Chapter 1
What is the dew point temperature?
The dew point temperature is the temperature at which the relative humidity reaches 100% when the temperature is lowered while keeping the amount water vapor in the air constant.
As the temperature drops further, some of water vapor that was present as a gas in the air changes into liquid water. This is called dew condensation.
The Japan Meteorological Agency defines the dew point temperature as the temperature at which, when water vapor pressure is kept constant and the temperature is lowered, the relative humidity reaches 100%, causing water vapor in the air to condense and form dew.
Let's consider a specific example.
For example, let's say the indoor air temperature is 25°C and the relative humidity is 60%.
The dew point temperature of this air is approximately 17°C according to general approximation calculations.
In other words, even if the air itself is 25°C, if the surface of metal or window glass that is in contact with that air cools down to about 17°C or below, dew condensation may form on the surface.
Air temperature 25℃ > Dew point temperature approximately 17℃
There is no dew condensation in the air yet.
Surface temperature is approximately 17°C or lower
The air near the cold surface may reach the dew point and dew condensation may occur.
Chapter 2
What is the difference between dew point temperature, humidity, and relative humidity?
While both "humidity" and "dew point temperature" are used to describe the state of moisture in the air, they represent different things.
| index | What does it represent? | unit | Main Applications |
|---|---|---|---|
| Relative humidity | How much water vapor is currently present relative to the amount water vapor that saturates the system at that temperature? | %rh | Indoor environment, air conditioning, general temperature/humidity control |
| Dew point temperature | At what temperature will the air become saturated and begin to dew condensation if the current amount of water vapor is maintained while cooling? | ℃ | dew condensation control, drying management, compressed air, process control |
Relative humidity changes with temperature.
When understanding relative humidity, it's important to remember that even with the same amount water vapor, relative humidity changes with temperature.
When air is cooled, the amount of water vapor that can saturate it at that temperature decreases, so even if the amount water vapor remains almost the same, the relative humidity increases.
The dew point temperature is the temperature at which the relative humidity reaches 100%.
Understanding the difference between relative humidity and dew point temperature becomes easier if you think of relative humidity as "how humid it is relative to the current temperature" and dew point temperature as "how cold it has to dew condensation before condensation occurs."
If you'd like to learn more about relative humidity and absolute humidity, please also see "What is Humidity?".
Chapter 3
What does it mean for the dew point temperature to be high or low?
A high dew point temperature generally means that there is a large amount water vapor in the air.
Conversely, a low dew point temperature indicates a low amount water vapor in the air.
The dew point temperature is high.
This is a condition where there is a lot of moisture in the air. Even a slight cooling of an object's surface can bring it close to the dew point temperature.
The dew point temperature is low.
This refers to a state where there is little moisture in the air. For dry air and compressed air, a low dew point temperature is used as a management indicator.
The U.S. National Weather Service also explains that a higher dew point temperature indicates a greater amount of moisture in the air.
Chapter 4
How do you calculate the dew point temperature?
The dew point temperature can be approximately determined from the air temperature and relative humidity.
While there are several calculation methods used in practical applications and in the meteorological field, one of the relatively easy-to-use methods is the Magnus-type approximation formula.
If the air temperature is T (°C) and the relative humidity is RH (%), the following approximation formula can be used as an example.
γ = ln(RH / 100) + 17.625 × T / (243.04 + T)
Dew point temperature Td = 243.04 × γ / (17.625 − γ)
The Magnus approximation is one method for approximating the relationship between saturated water vapor pressure and temperature. Improvements such as those by Alduchov and Eskridge are well known.
At 25°C and 60% RH
If we substitute an air temperature of 25°C and a relative humidity of 60%rh into the above approximation formula, the dew point temperature will be approximately 16.7°C.
Therefore, dew condensation may occur if the surface temperature of an object exposed to that air falls below approximately 16.7°C.
This formula is a general approximation. For high accuracy measurements, special temperature and pressure conditions, calibration, and quality assurance applications, please check specifications of the measuring instrument and computation method being used.
