Humidity Academy Theory 6 – The Capacitive Sensor

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The capacitive humidity sensor consists of a hygroscopic dielectric material placed between a pair of electrodes that forms a small capacitor. Most capacitive sensors use a plastic or polymer as the dielectric material, with a typical dielectric constant ranging from 2 to 15. When no moisture is present in the sensor, both this constant and the sensor geometry determine the value of capacitance. At normal room temperature, the dielectric constant of water vapor has a value of about 80, a value much larger than the constant of the sensor dielectric material. Therefore, absorption of water vapor by the sensor results in an increase in sensor capacitance. At equilibrium conditions, the amount of moisture present in a hygroscopic material depends on both the ambient temperature and the ambient water vapor pressure. This is true for the hygroscopic dielectric material used on the sensor.

By definition, relative humidity is also a function of both the ambient temperature and water vapor pressure. So there is a relationship between relative humidity, the amount of moisture present in the sensor and sensor capacitance. This relationship is the basis of the operation of a capacitive humidity instrument.

In a capacitive instrument, as in practically every other type of instrument, humidity is measured by a chain process and not measured directly. Instrument performance is determined by all of the elements of the chain, not by the sensor alone. Since the sensor and associated electronics cannot be considered separately, any factor that can disturb the chain process of measurement is bound to have an effect on the instrument performance.

Application Considerations - Capacitive Humidity Sensors

Newer humidity measurement techniques such as the HYGROMER IN-1 capacitive humidity sensor have greater accuracy than that of the wet and dry bulb technique and also offer superior control characteristics over a wide range of temperatures and humidity.

Choosing sensor technology that is compatible with your specific application is critical to achieving reliable, repeatable and accurate measurement.

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Capacitive Humidity Sensors Pros & Cons:

Pros:

  • • Wide measurement range
  • • Wide temperature range
  • • Excellent stability
  • • Fast response
  • • Full recovery from condensation
  • • Highly resistant to chemicals
  • • Small
  • • Low cost
  • • Requires very little maintenance

Cons:

  • •Can be limited by distance from sensor to the electronics
  • •Loss of relative accuracy below 5% RH
  • •Requires electronics to convert capacitance to relative humidity

Classification of Errors in Capacitive Humidity Sensors

Sytematic errors are predictable and repeatable, both in magnitude and sign. Errors resulting from a nonlinearity of the instrument or from temperature effects fall into this profile. Systematic errors are instrument specific.

Random errors are dependent on factors external to the instrument, which means that while systematic errors are predictable and repeatable, random errors are not. For example, errors resulting from sensor hysteresis, which we’ll define below, as well as those resulting from the calibration procedure, are random errors. Usually, random errors are estimated on the basis of statistical data, experience and judgment.

Linearity Errors

The typical response of a relative humidity sensor (between 0 and 100% RH) is non-linear. Depending on the effectiveness of the correction made by the electronic circuits, the instrument may have a linearity error. Assuming that both the sensor and associated electronics have reproducible characteristics, the linearity error is a systematic error.

Attention: Careless selection of the calibration values can result in a different distribution of the linearity error and can be detrimental to instrument accuracy!

Generally, the values recommended by the instrument manufacturer for calibration were determined with the goal of minimizing the linearity error. Calibrating at those values should produce an even plus and minus distribution of the linearity error.

Temperature Errors

Temperature can have a major effect on several elements of the chain process of measurement described earlier. In the specific case of a capacitive humidity instrument, the following effects can produce a temperature error. Sensor hygroscopic properties vary with temperature. A relative humidity instrument relies on the assumption that the relationship between the amount of moisture present in the sensor hygroscopic material and relative humidity is constant. However, in most hygroscopic materials, this relationship varies with temperature. In addition, the dielectric properties of the water molecule are affected by temperature. At 20 °C, the dielectric of water has a value of about 80. This constant increases by more that 8 % at 0 °C and decreases by 30 % at 100 °C. Sensor dielectrics properties also vary with temperature.

The dielectric constant of most dielectric materials decreases as temperature increases. Fortunately, the effect of temperature on the dielectric properties of most plastics is usually more limited than in the case of water.

Any length of cable connecting the sensor to the electronic circuits has its own capacitance and resistance. The electronic circuits cannot discriminate between the sensor and its connecting cable. Therefore, since the capacitance of the sensor and the cable can vary with temperature, the humidity values reported by the electronics must be compensated for the effects of temperature. Failure to do so can result in large measurement errors, sometimes up to 8 %rh or more.

Hysteresis

Hysteresis is the maximum difference that can be measured between corresponding pairs of data, obtained by running an ascending and a descending sequence of humidity conditions. Hysteresis determines the repeatability of a humidity instrument.

