Moisture In Gases In Liquid

Process Analyzers

Some of the industrial process analyzers are of the probe type (capacitance, fiber-optic infrared probes), others can through the process stream in the pipe (microwave); while the majority requires some form of a sampling system.

Sampling Systems

There are some sampling system features peculiar to moisture analyzers, and these are covered below. The function of any sampling system is to deliver a clean, representative sample to the measuring element at the required pressure, temperature, and flow rate. Measurement of moisture, although simple in principle, is complicated by the fact that all materials adsorb and desorb moisture. At moisture levels in the percent range, moisture absorption is not a serious problem; however, at moisture levels in the low parts per million (ppm) ranges, sampling system materials must be selected so as to minimize their effect on the moisture level in the sample. Materials which are least likely to contribute to the moisture in the sample are stainless steels, Teflon, Viton, Kel-F, nickel and nickel-plated materials, and cadmium and cadmium-plated materials. Materials that should be avoided are copper and its alloys, rubber, neoprene, and elastomers.
It is important to apply the material considerations to all parts of the sampling system, including the internals of valves and filters.

Electrolytic Hygrometer

The principle of measurement utilized involves the electrolysis of water into oxygen and hydrogen. Since two electrons are required for electrolysis of each water molecule, the electrolysis current is a measure of the water present in the sample. If the volumetric flow rate of the sample gas into the electrolysis cell is controlled at a fixed value, then the electrolysis current is a function of water concentration in the sample.

Capacitance Hygrometer

The principle of measurement utilized is the change of capacitance associated with a change of the sample dielectric constant between capacitor plates. The value of capacitance is a function of plate area, plate spacing, and the dielectric constant of the material between the plates. The dielectric constant of a material has a unique value for each substance and is related to the polar characteristics of the molecules. This property can, therefore, be used to detect the presence of a specific substance in a pipeline stream.
The measuring cell of the hygrometer consists of two electrically insulated concentric metal cylinders to form the measuring capacitor plates. The annulus between the cylinders is filled with alumina desiccant. Two porous metal discs support the cylinders and retain the desiccant in the annulus. The sample is allowed to flow through the annulus and process water is absorbed or desorbed by the desiccant, which remains in equilibrium with the sample in terms of percent saturation. The moisture content of the sample is thus amplified by the desiccant because the saturation level of the desiccant is very much higher than that of the sample. The measuring capacitor is part of an electrical circuit, which includes a reference capacitor. This circuit is powered by a 15-kilocycle, fixed-amplitude sine wave. The measuring and reference capacitors are switched alternately into the circuit, in such a way that its output voltage is a function of connected capacitance. The output signal is a 15-kilocycle sine wave whose amplitude varies with the measuring and reference capacitors as they are switched into the circuit. This difference in amplitude is related to the measured capacitance, which in turn is a function of the moisture content of the sample.

Thin-film Capacitance Probes

These units are suitable for explosion-proof area classifications and can be used in measuring the moisture content of liquid hydrocarbons. The probe units are suitable for installation in high-pressure processes and are provided with microprocessor-based electronics capable of self-diagnostics and of multiple measurements or unit conversions.

Impedance Hygrometer

This instrument measures the water content of a sample by means of a probe whose electrical impedance is a function of the vapor pressure of moisture in the fluid. Impedance is the apparent opposition to the flow of alternating current. The probe consists of an aluminum strip, which is anodized to form a porous layer of aluminum oxide. A thin coat of gold is applied over the aluminum oxide. Leads from the gold and aluminum electrodes of the probe connect the sensing element to the measuring circuitry.
Water vapor penetrates the gold layer and equilibrates on the aluminum oxide. The number of molecules adsorbed to the aluminum oxide is a function of the water vapor pressure in the sample. Each water molecule adsorbed contributes a distinct increment to the total conductivity of the aluminum oxide. The total probe impedance, the reciprocal of probe conductivity, is thus a measure of water vapor pressure in the sample. Water vapor pressure of a gas sample uniquely determines the dew-point temperature and moisture content of the sample. The output is normally calibrated to these units since they are more convenient to use than vapor pressure.

