Psychrometer and 3 important methods related to Relative humidity

What is a Psychrometer ?


A psychrometer is an instrument used to determine the air’s moisture quantity; this usually determine the dew point and the relative humidity. There are numerous techniques that this quantity could be measured. The basic is common and traditional, is to utilize a drybulb and a wetbulb thermometer in combination.

To measure the relative humidity and dew point, Two thermometer is required, a dry bulb thermometer is utilized for the measurement open air temperature. A wet-bulb thermometer is muslin saturated (i.e., soaked) dipped in water over the bulb section.

This works on the principle of “latent heat of evaporation technique” that will cause the bulb temperature to read lower than the dry one. This variance or change can then be exploited to estimate the relative humidity and dew point. The thermometers are permanently straddling in a Stevenson Screen or with a sling psychrometer. It is a transportable device that embraces the two thermometers to compute the amount of moisture of the air to spin it to permit the thermometers to calculate an accurate temperature.

“A psychrometer determine the relative humidity of atmosphere.”

A Sling Psychrometer
A Sling Psychrometer.
CambridgeBayWeatherSling psychrometer, marked as public domain, more details on Wikimedia Commons

A Psychrometer comprise of dual thermometers


“A quantity representing the amount of water vapour in the atmosphere or in a gas.”

Formula of Relative Humidity:

Relative Humidity = Specific Humidity divided by the Saturation Point.

Comparison between Relative Humidity and Dew point :

Dew Point Variation

Dew Point Variation
Dew Point Variation
MikeRunDew-point-exampleCC BY-SA 4.0

A Sling Psychrometer

A type of fundamental psychrometer instrument acts based on the evaporation procedure. There are devices, including aspirated and ventilated psychrometers, which can be made to work similar to this, too, defined to work with fans who ventilate the wet-bulb thermometer. The procedure improved evaporation rates, which produces a more precise reading.

“A psychrometer will whirl may be erroneous till the measurement of the wet-bulb thermometer reaches a constant value.”

How does a sling psychrometer determine relative humidity?

A sling-psychrometer is utilize to measure the relative humidity, characterized in proportional % form. The relative humidity is determined by multiplying the quantity of moisture in the air at that temperature, dividing by the max-amount of moisture (at that fixed temperature the air could contain), and then multiplying the quotient by a hundred, to represent in percentage term.

Proper care has to be there to use a psychrometer carefully to avoid any damage to the device.

Accuracy in measurement is one of the critical aspects of using a psychrometer, ensures the most accurate readings for relative humidity. People who rely on atmospheric readings for their experimental jobs, including training outdoors climate research, utilize this technologies as part of a weather meter apparatus created for use in the field. The calibration process of hygrometer, a psychrometer, is used to ensures the most accurate readings during the determination of relative humidity.


Psychometrics’, psychrometry, and hygrometry are terms for the field of engineering concerned with the physical and thermodynamics properties of vapor and gas mixture.

What is Psychrometer Chart?

The chart is an important analysis datasheet for understanding the connections between the parameters of the humidity and supply air. This standard model template enables a programmer or operator to “work backwards” in the desirable room relative humidity to the favourite condition of the atmosphere as it passes the distribution or supply duct.

The psychrometric chart is a datasheet with diagram-based tool that’s utilized to see the associations between moisture, dry air, and energy enclosed. To design or maintenance of air conditioning in big houses and office, a technical understanding and crystal-clear grasp of the psychrometric graph will always be useful to make job simpler for us.

A Psychrometric chart is made considering two basic perceptions.

Interior air is a mix of dry air and water vapor.

Specific amount of energy is present in the mix at a specific pressure and temperature.

To realize the connection among temperature, air, and water vapor, we will consider two statuses:

a. The amount of water vapor is increasing, although the air temperature is constant.

b. The amount of water vapor is constant, although the atmosphere temperature is dipping.

Hygrometer vs Psychrometer

  • Users and experimentalists get confused and leave them uncertain about which one to buy when they’re searching to buy between a psychrometer and a hygrometer. A psychrometer is a special type of hygrometer, though.
  • Different techniques determine the air’s moisture content; these may comprise thermal conductivity, resistance, or capacitance. These are quantified with devices labelled as hygrometers and will see them recorded in as dew point meters. If required for ideal selection for measuring the moisture content of the air. This will be dependent on our requirements. A meteorologist or someone else for whom precision is the main consideration, usually use it.
  • Other people will need to find out the moisture content of the atmosphere like technicians, painters. An electronic hygrometer could be the best alternative as it’s comparatively easy to operate. Also, it works fast. For approximate accuracy requirements, the digital version is good for many users.
  • The very best alternative, if somebody is concerned in the climate, is likely a hygrometer nor a psychrometer. Home climate conditions channels have a hygrometer feature and providing that they can quantify each other weather parameters, so they create a fantastic alternative for most users.

To learn more about Hygrometer , Visit here.

A motorized psychrometer
A motorized psychrometer
CambridgeBayWeatherStevenson screen interior, marked as public domain, more details on Wikimedia Commons

About Dr. Subrata Jana

I am Subrata, Ph.D. in Engineering, more specifically interested in Nuclear and Energy science related domains. I have multi-domain experience starting from Service Engineer for electronics drives and micro-controller to specialized R&D work. I have worked on various projects, including nuclear fission, fusion to solar photovoltaics, heater design, and other projects. I have a keen interest in the science domain, energy, electronics and instrumentation, and industrial automation, primarily because of the wide range of stimulating problems inherited to this field, and every day it’s changing with industrial demand. Our aim here is to exemplify these unconventional, complex science subjects in an easy and understandable to the point manner.
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