For any given instrument, the value of hysteresis depends on several things:

  • • the total span of the humidity cycle used to measure hysteresis
  • • exposure time of the sensor to each humidity condition
  • •temperature during the measurements
  • • criteria used to determine sensor equilibrium
  • • and previous sensor history
Usually, sensor hysteresis increases as the sensor is exposed to high humidity and high temperature over longer periods of time.

Attention: Temperature can change the capacitance of the sensor and the cable. Humidity values reported by the electronics must compensate for the impact of temperature on the sensor.

It’s only meaningful to state a sensor’s hysteresis values while also providing details on how the tests were performed. In actual measurement practice, conditions are extremely diverse and hysteresis may or may not reach its maximum value. Therefore, it is reasonable to consider hysteresis a random value that can be neither fully predicted nor compensated. When the accuracy of an instrument is specified, half the maximum value of hysteresis should be equally distributed as a positive and a negative error. However, instrument repeatability should not be specified at less than the full value of hysteresis.

Calibration Errors

Calibration consists of comparing the output of a measurement instrument against a reference and reporting the results. Adjustment consists of changing the output of an instrument being calibrated to match the output of the reference. In some cases, the service named “calibration” includes both calibration and adjustment.

The reference instruments used to provide known humidity and temperature values for calibration have their own accuracy, repeatability, and hysteresis values, which must be taken into consideration when specifying final instrument uncertainty. In addition, no adjustment made during a calibration service can perfectly replicate the value seen by the reference instruments. These errors must be considered and treated as random errors in the calculation of instrument uncertainty.

Long-Term Stability

One crucial factor is the instrument’s ability to return the same values for RH for a given humidity condition over a long period of time. This value, usually termed repeatability, measures an instrument’s ability to maintain its calibration in spite of shifting characteristics of the sensor and its associated electronics over long periods of time. Generally, one can split the problem of repeatability into two areas: the ability of the sensor to maintain its response to a given humidity condition at a given temperature and the stability of the electronics over time.

Attention:
• Long-term stability plays a critical role in the frequency of calibration required for a humidity instrument.
• The stability of the instrument significantly affects the value of the measurement data received from the instrument

Chemical Resistance

Capacitive polymer humidity sensors are sensitive to the presence of chemicals in the surrounding gas. The amount of the influence depends on a number of parameters:

  • • type of chemical
  • • concentration
  • • length of the influence
  • • amount of humidity and temperature
  • • and presence of other chemicals

Because it’s difficult to make predictions about the deviation and the lifetime of the sensor, it’s best to test. between calibration cycles.

Uncritical Chemicals

The following tables refer to the impact of these gases on the Rotronic IN-1 family of sensors:

  • • Argon (Ar)
  • • Carbon dioxide (CO2)
  • • Helium (He)
  • • Hydrogen (H2)
  • • Neon (Ne)
  • • Nitrogen (N2)
  • • Nitrous oxide (laughing gas, N2O)
  • • Oxygen (O2)

The following gases have no or little influence on the sensor and the humidity measurement:

  • • Butane (C4H10)
  • • Ethane (C2H6)
  • • Methane (CH4)
  • • Natural gas
  • • Propane (C3H8)

Critical Chemicals

In the following concentrations, the gases listed in the following table have no or little influence on the sensor or humidity measurement. The shown data are only guide values. The resistance of the sensor depends strongly on the temperature and humidity conditions and the length of the pollutant influence.

Allowed fault caused from the pollutant: +/- 2 %rh

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Application Examples

A) Humidity Measurement in Sterilisation Chamber (Ethylene oxide)

Customer application: Sterilisation of medical equipment Sensor: C-94
Concentration Ethylene oxide: 15% by volume
Carbon dioxide: 85% by volume
Pressure: 0.2 to 2.5 bar absolute
Temperature: app. 40 °C
Humidity: app. 80 %rh
Application experience: The sensors have a lifetime of approximately 3 months. The chamber is in continuous operation.

B) Humidity Measurement in Ozone Chamber

Sensor: HYGROMER HT-1
Concentration ozone: app. 500 ppm
Temperature: app. 23 °C
Humidity: app. 50 %rh
Application experience: The sensors have a lifetime of approximately 1 month at 500 ppm ozone.

C) Special Application: Humidity Measurement in Oil

Humidity measurement direct in oil is possible in principle, but the lifetime of the sensors depends strongly on the used oil. Measurements in oil are only possible with a special sensor, and plan for tests.

Learn more about humidity in the following video: “Relative Humidity Measurement Explained”

See previous blog posts:
Theory 1 – What is Humidity?
Theory 2 – Relative Humidity, Pressure and Temperature
Theory 3 – Humidity and Vapor Pressure
Theory 4 – Definitions of Humidity: Vapor Concentration
Theory 5 – Effect of Temperature and Pressure on % rh
Watch out for Humidity Academy Theory part 7 on the PST Blog




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