Humidity

What is a humidity sensor?

A humidity sensor (or hygrometer) detects, measures and reports both humidity and air temperature. The ratio of humidity in the air to the greatest amount of humidity at a particular air temperature is called relative humidity. Relative humidity becomes an important factor when looking for comfort.

Humidity sensors work by detecting changes that modify electrical currents or temperature in the air.

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What are the types of humidity sensors?

Capacitive humidity sensor

Principle of operation

The capacitive humidity sensor is a small capacitor made of hygroscopic dielectric material placed between a pair of electrodes. Most capacitive sensors use plastic or polymer as the dielectric material, with a typical dielectric constant ranging from 2 to 15. This constant and the geometry of the sensor determine the value of the capacitance.

At normal room temperature, the dielectric constant of water vapor has a value of about 80, a value much greater than the constant of the dielectric material of the sensor. Therefore, the absorption of moisture by the sensor results in an increase in the capacity of the sensor. Under equilibrium conditions, the amount of moisture present in a material depends on both the ambient temperature and the ambient water vapor pressure. This also applies to the hygroscopic dielectric material used in the sensor.

By definition, relative humidity is a function of both ambient temperature and water vapor pressure. There is a direct relationship between relative humidity, the amount of humidity present in the sensor and the capacity of the sensor. This relationship is the basis of the operation of a capacitive humidity instrument.

We know that relative humidity is the ratio of the actual water vapor pressure to the maximum water vapor pressure (saturated vapor pressure) possible at a given temperature. The dielectric material varies at a rate related to the variation of the relative humidity.

Measuring chain and performance

In a hygrometer using a capacitive sensor, humidity is measured by a chain process instead of being detected directly. The chain is made up of the following components:

1. Capacitive sensor

2. Probe

3. Cable

4. Electronic

5. Output signal


The performance of the instrument is determined by all the elements of the chain and not by the sensor alone. The sensor and the associated electronics cannot be considered separately. Any factor that could disturb the chain measurement process is likely to have an effect on the performance of the instrument.

Errors and uncertainties

Classification of errors affecting the final uncertainty of a hygrometer with a capacitive sensor. Measurement errors can be divided into two main categories:

Systematic errors are predictable and reproducible. Errors resulting from non-linearity of the instrument or temperature effects fall into this category. Systematic errors are specific to the instrument.

Random errors are not entirely predictable as they depend mainly on factors external to the instrument. Errors resulting from sensor hysteresis, as well as those resulting from calibration, are random errors. Usually, random errors are estimated on the basis of statistical data or on the basis of experience and judgment.

Because they are predictable, systematic errors can potentially be eliminated. However, random errors cannot be entirely eliminated.

Linearity errors. The typical response of a capacitive relative humidity sensor (between 0 and 100% RH) is not linear. Depending on the correction made by the electronic circuits, the instrument may have a linearity error. Assuming that the sensor and the associated electronics have reproducible characteristics, the linearity error is a systematic error.

Typically, the measurement points recommended by the manufacturer of the instrument for calibration are determined to minimize the linearity error. Calibration at these points should produce a more and less equal distribution of the linearity error.

Temperature errors. Temperature can have a major effect on several elements of the chain measurement process described above. The hygroscopic properties of the sensor vary with temperature. A relative humidity instrument works properly based on the assumption that the relationship between the amount of humidity present in the sensor dielectric and the relative humidity is constant. However, in most hygroscopic materials, this relationship varies with temperature.

Dielectric properties

The dielectric properties of the water molecule are affected by temperature. At 20 ° C, the dielectric constant of water has a value of about 80. This constant increases by more than 8% at 0 ° C and decreases by 30% at 100 ° C. Similar effects can be noted with regard to concerns other physical properties of water such as electrical conductivity.


The dielectric properties of the sensor also vary with temperature. The dielectric constant of most dielectric materials decreases as the temperature increases. The effect of temperature on the dielectric properties of most plastics and polymers is generally more limited.

Thermal humidity sensor

Two thermal sensors conduct electricity according to the humidity of the ambient air. One sensor is enclosed in dry nitrogen while the other measures the ambient air. The difference between the two measures humidity.

Resistive humidity sensor

Principle of operation

Resistive humidity sensors measure the variation of the electrical impedance of a hygroscopic medium such as a conductive polymer, salt or a treated substrate.

Resistive sensors are based on an interdigitated or two-wire winding. After depositing a hydroscopic polymer coating, their resistance changes inversely with humidity. The change in impedance is usually an inverse exponential relationship to humidity.

Resistive sensors generally consist of noble metal electrodes deposited on a substrate by photoresist techniques or electrodes wound on a plastic or glass cylinder. The substrate is covered with a salt or a conductive polymer. Alternatively, the substrate can be treated with activating chemicals such as acid.

The sensor absorbs water vapor and the ionic functional groups are dissociated, which increases the electrical conductivity. The response time of most resistive sensors varies from 10 to 30 s to reach 63% of the actual value. The impedance range of typical resistive elements ranges from 1 ohms to 000 ohms.

Most resistive sensors use a symmetrical AC excitation voltage without DC bias to prevent sensor bias. This response can be linearized by analog or digital methods. Typical variable resistance ranges from a few kilohms to 100 Mohms. The nominal excitation frequency is 30 Hz to 10 kHz.

Sensor calibration and accuracy 

The "resistive" sensor is not purely resistive in that the capacitive effects make the response an impedance measurement. A distinct advantage of resistive RH sensors is their interchangeability, generally within plus or minus 2% RH, which allows electronic signal conditioning circuits to be calibrated by a resistance at a fixed RH point. This eliminates the need for humidity calibration standards, so resistive humidity sensors are generally field replaceable.

The accuracy of individual resistive humidity sensors can be confirmed by testing in an RH calibration chamber or by a computerized DA system referenced in a standardized humidity controlled environment. The nominal operating temperature of the resistive sensors varies from -40 degrees C to 100 degrees C.

Sensor life

In residential and commercial environments, the life expectancy of these sensors is> 5 years, but exposure to chemical vapors and other contaminants such as oil mist can cause premature failure. Another disadvantage of some resistive sensors is their tendency to shift values ​​when exposed to condensation if a water soluble coating is used.

Resistive humidity sensors have significant temperature dependencies when installed in an environment with large temperature fluctuations. Simultaneous temperature compensation is integrated for more precision. The small size, low cost, interchangeability and long-term stability make these resistive sensors suitable for use in control and display products for industrial, commercial and residential applications.

Verification of operation over time

Precision

Each sensor must have its own calibration curve, based on a 9-point system.

Repeatability

The measurements of a sensor must be made so that they do not deviate. Repeatability is the successive measure of drift among measures of a single quantity.

Linearity

It indicates the voltage deviation from the BFSL value and the measured output voltage value, converted into relative humidity.

Reliability

Measurements often cause a desynchronization of the sensor. However, for a sensor to be useful, it must provide reliable measurements.

Response time

Generally, the time it takes for a sensor to reach 66% (rise time) or 33% (fall time) of the maximum output voltage is called the response time.

Application of sensors

The applications of humidity sensors are very varied. People with diseases affected by humidity, surveillance and preventive measures in homes use humidity sensors. A humidity sensor is also part of the heating, ventilation and air conditioning systems (HVAC systems). These are also used in offices, cars, humidifiers, museums, industrial spaces and greenhouses and are also used in weather stations to report and forecast the weather.

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