Chapter 5
Dew point temperature guidelines based on temperature and humidity
When air temperature and relative humidity change, the dew point temperature also changes. The following are approximate values based on the Magnus-type approximation formula.
| air temperature | 40% rh | 50% rh | 60% rh | 70% rh | 80% rh |
|---|---|---|---|---|---|
| 10℃ | Approximately -3℃ | Approximately 0℃ | Approximately 3℃ | Approximately 5℃ | Approximately 7℃ |
| 15℃ | Approximately 2℃ | Approximately 5℃ | Approximately 7℃ | Approximately 10℃ | Approximately 12℃ |
| 20℃ | Approximately 6℃ | Approximately 9℃ | Approximately 12℃ | Approximately 14℃ | Approximately 17℃ |
| 25℃ | Approximately 11℃ | Approximately 14℃ | Approximately 17℃ | Approximately 19℃ | Approximately 21℃ |
| 30℃ | Approximately 15℃ | Approximately 18℃ | Approximately 21℃ | Approximately 24℃ | Approximately 26℃ |
*The values are approximate values calculated using a general Magnus-type approximation formula. They are not guaranteed values for measurement or quality control purposes.
For example, at 25°C and 50% rh, the dew point temperature is approximately 14°C, but at the same 25°C but with 80% rh, it is approximately 21°C. This shows that the higher the humidity, the closer the conditions for dew condensation approach, even if the surface of an object is not particularly cold.
Chapter 6
What is the relationship between dew point temperature and dew condensation?
Understanding dew condensation requires paying attention not only to the air temperature but also to the "surface temperature of the object."
For example, even if the room temperature is 25°C, the surface temperature of a glass containing a cold drink will be significantly lower than 25°C.
When the air near the surface of a glass cools and the surface temperature falls below the dew point, water vapor in the air condenses into water droplets and appears on the surface.
| Surface temperature and dew point temperature | Concept of state |
|---|---|
| Surface temperature > Dew point temperature | Condition that is less prone to dew condensation |
| Surface temperature ≒ Dew point temperature | Conditions for dew condensation are approaching. |
| Surface temperature ≦ Dew point temperature | dew condensation is highly likely to occur. |
Therefore, in dew condensation prevention, in addition to "keeping humidity below ○%", there is also a method to check whether the surface temperature is kept above the dew point temperature.
Chapter 7
dew condensation risk should be considered as "surface temperature - dew point temperature".
When proactively monitoring dew condensation in factories and other similar environments, comparing the dew point temperature to the surface temperature of objects you want to prevent dew condensation from, rather than just looking at the dew point temperature alone, makes it easier to assess the situation.
Temperature difference until dew condensation = Surface temperature - Dew point temperature
If this difference is large, there is some margin between the current conditions and the dew point.
Conversely, the smaller the difference, the higher the likelihood that dew condensation conditions will be reached due to a decrease in surface temperature or an increase in humidity.
Example: What if a metal product is at 18°C and its dew point is 17°C?
When the temperature of a metal product is 18°C and the dew point temperature is 17°C, the temperature difference is only 1°C.
If the metal object cools further in this state, or if the amount moisture in the air increases and the dew point temperature rises, dew condensation conditions may be reached.
There is no single standard for what temperature difference is considered safe. Management standards are set for each site, taking into account the target product, material, surface condition, the rate of environmental change, and the time it takes to respond to abnormalities.
For actual dew condensation monitoring of metal products, see the section "dew condensation Countermeasures for Metal Products" for specific system configuration examples.
Chapter 8
Where is dew point temperature used?
Dew point temperature is used not only for weather forecasting but also in various industrial fields such as manufacturing, quality control, and equipment management.
dew condensation prevention measures for metal products
By comparing the metal surface temperature with the dew point temperature, we monitor the risk dew condensation that could lead to rust and corrosion.
Air conditioning and cleanrooms
temperature/humidity, and dew point temperature are controlled to prevent dew condensation from occurring on cooling surfaces and piping.
Drying process
This system allows us to understand the state of moisture remaining in the air and use that information to manage drying conditions and process requirements.
Compressed air
The dew point is used as an indicator to check the dryness of compressed air, preventing moisture from condensing inside pipes and other structures.
Pharmaceuticals and Food Products
It may be used to manage humidity and dew condensation risks in manufacturing and storage environments.
Constant temperature and humidity environmental testing
It is used to understand moisture content of the test environment and to manage the necessary temperature/humidity conditions.
Chapter 9
How do you measure or determine the dew point temperature?
The method for determining the dew point temperature varies depending on the application and the required accuracy.
computation from temperature and relative humidity.
We measure the temperature and relative humidity, and then calculate the dew point temperature from these values.
It can be used for general environmental monitoring and dew condensation monitoring.
Use a temperature/humidity that supports dew point temperature output.
Some devices computation and display /output the dew point temperature from temperature and humidity measurements.
Use a dew point meter
Depending on the application, such as low dew point regions or compressed air, a specialized method suitable for dew point measurement is used.
The system will continuously monitor the system.
One method involves continuously collecting temperature/humidity, and object temperature data, and then computation the dew point temperature and temperature difference to monitor the risk of dew condensation.
