Importance of Humidity measurement:
Measurement and control of moisture/Humidity is an important consideration in many industrial processes. The moisture content of gas streams directly affects the overall profitability and/or quality of many industrial processes.
Water vapour can shorten equipment life, affect product quality and significantly impact product cost. Thus, measuring and controlling moisture can help manufacturers cut fuel costs, optimize drying processes, produce consistently better product at a lower cost, and reduce capital expenditures and maintenance costs by increasing equipment life.
Salt cells:
Salts cells are bifilar windings of electrodes coated with a hygroscopic salt paste. Wind is allowed to pass through the electrode, air moisture content is determined from the current resulting from applying a voltage to the electrodes.
There are two different ways to measure the humidity, amperemetric which is continous measurement and coulemetric batch measurement.
Amperemetric method:
With the amperemetric measurement, as salt absorbs or desorbs water to achieve equilibrium with the air sample flowing over it, its electrolytic conductivity changes, increasing with increases in moisture content. The conductivity is measured using alternating current to minimize electrode polarization. Since the current depends not only on air moisture content but also such variables of cell construction as electrode length and spacing, salt thickness, etc., this is not a primary measurement and requires calibration.
Coulemetric method:
The coulemetric method does provide a primary measurement of air moisture content: a known volume of air is carried over the salt-coated electrodes by a carrier gas of constant, low moisture content. Confining flow to a narrow tube allows all water in the sample to diffuse into the salt paste and be electrolyzed. Shortly before the sample reaches the cell, a voltage sufficiently high to decompose water is applied to the electrodes. A “baseline” current resulting from moisture in the carrier gas is established.
When the sample reaches the cell, additional moisture content results in an increased current, which reaches a peak value and then diminishes back to baseline. The total charge, corresponding to the positive deviation from baseline current integrated over the time until baseline current is reestablished, is a direct
Advantages:
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Direct measurement
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Small variations in cell geometry, condition of electrodes, salt contamination, etc., do not affect the results provided.
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Provide stable result
Disadvantages:
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Direct current operation eventually produces a buildup of reaction products at the electrodes.
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The updates to changes in moisture content occur only once every few minutes
To know more about Density measuring devices