Relative Humidity Data: Why Is Temperature Necessary?

Dew drops on a surface

When you purchase a precision relative humidity sensor, often the temperature is also a data returned by the instrument. It is important to know that relative humidity changes as a function of temperature. Relative humidity is a ratio of water vapor pressure to saturation vapor pressure, both of which vary with temperature and atmospheric pressure. Let's forget about this last parameter for the moment and concentrate on the usefulness of knowing the temperature when we want to measure relative humidity.

Temperature and relative humidity data logger

Some hygrometers measure absolute humidity. However, the density of water molecules in a given volume does not account for the unique behavior of water, which can be in different phases at the same time. Indeed, when the air reaches a saturation point in water density in gaseous form, water condenses and becomes liquid. This is why the relative humidity is an important factor.

Relative humidity

relative humidity icon (%RH), with water drops

As we said in the introduction, relative humidity (φ) is a rate. It is

φ = 100% PH2O / PSAT(T)

We see here that when the water vapor pressure (PH2O) equals the saturation vapor pressure (PSAT), at a given temperature (T), the relative humidity is 100%. This means that from this point, water will begin to liquefy. This is the saturation point.

In the relative humidity equation, PSAT and PH2O are partial pressures, calculated in kilopascals (kPa) or millibar (mbar), for example. The saturation vapor pressure is a constant value at a given temperature. It is a proportion of the atmospheric pressure. For example, at 0 °C, PSAT is 0.6% of atmospheric pressure, while at 25 °C it is 3% of atmospheric pressure. This implies that between 0 °C and 25 °C, the air can contain 5 times more water vapor, before the saturation point is reached. Warning, this is not a linear rule.

Thus, at a given altitude, and therefore at a given atmospheric pressure, the way to influence the relative humidity is to change the temperature in order to alter PSAT, unless of course the water vapor pressure is adjusted directly.

So why worry about saturation vapor pressure and saturation point? Because your data logger is an electronic device that doesn't do well with condensation, just like many experimental environmental measurement conditions.

Dew point

The dew point is the temperature at which water vapor condenses. Knowing that condensation occurs when the water vapor pressure exceeds the saturation vapor pressure, at a given temperature, it is important to know this temperature. Traditionally, the dew point can be calculated with a psychrometric chart.

Now let's imagine that you want to measure the relative humidity of an enclosed area where there is a risk of condensation (very high humidity or very low temperature). By knowing the dew point, you know that the temperature of this space cannot go below this point, without condensation. So you need to keep an eye on the temperature of that area, while measuring its relative humidity. With an instrument that measures both temperature and relative humidity, you have all the data in hand.

To make your life easier, Dracal Technologies' temperature and humidity sensors even calculate the dew point for you. You can therefore know exactly the temperature not to be exceeded in the precise conditions of your experiment, in real time, preserving your experiments and at the same time your instrument. 

Electronics

For a detailed article on what to consider when taking a relative humidity measurement, see the following link.

Positioning

When we are interested in measuring instruments that go into the air, it is important that the sensor is not immersed. In order to avoid damage to the electronic components of your measuring instrument, it is necessary to understand how to position it in a humid environment with a high risk of condensation. The guideline is that water should not remain on the instrument. For example, the TRH320 type probes on Dracal instruments must be mounted vertically and downward so that water will drain off and not come in contact with the sensor.

Dracal temperature and humidity sensor probe_positioned downwards

In addition to this positioning precaution, it is relevant to keep an eye on the dew point, in order to avoid the formation of condensation on an instrument with sensitive electronics. Having said that, if a measuring instrument has been in contact with water and has dried out, it is possible that it will work again. But you will have lost valuable data during the time it was wet.

Calibration

relative humidity calibration icon

The calibration of an instrument measuring relative humidity is done in an environmental chamber, where temperature, air pressure and water vapor pressure can be controlled. Since temperature has an impact on relative humidity, it is critical to determine at what temperature the relative humidity calibration will be performed. For example, if the measurement uncertainty is ±2%RH between 0%RH and 90%RH at 25°C, it would be wise to calibrate the instrument at 25°C in order to properly assess whether the instrument meets the manufacturer's specifications. On the other hand, if the instrument will be used at a significantly different temperature, it may be advisable to evaluate its measurement uncertainty at the temperature of use.

Once the calibration temperature is determined, the laboratory will be able to do a relative humidity calibration. This time it is useful to know the relative humidity range where the instrument will be used. According to good practice, three calibration points should be determined: the first at the lower bound of relative humidity, the second at the upper bound and the third in the center of the range.

During calibration, the dew point must not be reached. Therefore, if a temperature and relative humidity sensor is to be calibrated for both parameters, the order of the temperature and humidity change is important. Always increase the temperature before increasing the humidity to prevent condensation.

Conclusion

A relative humidity and temperature data logger allows one to check if the environment will reach the dew point, that is the point of condensation of water vapor in the air. It is understood that for the electronics of the instrument, this data is very useful. In the case of storage or transport of humidity-sensitive goods, knowing the temperature together with the relative humidity level also helps to preserve the products or equipment. If the instrument also provides you with the precise dew point that should not be exceeded at any time, it is even easier to ensure that the environment will not ruin your experiments or equipment.

 

References

Frederick, J. E. (2008). Principles of Atmospheric Science. États-Unis: Jones and Bartlett Publishers.

Walker, R. and Cordner, A. (2022). Temperature and Relative Humidity Calibration System. [online] Fluke Calibration. Available at: https://us.flukecal.com/literature/articles-and-education/temperature-calibration/papers-articles/temperature-and-relative-h#:~:text=The%20calibration%20procedure%20specifies%20that,within%20the%20specified%20temperature%20range. [Accessed 15 Dec. 2022].