The micro moisture tester is an instrument used to detect moisture, including electrolysis method, resistance capacitance method, cold mirror method and optical fiber method. The measuring cell is detachable and easy to maintain.
The micro moisture tester is mainly divided into four types
1. Electrolytic method
The phosphorus pentoxide sensor utilizes the principle of electrolyzing water molecules into hydrogen and oxygen. The sensor is composed of a glass cylinder and two parallel electrodes. The electrode material (usually made of platinum or rhodium wire) is selected according to the specific application, and a very thin layer of phosphoric acid H3PO4 is coated between the two electrodes. The electrolytic current between the two electrodes makes the water in the acid decompose into H2 and O2. The final product of this process is phosphorus pentoxide. P2O5 is a highly hygroscopic material, so it absorbs water from oxygen. Through a continuous electrolysis process, the water content of the sample gas should be balanced with the water after electrolysis. The electrode current is proportional to the moisture content in oxygen. The signal is processed by the internal signal amplifier of the instrument, then displayed and read out. This principle is used to measure all gases. Including Cl2, HCl, H2S, H2SO4, HBr, SO2, SF6, CO2 and other gases and all inert gases, except for a few gases that react with phosphoric acid.
P2O5 probe is applicable to measuring various inert gases, hydrocarbons or corrosive gases such as HCl, Cl2 or SO2 according to the selected probe material. The material of the probe in contact with oxygen can be glass, platinum or rhodium, and other materials can also be provided.
The sample gas flows through the probe in a special way and combines with a high-quality interface. These designs are important for very low ppm level measurements to ensure rapid probe response and little interference. The sample gas flow rate through the probe is usually set at 20Nl/h (100Nl/h optional). The electrical connector with the analyzer is of waterproof and sealed structure. The user can easily regenerate the probe in five minutes. The probe can be easily installed anywhere with 3 M4 screws.
Advantages: high test sensitivity, suitable for very small amount of water/trace water test, and can also measure corrosive gases.
Disadvantages: The sensor needs to be recoated regularly, with large drift, and is vulnerable to background gases such as H2 and O2. Long balancing time and slow response.
2. Resistance capacitance method
A high-purity aluminum rod is used to oxidize its surface into an ultra-thin aluminum oxide film, which is coated with a layer of empty mesh gold film. A capacitance is formed between the gold film and the aluminum rod. Due to the water absorption characteristics of the aluminum oxide film, the capacitance value changes with the amount of water in the sample gas. The oxygen humidity can be obtained by measuring the capacitance value. The main advantage of this method is that the measuring range can be lower, even up to - 100 ℃. Another outstanding advantage is that the response speed is very fast, from dry to wet, the response can reach 90% in one minute, so it is mostly used in the field and rapid measurement occasions; The disadvantage is that the precision is poor, and the uncertainty is mostly ± 2~3 ℃. However, with the continuous efforts of various manufacturers, this method is gradually improved. For example, the stability of the sensor is greatly improved by changing the materials and improving the process, and the saturation linearity is achieved by compensating the response curve of the sensor, which solves the problem of automatic calibration.
Advantages: fast response.
Disadvantages: poor accuracy.
3. Cold mirror method
Let the oxygen flow through the condensing mirror in the dew point cold mirror room, and make the sample gas reach the saturated dewing state (there are liquid drops on the condensing mirror) through the isobaric refrigeration. The temperature of the condensing mirror at this time is the dew point of oxygen. The main advantage of this method is its high accuracy, especially when semiconductor refrigeration and photoelectric detection technology are used, the uncertainty can even reach 0.1 ℃; The disadvantage is that the response speed is slow, especially when the dew point is below - 60 ℃, and the balance time even reaches several hours. In addition, this method also has high requirements for the cleanness and corrosivity of oxygen, otherwise it will affect the photoelectric detection effect or cause measurement errors due to 'false condensation'.
Advantages: high precision.
Disadvantages: slow response.
4. Optical fiber method
This technology is a new measurement technology developed at the end of the 20th century, which has raised the micro water analysis technology to a new level. The surface of the optical fiber humidity sensor is a laminated structure composed of silica and zirconia with different reflection coefficients. Through advanced thermal curing technology, the aperture of the sensor surface is controlled at 0.3 nm, and 0.28 nm water molecules can penetrate. The controller emits a bunch of 790-820nm near-infrared light, which is transmitted to the sensor through the optical fiber cable. The water molecule entering the sensor will change the reflection coefficient of the light, thus causing the change of the wavelength. The change is proportional to the moisture content of the medium. By measuring the wavelength of the received light, the dew point and moisture content of the medium can be obtained.
Advantages: high precision, maintenance free, very stable, can measure corrosive media containing H2S, HCL, etc.
Disadvantages: The transmission optical fiber is easy to break and needs protection.