The psychrometer

Psychrometer measurement station

Today we come to describe the operation of another of the measurement devices in meteorology. We will talk about psychrometer. It is an instrument used to measure the content of water vapor in a column of air. Knowing the water vapor in the atmosphere is important to determine the degree of humidity.

If you want to learn how to handle the psychrometer, all its characteristics and the care it needs, this is your post.

What is the psychrometer

Parts of a psychrometer

As we mentioned in the introduction, it is a device for measuring water vapor in air. To do this, it consists of a pair of glass thermometers with a mercury column (like old thermometers). They are mounted on a plate. One of them is called a dry bulb and the other is called a wet bulb. It is named for having placed in the mercury bulb a cover or lining of cotton fabric called muslin, which needs to be wet to obtain the required indications.

The wet bulb is covered in clean muslin and is saturated with water prior to observation. When the bulb is ventilated, it will indicate the temperature of the wet bulb and the other that of the dry bulb.

How to use the psychrometer

Weather coat

To obtain the temperatures measured by both bulbs, the following steps need to be performed.

  1. We should read the dry bulb thermometer approaching to the tenth degree. This temperature marks the temperature of the environment.
  2. We wet the muslin of the wet bulb thermometer for as long or as many times as necessary with clean water until a drop forms at the bottom of it.

To wet the muslin we should not have a psychrometer that is fixed inside the weather coat. It has to be taken to a container with water to the psychrometer so that the bulb with the muslin is submerged in the liquid.

In general, the water must be kept in a glass container placed inside the weather shelter. We will describe the coat later to get to know it better. It is necessary to try to keep the container covered so that the water remains clean and the humidity inside the meteorological shelter is not altered.

There are times when the humidity is quite high. In this situation the muslin may appear damp, but it will need to be wet again. If the ambient temperature is high or the relative humidity is very low, You will need to wet the muslin long enough for it to dry. The observer can estimate that the bulb temperature will be 0 degrees or less in ambient cold.

Temperature and humidity

Psychrometer for humidity

If the muslin dries before the thermometer indicates the correct wet bulb temperature, we are making a wrong measurement.

There are numerous climates and temperatures around the world. Therefore, there are regions where temperatures are high and humidity is low. These are desert or semi-desert areas. On these occasions we must use fresh water to wet the muslin and avoid premature drying of it.

To keep the water fresh, it can be kept in a porous container, but trying to leave the container outside the coat to avoid altering the humidity inside it.

  • Another step to take to get the psychrometer to work is to run the fan to provide a constant flow of air. This air must pass through the bulbs of the thermometers for a correct measurement. In the case that measurements are made at night, a spotlight must be used. If the psychrometer we use is a sling, we must turn it at a speed of four revolutions per second. This spin speed is used to take a faster reading. This is when you should gently stand up and take the reading in the shade.
  • We must also ventilate enough for three minutes. The mercury in the thermometer should stop its descent and reach its minimum column length. The reading must be taken by approximating the values ​​to tenths. The value that we will obtain will be that of the wet bulb temperature.
  • We will turn off the fan and if we make night observations, we will turn off the focus.
  • If the air temperature is less than or equal to 3 degrees, it is necessary to moisten the muslin with water at a higher temperature. This will completely melt any ice buildup on the thermometer bulb or on the muslin itself.

Cautions that must be taken to do the readings well

Readings

If we want to take data as reliable as possible, we must take into account some aspects:

  • When we read the thermometers, we must stand at a convenient distance of about 30 cm or more to avoid that the heat of our body influences the temperatures of the thermometer. In this way we will get a correct reading
  • At the same time, make sure that the line of sight is tangent to the meniscus of the liquid and perpendicular to the thermometers. This way we will be avoiding parallax errors.
  • If the thermometer reading is done at night, we have to keep the electric lamp on for the shortest time possible and not bring it near the device. Otherwise it will affect the taking of temperatures.
  • If a sling psychrometer is used, it is less to do so outdoors and in the shade near the sensory observation site.

Necessary maintenance

The meteorological shelter is the instrument whose presentation is the best indicator of the care that an observer has of his Station. Both need some care to protect them. These are the cares:

  1. Coat cleaning at least once a day remove dirt and dust that may settle.
  2. The paint must be in good condition. It is enough to paint it every six months. If the station is near the coast it is better to paint it every three months.
  3. After finishing the last observation of the day, change the water used to wet the muslin wet bulb thermometer. We will also wash the container that contains it.
  4. Once a week change the muslin.

With this information you can learn to use a psychrometer.


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  1.   Diego said

    Hello,

    Very good article, comparing with the rest that I have found, this one is very well organized. I have a question about a job that I have to do. I need to measure the wet bulb temperature of a boiler whose maximum temperature is in the range of 100-120ºC. For this I have been looking for a psychrometer that adapts to that range of temperatures among different providers, but I cannot find it. Do you know any? On the other hand, to manufacture the device myself, I have been looking for different fabrics that can withstand humid conditions at high temperatures, would it be enough if the water to wet the fabric was cool?

    Thank you all.