Types of Trace Moisture Analyzers

A trace moisture analyzer detects moisture using electrolytic, capacitive (blocking), chilled-mirror, or optical-fiber measurement methods; its measuring cell is removable for easy maintenance. Let's take a look at the types of trace moisture analyzers.!

1. Electrolytic method

 The phosphorus pentoxide sensor works on the principle of electrolyzing water molecules into hydrogen and oxygen. It consists of a glass cylinder with two parallel electrodes—the electrode material chosen for the application (usually platinum or rhodium wire)—with a thin layer of phosphoric acidH3PO4 coated between them. The electrolytic current between the electrodes decomposes the water in the acid into H2 and O2. The end product of this process is phosphorus pentoxide; P2O5 is strongly hygroscopic, absorbing moisture from the sample gas, and through continuous electrolysis the moisture in the sample gas and the electrolyzed water remain in balance. The electrode current is proportional to the moisture content of the sample gas; the signal is processed by the instrument's internal amplifier, then displayed and read out. This principle measures virtually all gases, including Cl2, HCl, H2S, H2SO4, HBr, SO2, SF6, CO2, and all inert gases, except the very few that react chemically with phosphoric acid.

The P2O5 probe suits inert gases and hydrocarbons, or, depending on the probe materials chosen, corrosive gases such as HCl, Cl2, or SO2. Materials contacting the sample gas can be glass, platinum, or rhodium; other materials are available.

The sample gas flows through the probe in a special transfer arrangement, combined with a high-quality interface, ensuring fast probe response and minimal interference—design features that are essential for very lowppm-level measurement. The sample gas flow through the probe is typically set at 20 Nl/h (100 Nl/h optional). The electrical connector to the analyzer is waterproof and sealed. Users can easily regenerate the probe within five minutes, and the probe mounts anywhere with three M4 screws.

Advantages: high sensitivity, suitable for very trace moisture/trace water testing, and capable of measuring corrosive gases.

Disadvantages: the sensor needs periodic recoating, drifts considerably, and is susceptible tobackground gases such as H2 and O2; long equilibration time and slow response.

2. Capacitive (blocking) method

A high-purity aluminum rod is oxidized to form an ultra-thin aluminum oxide film, coated with a porous gold mesh; a capacitor forms between the gold film and the rod. Because the alumina film is hygroscopic, the capacitance varies with the moisture in the sample gas, and measuring it yields the gas humidity. The main advantage is an even lower measurement range, down to100 °C; another outstanding advantage is very fast response—90% response from dry to wet within one minute—making it popular for field and rapid measurements; the disadvantage is lower accuracy, with uncertainty mostly ±2–3 °C. But with manufacturers' continued efforts the method keeps improving: better materials and processes have greatly improved sensor stability, and compensation of the sensor response curve has achieved saturation linearity, solving auto-calibration.

Advantages: fast response.

Disadvantages: lower accuracy.

3. Chilled-mirror method

The sample gas flows through the condensing mirror of a dew-point chilled-mirror chamber; by isobaric refrigeration the gas reaches saturation and condenses (droplets appear on the mirror), and the mirror temperature at that moment is the dew point. The main advantage is high accuracy—with semiconductor cooling and photoelectric detection, uncertainty can reach0.1 °C; the disadvantages are slower response—at dew points below 60 °C, equilibration can even take hours—and demanding requirements for sample gas cleanliness and non-corrosiveness, otherwise photoelectric detection suffers or 'false dew' creates measurement error.

Advantages: high accuracy.

Disadvantages: slow response.

4、Optical fiber method

This technologyemerged at the end of the 20th century, raising trace moisture analysis to a new level. The surface of the fiber-optic humidity sensor is a layered structure of silica and zirconia with different reflection coefficients; advanced thermal curing controls the surface pore diameter to 0.3 nm so that water molecules of 0.28 nm can penetrate. The controller emits a 790–820 nm near-infrared beam delivered through fiber cable to the sensor; water molecules entering the sensor change the light's reflection coefficient, shifting the wavelength by an amount proportional to the moisture content of the medium. Measuring the received wavelength yields the medium's dew point and moisture content.

Advantages: high accuracy, maintenance-free, very stable, capable of measuringcorrosive media such as H2S and HCl.

Disadvantages: the transmission fiber is fragile and needs protection.

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