Coulometric humidity sensor

A coulometric humidity sensor or Keidel measuring cell is a humidity sensor that measures the water vapor content of the air ( air humidity ) in a very low concentration ( trace humidity ) . The measurement is based on the property of diphosphorus pentoxide ( ) to absorb the water vapor contained in the environment. The classic measuring method of the electrochemical decomposition of water on the strongly hygroscopic phosphorus oxide layer goes back to Keidel [1] . has long been known as an excellent desiccant for binding residual moisture. If a gas stream with a low moisture content is led past this strongly hygroscopic layer, the water vapor contained in the gas stream is absorbed. If an electric field is applied to this sensitive layer, the phosphorus compound will decompose and, according to Faraday's law , a current flow equivalent to the amount of water vapor converted ( ): ${\displaystyle \mathrm {P_{2}O_{5}} }$ ${\displaystyle \mathrm {P_{2}O_{5}} }$ ${\displaystyle \mathrm {H_{2}O} }$

${\displaystyle Q=I\cdot \mathrm {d} t=F\cdot a=F\cdot z{\frac {m(\mathrm {H_{2}O} )}{M(\mathrm {H_{2}O} )}}\qquad \qquad (1)}$

The individual symbols stand for the following quantities :

• ${\displaystyle Q}$- amount of cargo
• ${\displaystyle I}$- electric current
• ${\displaystyle z}$ - number of electrons exchanged
• ${\displaystyle F}$- Faraday constant = 96484 As / mol
• ${\displaystyle m(\mathrm {H_{2}O} )}$- mass of water vapor
• ${\displaystyle M(\mathrm {H_{2}O} )}$- molar mass of the water vapor

If complete electrochemical decomposition of the water vapor is not achieved, a dynamic equilibrium is established between the amount of water absorbed and the amount of water that has been electrolyzed at a constant gas flow and constant pressure, so that the measured current is proportional to the moisture content in the measuring gas. With this method, water vapor concentrations can or a frost point by (-90 ... -13) ppmv in gases in the range of (2000 ... 0.1) ° C are measured. From this, Keidel developed an industrial measuring cell for measuring trace moisture. The water is broken down under the influence of electrical current. The reaction takes place in equilibrium:

${\displaystyle \mathrm {P_{2}O_{5}+H_{2}O\rightarrow 2HPO_{3}} \qquad \qquad (2)}$

${\displaystyle \mathrm {2HPO_{3}\rightarrow H_{2}+{\frac {1}{2}}O_{2}+P_{2}O_{5}} \qquad \qquad (3)}$

The measurement of trace moisture in a gas flowing past is traced back to a current measurement. The electrical current at the electrodes is directly dependent on the amount of water converted. According to Faraday's law, an electrical charge of 96484 As is required for the conversion of 1 mol of water. When the water is completely converted through the flows a current of ${\displaystyle \mathrm {P_{2}O_{5}} }$

${\displaystyle I=1{,}0712104\cdot a\cdot V\;\mathrm {As/g} \qquad \qquad (4)}$

The individual symbols stand for the following quantities:

• ${\displaystyle a}$= absolute humidity in g ( W ) / kg ( tr.L )
• ${\displaystyle V}$ - Volume flow of the moist gas

The technical design of a sensor based on coulometric measurement consists of the following components:

• sensitive surface layer made of electrodes to which a constant direct voltage U> 2 VDC is applied ${\displaystyle \mathrm {P_{2}O_{5}} }$
• constant gas flow over this surface; Gas flow at a short distance and with a constant flow rate
• constant gas volume (set via volume flow controller)
• Ammeter for measuring the converted current.

The determination of the water content is based on the measurement of the ohmic resistance , which is variable over a wide range. The advantages of the measurement method are that it is very robust and the sensor is relatively simple. The sensor can be regenerated very easily by applying the phosphorus solution. A subsequent calibration of the measuring cell is not necessary, as this is a direct measuring method for measuring the absolute moisture content. Gases with aggressive components ( HCl ) can be measured without any problems. As long as the aggressive components do not destroy the equilibrium described above (e.g. ), measurements are possible. The disadvantage of the measuring method is that the measuring cell usually cannot be inserted directly into the gas flow. ${\displaystyle \mathrm {P_{2}O_{5}} }$ ${\displaystyle \mathrm {NH_{4}} }$

literature

• PA Keidel: Analytical Chemistry. S. 31/59
• R. Wernecke: Industrial moisture measurement. Wiley-VCH Verlag, Weinheim 2003, ISBN 9783527302857
• D. Sunday: hygrometry; A manual for measuring moisture in air and other gases. Academy Publishing House; Berlin 1966–1968
• T. Huebert; R. Wernecke: Quality for industrial coulometric trace humidity measurement. NPL-Tagung 2002: Papers and Abstracts from the Fourth International Symposium on Humidity and Moisture; Taiwan 2002