Piezoelectric Hygrometer

Piezoelectric quartz crystals have a number of uses: in communications, to control frequencies and, in industry, to measure temperature and thickness of metal films, and to generate ultrasonic waves. In moisture measurement, advantage is taken of the oscillating crystal's sensitivity to deposits of foreign material on its surface. Commercially available crystals will show a frequency change of 2000 cycles per second (cps) per microgram of material deposited. For moisture measurement, the quartz crystals are coated with a hygroscopic material and exposed to the sample; water from the sample is absorbed by the crystal coating, thus increasing the total mass and decreasing the oscillating frequency of the crystal. In order to measure changes of decreasing moisture concentration and to simplify the frequency measurement, two crystals are used. One crystal is exposed to wet sample and the other to a dry reference gas for a short period. Then sample and reference gas flows are switched so that moisture is absorbed by one while being desorbed by the other crystal. Sample switching is controlled by a cycle timer in the instrument. The frequency difference between the two crystals is in proportion to their mass difference and the moisture content of the gas.

Heat Adsorption Hygrometer

The process of adsorption and desorption involves an exchange of energy. During adsorption, energy is released to and during desorption energy is removed from the environment. When wet gas is passed through a column of adsorbent, which selectively adsorbs moisture, the temperature rise due to heat liberation is in proportion to the amount of moisture adsorbed. Other factors which affect the heat of adsorption, such as the nature of the adsorbent and operating temperature, are selected to maximize the heat of adsorption.

Infrared Absorption Hygrometer

Water absorbs electromagnetic radiation in the infrared (IR) region of the spectrum. Specifically, infrared radiation of 1.4 and 1.93 micron wavelength is absorbed strongly by water. By measuring the attenuation (decrease of light intensity) of a beam of this wavelength as it passes through a sample, the moisture content of the sample can be determined. However, other factors such as reflection and dispersion of the radiant energy will contribute to the attenuation. Therefore, it is necessary to either calibrate these factors out of the measurement, or to use a reference wavelength which is not absorbed by moisture but is affected by all other factors to the same extent as the measuring wavelength. The difference in attenuation of the measurement and reference wavelength is then a function of moisture content only.
The sensing element consists of three groups of components: an IR radiation source, sample cell, and radiation pickup. The radiation source consists of a lamp, filters to pass the measuring and reference wavelengths, and optics to direct the beam through the glass sample cell. The radiation pickup consists of optics to collect the transmitted radiation and a photocell to convert the electromagnetic energy to an electric current. The measuring and reference wavelengths are allowed to impinge alternately on the photocell, so that two sets of current pulses are produced. These pulses are converted into two direct current (DC) signal levels whose ratio represents the moisture content of the sample

Microwave Absorption Hygrometer

In the microwave frequency band of 20 to 22 0Hz (K band), the wavelength is about 13 to 15 mm. In this frequency band moisture (free water) is most specific, as it is only the water molecule that produces molecular resonance. The microwave radiation can be guided by waveguides or be transmitted through the process stream from source to receiver.
The principle of operation of this instrument is the same as that of the infrared absorption hygrometer; namely, selective absorption of electromagnetic energy by moisture in the sample. However, in this case, radiant energy in the microwave length is used. A transmitter provides a microwave beam, which can either be transmitted through or reflected by the sample material. The receiving unit senses wave attenuation and phase shift and provides a readout. The unit senses the mass of moisture in the beam path so that the readout is normally in terms of the mass of moisture per unit volume. However, with calibration, readout in weight percent can be achieved.
Several probe configurations are possible depending on the application. On type has the transmitting and receiving probes welded into a section of pipe. Internally, Teflon contacts the sample. This type of arrangement can be used to advantage on slurries and pastes since there are no obstructions to flow. The units described above measure the attenuation by moisture of a microwave beam in the sample. The reflectance-type unit contains the source and detector in one housing. Rather than detecting the transmitted microwave energy, this design measures the amount of energy reflected by the sample. The probe of the reflectance unit does, however, require contact with the process material.