Elements of Psychrometric Chart

  • Be able to understand the basic terminology and definitions related to air properties;

  • Be able to read psychrometric chart; and

  • Be able to apply the psychrometric charts on air-conditioning or drying applications.

  • Identify the four basic parts of the Psychrometric chart

  • Plot a point on the psychrometric chart using two measured conditions of the air.

  • Recognize the relationship of the conditions of the air and the operation of the heating and cooling equipment.

Dry Bulb Temp Lines

  • Dry Bulb Temp.: The temp.of air readon a standard thermometer. Lines of constant DBT are straight, vertical lines on the chart. The DBT scale is at the bottom of the chart. Units are Deg. C.

Wet Bulb Temp Lines

  • Wet Bulb Temp.: It is indicated by a thermometer whose bulb is covered by a wet wick and exposed to a stream of air moving at a velocity of 5m/s. The WBT scale is on the curved line at the left of the chart. Units are Deg. C.

  • Humidity Ratio: It is the ratio the mass of water vapor to the mass of dry air. Lines of constant humidity ratio are straight, horizontal lines at right angle to the DBT lines. The humidity ratio scale is at the right of the chart. Units are grains of moisture per kg of dry air.

Absolute Humidity

  • Enthalpy: It is a measure of heat energy in the air. Units are KJ/kg of dry air.

  • Specific Volume: It is the volume per unit mass. Units are m\(^3\)/kg. Lines of constant specific volume indicated along the bottom axis of the psychrometric chart with the constant volume slanting upward to the left.

  • Dew Point Temp.: Units are Deg. C. It is the temp. at which water vapor has reached the saturation point (100% Relative Humidity)

  • Relative Humidity: Lines of constant relative humidity curve upward from left to right. It is the ratio of mole fraction of water vapor to the mole fraction of water vapor saturated at the same temp. and barometric pressure.

Source: https://tex.stackexchange.com/questions/59051/tikzhow-to-consider-a-curve-as-an-axis-in-a-chart

Solved Examples

Solved Example:


The temperature of air recorded by a thermometer, when it is not effected by the moisture present in the air, is called: (MPSC AMVI Paper 3- Set A 2011)

Correct Answer: B

Solved Example:


The relative humidity is defined as: (Based on ISRO Scientist Civil 2020)


Relative humidity: It is defined as the ratio of the mass of water vapour ($m_v$) in a certain volume of moist air to the mass of water vapour ($m_{vs}$) in the same volume of saturated air (air having the maximum amount of water vapor without condensing) at the same temperature.

Correct Answer: C

Solved Example:


The humidity ratio or specific humidity is the mass of water vapour present in:

Correct Answer: A

Solved Example:


One kg of steam sample contains 0.8 kg dry steam; its dryness fraction is: (JKSSB JE ME 2015)

Correct Answer: B