What is a Sling Psychrometer?

Sling Psychrometer is used to measure both the dry bulb and wet bulb temperatures at time. These temperatures are a measure of humidity content in air.

Description of Sling Psychrometer

The main parts of the instrument are

There are 2 thermometers in this instrument.

1.Dry bulb thermometer, whose sensor bulb is in direct contact with air (mercury is the sensing element)

2.Wet bulb thermometer: the sensor bulb of this thermometer is covered with a wick of cotton or muslin moistened with pure water
A glass frame is provided as the housing for the instrument.

A rotating handle is attached to the glass frame housing – the arrangement of the thermometer to ensure that the air in the wet bulb always in immediate contact with the wet wick.

When a thermometer bulb is directly exposed to a mixture of air-water vapor, the temperature indicated by the thermometer is the dry bulb temperature.

When the bulb of a thermometer is covered by a constantly wet wick and if the bulb covered by the wet wick is exposed to the mixture of air vapor and water, the temperature indicated by the thermometer is the temperature of the wet bulb.

A sling psychrometer is a simple device used to measure air humidity. It features two thermometers: one “dry bulb” thermometer and one “wet bulb” thermometer. The wet bulb thermometer is covered with a wick soaked in water.

When the device is swung rapidly through the air, evaporation of water from the wick cools the wet bulb thermometer, causing it to read a lower temperature compared to the dry bulb thermometer. After allowing sufficient time for the thermometers to stabilize, the temperatures are read.

These readings are then used with a psychrometric chart or table to determine humidity levels. The wet bulb temperature roughly equals the adiabatic saturation temperature,

so. TWB​≈T2​=T3​ in relevant equations.

Operation of Sling Psychrometer

In order to measure the dry bulb and the temperature of the wet bulb, the disposition of the thermometer of the psychrometer- glass cover – is rotated from 5 m / s to 10 m / s to obtain the necessary air movement.

Note: An important condition is that the correct / accurate measurement of the wet bulb temperature is obtained only if the air moves with the speed around the wet wick. To get this air velocity, the psychrometer is spinning.

The thermometer whose bulb is empty contacts the air indicates the temperature of the dry bulb.

At the same time, the thermometer whose bulb is covered with the wet wick comes in contact with the air and when this passes the wet wick present in the thermometer bulb, the moisture present in the wick begins to evaporate and a cooling effect occurs produced in bulb. Now the temperature indicated by the thermometer is the wet bulb thermometer that will naturally be lower than the temperature of the dry bulb.

Note: If the psychrometer is rotated for a short period, then the registered wet bulb temperature will not be adequate.

Note: If the psychrometer is turned for a longer period, the wick will dry soon and the temperature of the wet bulb will not be at its minimum value.

What is a Sling Psychrometer used for?

A sling psychrometer is used to measure the relative humidity of the air. It provides accurate readings by comparing the temperature difference between a dry bulb thermometer and a wet bulb thermometer. The readings can be used to determine humidity levels, dew point, and other related atmospheric conditions.

What is the Principle of a Psychrometer?

The principle of a psychrometer is based on the cooling effect of evaporation. The device uses two thermometers: a dry bulb and a wet bulb. The wet bulb thermometer is covered with a moist wick. When the psychrometer is spun in the air, water evaporates from the wick, cooling the wet bulb thermometer. The difference in temperatures between the dry bulb and wet bulb thermometers is used to calculate relative humidity. Evaporation cools the wet bulb, and the larger the temperature difference, the lower the humidity.

What are the uses of a Psychrometer?

• The primary use is to determine the relative humidity in the air.
• Psychrometers can be used to estimate the dew point temperature, which is the temperature at which air becomes saturated with moisture and dew forms.
• Used in weather stations and research to assess atmospheric moisture.
• Used to monitor and control indoor air quality by measuring humidity.
• Helps in managing greenhouse conditions and ensuring optimal humidity levels for crops.

Method for using a Sling Psychrometer

Preparation:

• Begin by soaking the wick of the wet-bulb thermometer in water. Ensure that the wick is thoroughly saturated for accurate measurements.

Whirling the Psychrometer:

• Hold the handle of the psychrometer and whirl it in the air. This action generates airflow around both the wet and dry bulbs.
• As the psychrometer is whirled, the water from the wet bulb’s wick begins to evaporate, which cools the wet bulb thermometer. The dry bulb thermometer remains unaffected by this process and continues to display the ambient air temperature.

Temperature Measurement:

• After spinning the psychrometer for a sufficient time, stop whirling and quickly read the temperatures displayed by both thermometers.
• The difference between the dry bulb and the wet bulb temperatures is known as the Wet-Bulb Depression.

Calculating Relative Humidity:

• Record the temperature readings from both the dry bulb and the wet bulb thermometers.
• Calculate the Wet-Bulb Depression using the formula:
• Wet-Bulb Depression=Dry-Bulb Temperature−Wet-Bulb Temperature
• Using the dry bulb temperature and the wet-bulb depression, refer to a psychrometric chart to determine the relative humidity of the surrounding air.

This method allows you to accurately measure the relative humidity by taking advantage of the cooling effect that occurs as water evaporates from the wet bulb

How long do you Spin a Sling Psychrometer?

You should spin a sling psychrometer for about 1 to 2 minutes, or until the temperatures on the thermometers stabilize. This allows sufficient time for the wet bulb thermometer to reach equilibrium through evaporation, ensuring an accurate humidity reading. After spinning, quickly read the temperatures before they change

What is the Unit of a Psychrometer?

A psychrometer itself does not have a unit since it is a measuring instrument. However, the measurements taken with a psychrometer are typically expressed in degrees Celsius (°C) or degrees Fahrenheit (°F) for temperature, and in percentages (%) for relative humidity.

Application of Sling Psychrometer

1. It is used for checking humidity level in air-conditioned rooms and installations.
2. It is used to set and check hair hygrometer.
3. It is used in the measurement range of 0 to 100% RH.
4. It is used for measuring wet bulb temperature between 0’C to 180’C.

Limitation of Sling Psychrometer

1. The measured medium is altered due to the act of measurement. The evaporation process in the wet bulb will add moisture to the air.
2. Can not be used in situations of automation requirements.
3. Can not be used for continuous recording purposes.
4. If the wick is covered with dirt, the wick will harden and its water absorption capacity will be reduced; however, a stiff / dirty wick will return to normal when boiling in hot water

What is the difference between a psychrometer and a hygrometer?

A psychrometer is a type of hygrometer that specifically uses a wet-bulb and dry-bulb thermometer to measure relative humidity. While a hygrometer is a broader term encompassing various instruments designed to measure humidity, psychrometers are one of the traditional methods within this category.

What is the difference between a sling psychrometer and a stationary psychrometer?

A sling psychrometer requires manual movement (whirling) to create airflow over the thermometers, while a stationary psychrometer uses a fan or remains still with ambient air flowing over it. Sling psychrometers are portable, whereas stationary ones are typically fixed in place.

What is the purpose of the digital psychrometer?

A digital psychrometer is used to quickly and accurately measure temperature and relative humidity, often providing additional data such as dew point and absolute humidity. It simplifies the process by eliminating the need for manual readings and calculations.