Electrical determination of composition and content

11.1 Electrical measurement of moisture and humidity

11.1.1 Definition and expression of moisture and humidity
1. Gas humidity
1. Absolute humidity
Under certain humidity and pressure conditions, the amount of water vapor contained in each unit volume of mixed gas, its unit is.
2. Relative humidity
The percentage of the ratio of the amount of water vapor contained in a unit volume of mixed gas to the amount of saturated water vapor at the same temperature, generally
expressed by the symbol %RH.
3. Dew (frost) point temperature
When the temperature of the air drops to a certain temperature, the water vapor in the air condenses into dewdrops (or condenses into frost), and this
specific temperature is called the dew point temperature (or frost point temperature) of the air. The dew point of the air has been measured as Ta, and the temperature of the air to be measured is Tw. By looking up the table, the saturated water vapor pressure of water at temperatures Ta and Tw is obtained, and the ratio of the two is the relative humidity of the air to be measured.
2. Solid humidity
The humidity of a solid is also called water content (or moisture), which is usually expressed as a percentage of the mass (or weight) of water contained in the substance to the total mass (or total weight).

11.1.2 Solid moisture electrical measurement method
1. Infrared type
Two wavelengths of infrared radiation, one easily absorbed by water and the other not absorbed by water, are used to alternately pass through the solid being measured, and the ratio of the radiation intensity passing through the solid being measured is taken to determine the moisture content of the solid being measured.
2. Resistive type

Figure 11-1-1 Using
the property that the resistance value of solid materials varies with the water content, its humidity can be measured.

Figure 11-1-2
shows the relationship between the dielectric constant and moisture content of a material. The moisture content of the material can be determined by measuring the capacitance of a capacitor with the material as the dielectric.

11.1.3 Electrical measurement of gas humidity
1. Temperature measurement method - dry-bulb and wet-bulb hygrometer

Figure 11-1-3

Principle: Determine the relative humidity of the air based on the difference between the measured dry bulb temperature T1 and the wet bulb temperature T2.
1. Traditional method - use a mercury thermometer to measure the dry and wet bulb temperatures, and check the corresponding table to determine the humidity of the gas.
2. Use two thermocouples or two thermal resistors to measure the dry and wet bulb temperature difference Figure 10-2-10 Figure 10-2-11
3. "Electronic dry and wet" humidity sensor

Figure 11-1-4[page]

3. Capacitive type
The capacitance value of polymer humidity sensitive capacitor is linearly related to the relative humidity in the gas.
4. Quartz vibration type
A polymer film is coated on the surface of the quartz chip. When the film absorbs moisture, the oscillation frequency of the quartz chip changes. Different frequencies represent different degrees of humidity.
5. Porous Al2O3 humidity
sensorAl2O3

Figure 11-2-1
When the density of the liquid increases, the buoyancy increases and the float rises. The density of the liquid can be measured by measuring the position of the float.
(III) Differential pressure method
Two pressure holes with a vertical distance of a constant value H are set on the container wall below the liquid surface to be measured. The differential pressure of the two pressure holes is measured by a differential pressure sensor to measure the density of the liquid.

11.2.2 Concentration electrical measurement method
If the mass of solute B in a solution with a total volume of V is m(B), then the mass concentration of solute B is expressed as:

1. Conductivity type
By measuring the conductivity of the solution in the low concentration area and the high concentration area (the concentration in the middle cannot be measured), the corresponding solution concentration C can be obtained.
1. Conductivity cell

Circuit diagram for measuring solution conductivity 11-2-2
2. Electromagnetic induction

Figure 11-2-3
uses the solution to be tested to form a short-circuit coil, and couples two transformers T1 and T2. T1 is an excitation transformer. When an AC voltage is passed through its primary, the conductivity of the solution can be obtained by measuring i2, and the corresponding concentration can be obtained.
2. Photoelectric type
1. The concentration of the solution can be indirectly measured by measuring the refractive index of the solution with a photoelectric refractometer.
2. The optical rotation of the solution can be measured using a photoelectric automatic polarimeter to obtain the concentration of the solution.
3. Quartz crystal microbalance
When the gas sample to be tested contacts the surface coating of the piezoelectric vibrator, the higher the gas concentration, the more gas molecules are absorbed into the coating. Therefore, the change in the vibration frequency of the piezoelectric vibrator is proportional to the concentration of the gas to be tested.

