Choosing the right humidity sensor

There are two very different physical quantities that can be used to measure atmospheric humidity or the humidity of a gas: absolute humidity and relative humidity.

Absolute humidity

Absolute humidity is defined as the mass of water vapor contained in a certain volume of dry air or gas. Generally expressed in grams per unit volume, absolute humidity does not depend on the temperature of the air or gas.

Relative humidity

Relative humidity is expressed as a percentage and refers to the ratio between the mass of water vapor present in the air (absolute humidity) and the maximum mass of water vapor that the air can contain (above which condensation of water vapor occurs).
This maximum mass of water vapor (before condensation) varies according to the temperature. The hotter the air or gas, the more water vapor it can contain. This means that the relative humidity of the air or of a gas containing the same mass of water vapor varies according to its temperature.
The temperature at which a relative humidity of 100% is reached is called the dew point. Below this temperature, condensation occurs.

There are three main types of sensors:

Capacitive humidity sensors

They consist of a layer of moisture-sensitive dielectric material placed between two electrodes, thus forming a capacitor whose capacitance varies linearly with relative humidity.

They measure relative humidity from 0% to 100%.

They also provide high-precision atmospheric measurements. This type of sensor is used in industry and meteorology.

Resistive humidity sensors

Measure the variation of the electrical impedance of a hygroscopic element (which can be a conducting polymer or a salt) by means of ions. The resistance of this sensitive element varies according to humidity.

These sensors offer little precision for measuring relative humidity values of less than 5%. They are, however, more affordable. They are therefore a good option if high-precision measurements are not required.

Dew point sensors

They measure the dew point temperature, i.e. the temperature at which the water vapor in the air begins to condense, thus providing an indication of humidity.

They are more complex than the types of sensors mentioned above and are mainly used in laboratories.

Rotronic capacitive humidity sensor

Capacitive humidity sensors have as their sensitive element a hygroscopic dielectric material positioned between a pair of electrodes, thus functioning as a capacitor in contact with the atmosphere (air or gas) whose humidity you want to measure.

By absorbing moisture from the atmosphere, the dielectric material causes the capacitance value of the capacitor to vary according to the relative humidity of the atmosphere.

Advantages:

• They are reliable, precise and stable;
• They have a fast response time;
• They offer a wide relative humidity measurement range (from 0% to 100%);
• They can operate over a wide temperature range (from -70°C to +200°C);
• They are compact and easy to integrate;
• They are resistant to condensation, dust and chemicals.

Disadvantages:

• They are less accurate when measuring very low humidity levels (below 10%);
• They are more expensive than resistive sensors.

Rotronic resistive humidity sensor

Resistive humidity sensors measure changes in the electrical impedance of a hygroscopic material, such as a conductive polymer or salt. Resistive sensors are generally made up of noble metal electrodes deposited on a substrate or electrodes wrapped around a plastic or glass cylinder. The substrate is coated with a salt or conductive polymer whose electrical resistance varies according to the humidity of the atmosphere around the sensitive element.

These sensors are more economical and offer the advantage that they can be replaced in the field and used immediately, without having to be calibrated first.

Advantages:

• They are a little more economical than capacitive ones;
• They can be replaced in the field without the need for calibration;
• They are suitable for a medium relative humidity range (from 20% to 90%).

Disadvantages:

• They are not suitable for extreme operating conditions;
• They are sensitive to condensation and chemicals.

Ahlborn dew point sensor

Dew point sensors work on an optical measuring principle to determine the dew point temperature of the atmosphere. They are based on a mirror that reflects a beam of light towards a sensor. The surface of the mirror is cooled by a thermoelectric element (Peltier). When the dew point is reached, condensation forms on the mirror, which then reflects less light to the optoelectronic sensor. A temperature sensor then measures the temperature of the mirror (dew point temperature), which is used to calculate the humidity of the air.

Advantages:

• They have excellent long-term stability;
• They react quickly to changes in humidity;
• They are resistant to condensation.

Disadvantages:

• They need a clean environment to function properly;
• They do not show linear behavior at very low levels of relative humidity;
• They are expensive compared to other technologies.

Once you have decided which type of humidity sensor is best suited to your intended use, you should select a specific model based on the usual selection criteria for any other sensor:

• Measuring range
• Precision
• Resolution
• Output signal and type of electrical connector
• Type of mounting and process connection
• Operating temperature and degree of protection according to ambient operating conditions