this subject. It does not matter if you remember it or can spell it, as long as you understand what it means. I am not feeling so hot, so I decided to just copy and paste an article from elsewhere <sorry I realize that copy and paste is cheap but it is the best I can do right now>
Here it goes:
A psychrometric chart is a graph of the physical properties of moist air at a constant pressure or often equated to an elevation relative to sea-level. The chart graphically expresses how various properties relate to each other, and is thus a graphical 'equation of state'.
Dry-bulb temperature is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry.
Wet-bulb temperature is that of an air sample after it has passed through a constant-pressure, ideal adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. In practice, this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air.
Dew point temperature is that at which a moist air sample at the same pressure would reach water vapor saturation. At this saturation point, water vapor would begin to condense into liquid water fog or (if below freezing) solid hoarfrost, as heat is removed. The dewpoint temperature is measured easily and provides useful information, but is normally not considered an independent property. It duplicates information available via other humidity properties and the saturation curve.
Relative Humidity is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure. The notion that air "holds" moisture, or that moisture dissolves in dry air and saturates the solution at some proportion, is an erroneous (although widespread) concept.
Humidity Ratio is the proportion of mass of moisture present in a unit mass of air at the given conditions (DBT, WBT, DPT, RH, etc.).
Specific Enthalpy symbolized by h, also called heat content per unit mass, is the sum of the internal energy of a thermodynamic system. These values correspond to the saturated state and are to be read parallel to WBT values.
Specific Volume, also called Inverse Density. Volume per unit mass.
The versatility of the psychrometric chart lies in the fact that by knowing three independent properties of some moist air, one of which is the chart's pressure, the other properties can be determined. Changes in state, such as when two air streams mix, can be modeled easily and somewhat graphically using the correct psychrometric chart for the location's air pressure or elevation relative to sea level. For locations at or below 2000 ft (600 m), a common assumption is to use the sea level psychrometric chart. The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. ... Wet-bulb temperature ... Dew on a spider web The dew point or dewpoint of a given parcel of air is the temperature to which the parcel must be cooled, at constant barometric pressure, for the water vapor component to condense into water, called dew. ...
Willis Carrier, considered the 'father' of modern air-conditioning, rearranged the Mollier diagram for moist air (its T-s chart) to allow these graphical solutions. Many variations and improvements to the psychrometric charts have occurred since, and most charts do not show the specific entropy (s) like the Mollier diagram. ASHRAE now publishes what are considered the modern, standard psychrometric charts, in both I-P and SI units, for a variety of elevations or air pressures. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) is an international voluntary organization for people involved in heating, ventilation, air conditioning, or refrigeration (HVAC&R). ...
How to read the chart
The most common chart used by practitioners and students alike is the "ω-t" (omega-t) chart in which the Dry Bulb Temperature (DBT) appears horizontally as the abscissa and the humidity ratios (ω) appear as the ordinates.
In order to use a particular chart, for a given air pressure or elevation, at least two of the six independent properties must be known (DBT, WBT, RH, Humidity Ratio, Specific Enthalpy, and Specific Volume). This gives rise to 6C2 or 15 possible combinations.
DBT : This can be determined from the abscissa
DPT : Follow the horizontal line from the point where the line from the horizontal axis arrives at 100% RH, also known as the saturation curve.
WBT : Line inclined to the horizontal and intersects saturation curve at DBT point.
RH : Hyperbolic lines drawn asymptotically with respect to the saturation curve which corresponds to 100% RH.
Humidity Ratio : Marked on Ordinate axis.
Specific Enthalpy : lines of equal values, or hash marks for, slope from the upper left to the lower right.
Specific Volume : Equally spaced parallel family of lines.
This wikipedia entry about psychometric charts is also useful. http://en.wikipedia.org/wiki/Psychrometrics