11.3 Gas Analysis and Detection

11.3.1 Gas Analysis
1. Thermal Conductivity Gas Analyzer
1. Principle: Assume that two gases with thermal conductivity coefficients λ1 and λ2 are mixed. λ1 and λ2 are known. If the thermal conductivity coefficient λc of the mixed gas is measured, the percentage content α1 and α2 of the two gases can be calculated:

2. Thermal Conductivity Cell Figure 11-3-1(a)
- A component that converts the change in the thermal conductivity of the mixed gas into the change in the resistance value of the thermal resistor.
3. Thermal Conductivity Gas Analyzer

Figure 11-3-1 (b)
Analysis chambers Rk1 and Rk2 are reference chambers, which are filled with clean air, and
analysis chambers Rx1 and Rx2 are filled with the mixed gas to be analyzed.
The four analysis chambers form a bridge circuit. The bridge output is a function of the mixed gas components. [page]

2. Magnetic oxygen analyzer
1. Principle: It is based on the measurement of paramagnetism of oxygen.
Volume magnetic susceptibility of mixed gas
Where: k1 is the volume magnetic susceptibility of oxygen;
k2 is the average volume magnetic susceptibility of each component gas in the mixed gas except oxygen;
C is the oxygen content
Conclusion: According to the size of the volume magnetic susceptibility of the mixed gas, the oxygen content C can be determined.
2. Thermal magnetic convection detector

Figure 11-3-2
The higher the oxygen content of the gas being analyzed, the stronger the magnetic wind, the greater the temperature difference between the two hot wire elements, and the greater the corresponding resistance change. The output voltage of the bridge can be used to indicate the concentration of oxygen contained in the gas being analyzed.
3. Optical absorption gas analyzer
(I) Working principle
When a substance absorbs light radiation of a characteristic wavelength, the relationship between the transmitted light energy and the incident light energy is:

where W is the transmitted light energy, W0 is the incident light energy; a is the absorption rate; b is the optical path length; c is the concentration of the absorbing substance in the sample. If a and b are known values, the concentration of the absorbing substance can be determined by measuring the ratio of the transmitted light to the incident light energy.
(II) CO2 infrared gas analyzer

Figure 11-3-3
During measurement, the gas to be measured passes through the sample chamber, and the reference chamber is filled with CO2-free atmosphere. After calibration, the CO2 content can be determined from the output signal.
(III) Photoelectric Colorimeter

The left half of Figure 11-3-4
is the reference medium optical path. The cuvette contains a certain liquid (or gas) that does not contain the measured component and has no absorption effect on the wavelength of the light beam.
The right half is the measurement optical path of the measured medium. The cuvette contains the measured sample and has a certain absorption effect on the wavelength of the light beam.
The outputs of the detection elements of the two optical systems are different, and the content of the measured medium is displayed after comparison and amplification. [page]

11.3.2 Practical gas detectors
1. Harmful gas detection and exhaust control circuit

Figure 11-3-5
When the HQ-1 gas sensor detects that the concentration of harmful gases exceeds the safe value, the circuit will sound an alarm and turn on the exhaust fan at the same time
.

Figure 11-3-6
If the driver is drunk, the resistance of the gas sensor drops sharply, the relay coil is energized, its normally open contact closes, the light-
emitting diode is turned on, and it emits red light as a warning. At the same time, the normally closed contact of the relay is disconnected, making it impossible for the driver to start the engine.
III. Practical gas alarm

Figure 11-3-7
When there is gas in the surrounding air, the resistance of the gas sensor decreases rapidly, the 4th pin of the 555 integrated circuit becomes a high level, the oscillator circuit starts to oscillate, and the speaker sounds an alarm to remind people to take appropriate measures to prevent accidents.