Electrolysis Method
The Phosphorus Pentoxide Sensor utilizes the principle of electrolysis of water molecules to produce hydrogen and oxygen. This sensor consists of a glass cylinder and two parallel electrodes, with the material of the electrodes chosen based on the specific application (typically made of platinum or rhodium wires). A very thin layer of phosphoric acid, H3PO4, is coated between the two electrodes. The electrolytic current that appears between the electrodes causes the water in the acid to decompose into H2 and O2. The final product of this process is Phosphorus Pentoxide, P2O5, which is a highly hygroscopic substance, thus absorbing moisture from the sample gas. Through a continuous electrolysis process, the moisture content of the sample gas should be balanced with the moisture after electrolysis. The electrode current is proportional to the moisture content in the sample gas. The signal is processed by the internal signal amplifier of the instrument and then displayed and data read out. This principle is used to measure all gases, including Cl2, HCl, H2S, H2SO4, HBr, SO2, SF6, CO2, and all inert gases, with the exception of a few gases that may react chemically with phosphoric acid.
The P2O5 probe is suitable for measuring various inert gases, hydrocarbons, or corrosive gases such as HCl, Cl2, or SO2, depending on the probe material chosen. Materials that can come into contact with the sample gas include glass, platinum, or rhodium, with other materials also available.
Sample gas flows through the probe in a unique manner, combined with a high-quality interface, ensuring rapid response and minimal interference. These designs are crucial for low ppm level measurements. The flow rate of the sample gas through the probe is typically set at 20Nl/h (with an optional 100Nl/h). The electrical connector to the analyzer is waterproof sealed. Users can easily regenerate the probe within five minutes. The probe can be easily installed anywhere with three M4 screws.
Advantages: High sensitivity for testing, suitable for ultra-micro water/trail water detection, and capable of measuring corrosive gases.
Flaws: Sensors require regular recoating, exhibit significant drift, and are susceptible to background gases like H2 and O2. Long balancing time, slow response.
2. Capacitive Method (Capacitance Method)
Utilizing a high-purity aluminum rod, its surface is oxidized to form an ultra-thin aluminum oxide film, which is then coated with a porous network of gold. An electric capacitor is formed between the gold film and the aluminum rod. Due to the hydrophilic nature of the aluminum oxide film, the capacitance value changes with the moisture content of the sample gas. By measuring this capacitance value, the humidity of the sample gas can be determined. The main advantage of this method is that the measurement range can be even lower, down to -100°C. Another significant advantage is the rapid response speed, with a 90% change from dry to wet in just one minute, making it suitable for on-site and rapid measurement applications. However, the downside is the relatively poor accuracy, with an uncertainty of ±2 to 3%. Nevertheless, with continuous efforts from various manufacturers, this method is gradually being refined. For instance, by altering materials and improving processes, the stability of the sensors has been significantly enhanced. Compensation of the sensor's response curve has achieved saturation linearity, solving the automatic calibration issue.
Advantages: Fast response time.
Flaw: Poor accuracy.
3. Cold Mirror Method
The method involves passing the sample gas through a condensation mirror in a dew point cold mirror room, achieving a saturated condensation state (with liquid droplets forming on the condensation mirror) through isobaric refrigeration. Measuring the temperature of the condensation mirror at this moment is equivalent to measuring the dew point of the sample gas. The main advantage of this method is its high precision, especially after incorporating semiconductor refrigeration and photoelectric detection technology, where the uncertainty can even reach 0.1℃. However, the drawback is its relatively slow response speed, particularly below -60℃ dew point, where the equilibration time can last several hours. Additionally, this method requires high cleanliness and corrosiveness of the sample gas, as otherwise, it may affect the photoelectric detection effectiveness or cause 'false condensation' leading to measurement errors.
Advantages: High precision.
Shortcomings: Slow response time.
4. Fiber Optic Method
This technology, developed at the end of the 20th century, elevates the micro-water analysis technique to a new level. The surface of the fiber optic humidity sensor is a layered structure composed of silicon dioxide and zirconium oxide with varying reflectivity coefficients. Advanced thermal curing technology ensures the sensor's surface pore size is controlled at 0.3nm, allowing 0.28nm water molecules to penetrate. The controller emits a beam of near-infrared light at 790-820nm, transmitted through a fiber optic cable to the sensor. The entering water molecules alter the light's reflectivity coefficient, causing a wavelength shift proportional to the medium's moisture content. By measuring the received light's wavelength, the dew point and moisture content of the medium can be determined.
Advantages: High precision, maintenance-free, extremely stable, capable of measuring corrosive media containing H2S, HCL, and more.
Flaws: Transmission optical fibers are prone to breakage and require protection.
