Ancient hygrometers[ edit ] Prototype hygrometers were devised and developed during the Western Han dynasty in Ancient China to study weather. The differences in weight were used to tally the humidity level. Other techniques were applied using mass to measure humidity, such as when the air was dry, the bar of charcoal would be light, while when the air was humid, the bar of charcoal would be heavy. By hanging a lump of earth and a bar of charcoal on the two ends of a staff separately and adding a fixed lifting string on the middle point to make the staff horizontal in dry air, an ancient hygrometer was made. In these devices, water vapor is absorbed by a salt-impregnated paper strip attached to a metal coil, causing the coil to change shape. These changes analogous to those in a bimetallic thermometer cause an indication on a dial.
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A sling psychrometer is a simple instrument that can be used to measure the humidity in the air. Here, we shall find out how a sling psychrometer works. Weather prediction is a tricky affair. So while you may come across certain individuals who can precisely tell whether it is going to rain or not simply by flicking their tongues to taste the air, or twitching their nostrils to smell it, others like us who are less endowed have to rely on the use of special instruments of measurement to make such predictions.
The sling psychrometer is one such innovative instrument, which helps us measure the levels of humidity in the air. It has a simple design, and is accurate enough to be used for predicting weather patterns. In the following sections, we shall take a closer look at the sling psychrometer, and study its working. What is a Sling Psychrometer? For measuring the relative humidity in the air, an instrument known as a hygrometer is used.
Of the several different types of hygrometers available for use, the sling psychrometer is the simplest, both in terms of design and ease-of-use. This process allows the device to measure a temperature difference, which is directly proportional to the humidity in the air. Thus, the user is able to determine the value of humidity with an appreciable level of accuracy. History Sir John Leslie , a reputed meteorologist of his time, is credited with the invention of the sling psychrometer.
Towards the end of the 19th century, German meteorologist and physician Adolph Richard Assmann modified the design, and introduced a type of wet-dry bulb psychrometer with improved accuracy. Principle of Working This instrument works on the principle of evaporation of water. When water on a surface evaporates, it extracts a significant amount of heat from it, cooling the surface down. This is how sweat is able to cool down our body and skin when it gets hot.
However, air can only hold a limited amount of water, after which it becomes saturated. The psychrometer is designed to make use of both these facts, to help us measure the humidity of air. Basic Design The psychrometer is the simplest type of hygrometer. Its design comprises two thermometers which are mounted securely on a platform. The bulb of one of the thermometers is left exposed to the air.
This is known as the dry-bulb thermometer. This one is known as the wet-bulb thermometer. It is used to sling the device around in circular motions. As mentioned earlier, water evaporation leads to a drop in temperature. This is measured by the wet-bulb thermometer. At the same time, the dry-bulb thermometer provides the original reference temperature reading.
Taking the difference between these two values, one can obtain the exact drop in temperature. This temperature drop will be directly proportional to the amount of water that gets evaporated into the air.
If the surrounding air is relatively dry, more moisture will evaporate into it, and as a result, the measured drop in temperature will be greater. Thus, more temperature drop indicates lower humidity levels.
Conversely, if the air is more humid, the amount of evaporation of water will be less, resulting in a lower temperature drop. How to Use Step 1: Pre-preparation Before using the psychrometer, it is advisable to perform an initial pre-preparation step, to ensure that you get accurate readings out of it. Basically you have to ensure that both, the thermometers on the device are displaying the same initial reading, so that the measurement commences from the same starting point.
A good way to do this is to shake the psychrometer rapidly for about a minute. Step 2: Prepare the Wick The device uses a wet bulb thermometer in addition to a normal dry one. Step 3: Whirl the Psychrometer Once you have readied your instrument by following the steps given above, you can start using it. You have to swing it around in circles for approximately a minute to allow the water from the wick to get evaporated into the surrounding air, and thus cool the wet-bulb thermometer.
Step 4: Take the Readings After swinging it for a minute, note down the readings from both the thermometers, starting first with the wet one. Step 5: Calculate the Humidity To calculate humidity, simply take the difference between the temperature values obtained in the previous step, and then refer to a psychrometric chart to know the humidity value.
How to Read a Sling Psychrometer