Some Chino temperature/humidity can output not only relative humidity but also temperature and dew point temperature. The appropriate model should be selected based on the application, temperature range, pressure conditions, etc.
Chapter 10
Points to note when managing dew point temperature
| Check items | Why is it important? |
|---|---|
| Measurement location | Even within the same room, temperature/humidity can vary due to factors such as air conditioning, outside air, and equipment. |
| surface temperature | When it comes to preventing dew condensation, not only the air temperature but also the surface temperature of the object is important. |
| Humidity sensor accuracy | When computation the dew point temperature from temperature and humidity, errors in the original measurement values will affect computation result. |
| response speed | In environments with rapid changes temperature/humidity, the sensor's response will affect management. |
| Calibration | If you plan to use the measurement data for long-term management, you will need to periodically check the sensor's status. |
| Operating environment | We will check if the equipment can withstand high temperatures, high humidity, dew condensation, dust, and pressure. |
In particular, when monitoring dew condensation, prolonged dew condensation of the humidity sensor itself to condensation can affect its measurement performance. Please check specifications and working conditions of the sensor you are using.
FAQ
Frequently Asked Questions about Dew Point Temperature
What is dew point temperature in simple terms?
This is the temperature at which water vapor begins to turn into water droplets when the air is cooled. It can also be described as the temperature at which the relative humidity reaches 100%.
Are dew point temperature and humidity the same?
They are not the same. Relative humidity is expressed as a percentage of water vapor relative to the current temperature, while dew point temperature is expressed in degrees Celsius as the temperature at which dew condensation begins with the current amount water vapor.
What happens if the dew point temperature is high?
Generally, this refers to a state where there is a high amount water vapor in the air. If the surface temperature of the object is close to the dew point temperature, attention should also be paid to the risk dew condensation.
Does dew condensation when the humidity reaches 100%?
100% relative humidity means the air is saturated at that temperature. If it is further cooled or comes into contact with a cold surface, water vapor will condense to form water droplets.
At what humidity level does dew condensation?
Dew point temperature cannot be determined solely by humidity. It's necessary to calculate the dew point temperature from the air temperature and relative humidity and compare it to the surface temperature of the object you don't want to dew condensation.
What is the dew point temperature at 25°C and 50% RH?
The general Magnus approximation formula gives a temperature of approximately 14°C. Therefore, dew condensation may occur if the surface temperature of an object exposed to that air falls below approximately 14°C.
What is the dew point temperature at 25°C and 60% RH?
The general Magnus-type approximation gives approximately 16.7°C. It might be easier to understand if you think of it as roughly 17°C.
How can I prevent dew condensation?
Basically, methods include keeping the surface temperature of the object above the dew point, lowering the dew point through dehumidification, and suppressing rapid temperature changes. Actual countermeasures will vary depending on the object and equipment conditions.
Can the dew point temperature be determined using temperature/humidity?
The dew point temperature can be computation from temperature and relative humidity. Some temperature/humidity can computation, display, and output the dew point temperature.
Summary
Knowing the dew point temperature helps predict "when dew condensation will occur."
The dew point temperature is the temperature at which, when air is cooled, the relative humidity reaches 100%, and water vapor begins to condense.
Relative humidity alone does not directly tell us "at what temperature dew condensation will occur on an object." By determining the dew point temperature and comparing it to the surface temperature of the object, we can more specifically assess the risk of dew condensation.
In factories in particular, dew point temperature is an important control indicator for metal products, piping, air conditioning equipment, clean rooms, and drying process.
Relative humidity = How humid it is relative to the current temperature.
Dew point temperature = At what temperature does dew condensation begin?
One way to estimate the margin before dew condensation occurs is by subtracting the dew point temperature from the surface temperature.
Reference
For those who want to know more about dew point temperature
For information on the definition of dew point temperature and its treatment in the meteorological field, refer to information from public and specialized organizations.
- Japan Meteorological Agency: "Temperature, Humidity"
- NOAA/National Weather Service “Dew Point”
- NOAA/National Weather Service “Dewpoint and Wet-bulb from Relative Humidity”
*You will be redirected to an external website.
I would like to consult about measuring dew point and dew condensation.
The appropriate measurement method varies depending on the object you want to prevent dew condensation from forming on, the operating temperature and humidity range, the measurement location, and the need for continuous monitoring.
Please contact us if you have any problems with temperature/humidity, dew point temperature measurement, or dew condensation monitoring systems.
I would like to consult about temperature/humidity, and dew point measurement.
