Measure residual moisture | Brabender Messtechnik

The current AQUATRAC product range symbolizes state of the art, together with the Brabender Messtechnik attributes robustness, suitability for everyday industrial use and outstanding service. Here we explain our measuring methods in detail.

Measuring residual moisture with water-selective dew point measurement

It is well known that measuring moisture using analytical balances (so-called gravimetric analysis using halogen or infrared lamps) can be faulty: These methods tend to measure other volatile sample components such as additives and thus overestimate the water content. It is also well known that chemical methods such as Karl Fischer titration are too costly for rapid residual moisture analysis in hectic industrial environments. With the water-selective dew point measurement Brabender Messtechnik now adds another, extremely precise method to the residual moisture determination by means of calcium hydride.

Dew point measurement - measurement sequence

The dew point is by definition a temperature. Gaseous water can condense on a surface. The temperature from which the moisture contained in the air condenses on a cool surface is the dew point. The dew point is measured with a capacitive sensor. Since water is a strongly polar molecule, it causes a capacitance change at the sensor. Therefore water can be measured excellently with a capacitive sensor. The dew point is physically exactly linked to the water content of the air and the gas temperature. The absolute amount of water in the AQUATRAC station is calculated from these measured values. And thus the precise water content of the sample.

The water is extracted from the sample particularly effectively and quickly by means of negative pressure and increased temperatures. The vacuum not only creates a high concentration gradient between the sample and the measuring chamber, which ensures that the water contained in the sample evaporates completely from the material. It also ensures that the water from the sample cannot deposit on the walls of the instrument. The interior of the AQUATRAC station is made of selected hydrophobic, i.e. water-repellent, materials. The water vapour desorption rates are known and are taken into account in the evaluation of the measurement. At the same time, the vacuum increases the accuracy of the analysis, as the saturation vapour pressure and the dew point temperature are exponentially linked.

Measuring residual moisture with the calcium hydride method

Calciumhydrid-measrurement - measurement sequence

Water and calcium hydride react with each other to form hydrogen; following reaction equation: CaH₂ + 2 H₂O → Ca(OH)₂ + 2 H₂ In the AQUATRAC-V this reaction takes place in a closed vessel, the reaction vessel. It is evacuated by a built-in vacuum pump before the measurement and electrically heated during the measurement. The water vapour released by the heating reacts with the calcium hydride and the pressure in the measuring system increases due to the developing hydrogen. The gas pressure is proportional to the amount of water introduced, because the reaction is specific for water. The gas pressure is the measured variable of this method. The ratio of water quantity and sample weight is calculated and the result is displayed in %, ppm and mg water content. Other volatile substances are condensed in a cold trap at room temperature and therefore do not participate in the measurement. Therefore, this is an absolute measuring method exclusively for water as also described in the standard DIN EN ISO 15512:2019 as method E – determination of the water content by the calcium hydride method.

Calcium hydride method - Reagent

Calcium hydride in granular form is used as water-specific reagent. It is non-toxic and environmentally friendly, so that no costly disposal is necessary. The reagent is filled into the reagent insert in the AQUATRAC-V with the aid of a metering spoon. Contrary to conventional handling, only approx. 0.3 g of calcium hydride is now added for each measurement and replaced during the next measurement. This makes using the AQUATRAC-V even more intuitive. In addition, the amount of hydrogen produced is guaranteed to be minimal for each measurement.