Psychrometry and Psychrometric Chart. I am having trouble pronouncing it

But it sounds cool – psychrometric and psychrometric chart, but I almost gave up trying to pronounce it the first time around. The pronunciation goes like this,


Now, that should do the trick! I have been using this word over and over again every since I knew how to pronounce it. It’s just so cool.

Before I get carried even more, let me take you through other, even more powerful uses of psychrometry and psychrometric chart.

Ok, one final practice of the pronunciation. Psychrometry. “Aaah”

Function of psychrometry and psychrometric chart:

Air conditioners, as the name suggests, are used to condition air. These equipment will be used to control properties of air.

Use of psychrometry and psychrometric chart is vital for designing and sizing air conditioners.



Your visit to air properties page gave you the insight to different characteristic of air, at different temperatures.

In summary, these are;

  • dry bulb temperature,

  • wet bulb temperature,

  • specific humidity,

  • dew point temperature,

  • specific volume,

  • and enthalpy

Psychrometry is defined as the study of air property.

Psychrometric chart

But why a chart?

There are six dimensions of parameters involved in defining an air condition. These parameters, are inter-related between each other.

Additionally, there are numerous data points from experiments, that need to be recorded.

Putting these data into a set of table (similar to Roger & Mayhew’s ”Thermodynamic and Transport Properties of Fluids”) is possible.

However, the drawback is, air conditioner designers and contractors have to flip-and-flip-back the pages to get necessary data.

A chart, named known as the psychrometric chart is formed, incorporating all six parameters. Data referencing made easy.

Psychrometric chart is defined as the representation of air properties, in graphical form.

More about psychrometric chart:

Psychrometric chart can either be represented in S.I. or metric units. Properties of the chart includes,

  1. bounded by two perpendicular axes, and a curved line these are,

    • dry bulb temperature line on one axis,

    • specific humidity line on another axis,

    • and dew point, or 100% relative humidity curve

  2. different values of dry bulb temperature along the dry bulb axis,

  3. different values of specific humidity along the specific humidity axis,

  4. different curves of relative humidity. 100% being farthest from dry bulb temperature axis, and 0% being on the axis itself,

  5. enthalpy values, on a slanted axis,

  6. wet bulb temperature, on almost coincidental lines as the enthalpy values,

  7. specific volume slopes, inclined towards specific humidity axis,

  8. and the sensible heat to total heat ratio, vis-á-vis, room slope

Psychrometric chart

Interpreting occurrences on psychrometric chart:

We will often have changes of air temperature and humidity, throughout air conditioning cycle.

Interpreting the air behaviour during these changes, is possible by plotting the data.

Main variations in a psychrometric chart are,

  1. sensible heating, and cooling along cooling, or heating coil

    • sensible cooling happens when the cooling or air, does not alter the specific humidity. However, the relative humidity will increase.

      Sensible cooling can only happen under condition that, the cooling coil's temperature, is not lower than the dew point temperature of entering air

    • sensible heating is similar to sensible cooling (alteration in dry bulb temperature), but dry bulb temperature is increased in this case.

      The condition that has to be met is, there should be no water within the heating system, to avoid increase in specific humidity

    Psychrometric chart on sensible cooling and heating

  2. adiabatic humidification, and dehumidification using humidifier, or chemical dehumidifier

    • adiabatic humidification occurs when water vapour is added to the atmosphere.

      Specific humidity will increase, along the wet bulb temperature line. Reduction in dry bulb temperature will happen as, the evaporated water will absorb heat.

      A humidifier will perform this function.

    • adiabatic dehumidification occurs similar to dehumidification, but on the opposite direction.

      This function is usually achieved by use of chemical agents, like absorbents or adsorbents.

      Specific humidity will be reduced along the wet bulb temperature line, whilst dry bulb temperature will increase, as a result of chemical reaction

    Psychrometric chart on adiabatic humidification and dehumidification

  3. cooling and heating with change in specific humidity using conventional air conditioners

    • cooling with change in specific humidity, can be detected in psychrometric chart, by noticing reduction in dry bulb temperature, with reduction in specific humidity.

      Circumstance for such occurrence is that, the evaporator’s temperature is lower than the dew point temperature of the entering air.

      Water will condensate, and drained off. You’ll get dry, cool air.

    • heating with change in specific humidity happens as the dry bulb temperature increases, with increase in specific humidity.

      This is achieved in air conditioners for heating (or famously known as heat pumps), by adding water vapour, after being heated

    Psychrometric chart on cooling and heating with change in specific humidity

  4. air mixing. Multiple air mixing from different ducts, are normally performed in central air conditioners. The most common mixing, is between re-circulated air, and fresh air.

    Fresh air and re-circulated air mixing is very important in maintaining low operating cost of an air conditioner.

    You can determine the condition of 2 mixed air stream, by,

    1. plotting conditions of air before mixing,

    2. draw a straight line between these points, and measure the length,

    3. get the ratio of mixing, and add these numbers,

    4. divide the air proportions of each stream, to the added number,

    5. multiply each divided number in (4), with the measured line length in (2), to get two different lengths,

    6. measure the longest distance along the line in (2) from the lowest proportion of air stream,

      (or the shortest distance from the highest proportion of air stream)

    7. mark at the end of the length,

    8. this new point is your mixed air condition on a psychrometric chart

    Psychrometric chart on air mixing

  5. room slope, or sensible heat to total heat ratio, indicates the ”behaviour of room” under a conditioned space.

    If the slope is,

    • positive, then, the air conditioner has to take care of the latent heat, and sensible heat increase

    • negative, then, the air conditioner has to take care of the latent heat increase, and sensible heat decrease

    • 1, then, the air conditioner has to take care of sensible heat increase or decrease

    • 0, then, the air conditioner has to take care of latent heat increase or decrease

    Psychrometric chart on room slope

Those are the main variations in psychrometric chart. Of course, the variations may be combined to achieve suitable air condition within your space.

Software are being developed with electronic format of psychrometric chart, to make the calculations much quicker, but these software will not be useful, unless we equip ourselves with the basic knowledge of psychrometry.

You can still use the paper chart, with pencil and ruler – in case of power disruption. It is indeed, a powerful tool.

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