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h2osensorgb [2026/04/06 19:13] – [How does it work?] administratorh2osensorgb [2026/04/07 01:43] (aktuell) – [How does it work?] administrator
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 ===== How does it work? ===== ===== How does it work? =====
-The principle behind this sensor is based on the fact that <chem>P2O5</chem> is extremely hygroscopic - one of the most hygroscopic substances there is. This means that all the moisture is basically absolutely absorbed into. <chem>P2O5</chem> then forms polyphosphoric acids with water until phosphoric acid (<chem>H3PO4</chem>) is reached. Therefore the probe head ({{ref>probe}}) is impregnated with phosphoric acid. The probe head is a ceramic cylinder with conductive traces deposted on it ({{ref>probe}}).+The principle behind this sensor is based on the fact that <chem>P2O5</chem> (or better <chem>P4O10</chem>) is extremely hygroscopic - one of the most hygroscopic substances there is. 
 +((A. F. Hollemann, E. Wiberg, N. Wiberg, //Lehrbuch der anorganischen Chemie//, 102. Auflage, 2007, Walter de Gruyter & Co., ISBN 978-3-11-017770-1, S. 786-788)) This means that all the moisture is basically absolutely absorbed into. <chem>P4O10</chem> then forms polyphosphoric acids with water until phosphoric acid (<chem>H3PO4</chem>) is reached.((A. F. Hollemann, E. Wiberg, N. Wiberg, //Lehrbuch der anorganischen Chemie//, 102. Auflage, 2007, Walter de Gruyter & Co., ISBN 978-3-11-017770-1, S. 786-788))  
 + 
 +<figure center|h3po4> 
 +{{:wartung:wassersensor:1775501152.png?direct&350 |title}} 
 +<caption>Hydrolysis of <chem>P4O10</chem>.</caption> 
 +</figure> 
 + 
 +Therefore the probe head ({{ref>probe}}) is impregnated with phosphoric acid. The probe head is a ceramic cylinder with conductive traces deposited onto it ({{ref>probe}}).
  
 <figure center|probe> <figure center|probe>
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 <caption>Probe head of of a phosphoric acid-based moisture sensor. The thin conductive traces are elektrodes made from platinum.</caption> <caption>Probe head of of a phosphoric acid-based moisture sensor. The thin conductive traces are elektrodes made from platinum.</caption>
 </figure> </figure>
 +
 +Each one of this conductive traces acts as an electrode with alternating polarity next to each other. By applying an electrical voltage to the electrodes the phosphoric acid, or better the amount of water it contains, is broken down via electrolysis into oxygen and hydrogen ({{ref>scheme}}).(([[https://www.dks-engineering.com/de/wissen/spurenfeuchtemessung-definition-und-messprinzipien|Coulometrischer Feuchtesensor / Spurenfeuchtemessung, 06.04.2026 (external link)]]))
  
 <figure center|scheme> <figure center|scheme>
 {{:wartung:wassersensor:elektrolyse.drawio.png |}} {{:wartung:wassersensor:elektrolyse.drawio.png |}}
-<caption>Schematic display of the working principle of a phosphoric acid-based moisture sensor.</caption>+<caption>Schematic display of the working principle of a phosphoric acid-based moisture sensor. U is a voltage source and A is an ampere meter connected in series.</caption>
 </figure> </figure>
 +
 +The electric current flowing in this electrolysis can be measured by using an ampere meter (A). According to the Faraday law the current **//I//** is proportional to the amount of substance **//n//** converted at the electrodes ({{ref>equation}}). This enables the possibility to determine the moisture in a system down to the ppm level.
 +
 +
 +<figure center|equation>
 +{{:wartung:wassersensor:1775088659.png?direct&400 |title}}
 +<caption>The Faraday law rearranged towards the amount of substance //n//.</caption>
 +</figure>
 +
 +The probe head needs a constant flow of gas to work correctly. A standing atmosphere over the probe head will inherently dry itself and therefore provide false data.
 +
 +Basic gases and fumes will form phosphates with the phosphoric acid (e. g. amines). This leads to the deterioration of the acid which is the reason why the sensor head needs to be cleaned and impregnated with fresh acid on a regular basis.