Axis AGS200/T250 User Manual

Axis AGS200/T250 User Manual Manual pdf 10 page image

10 USER MANUAL10. Description of thermogravimetric analysisThis section gives some practical details about moisture analysis using infraredradiation for reliable results and easier use of moisture analyser. The description isbased on a pre-production experience and customers‟ suggestions.Moisture in substances is an essential quality factor of technical and economicalimportance.Methods of determining moisture may be grouped in two main categories: absolute anddeductive.Absolute methods are based on simple relations, e.g. weight decline during drying.Thermogravimetric analysis used in AXIS moisture analyser is an example of thismethod.Deductive (indirect) methods measure physical quantity related with moisture, e.g.electromagnetic waves absorption, electrical conductance, acoustic wave speed. Someof these methods, unlike thermogravimetric analysis, enable to determine water content.Thermogravimetry - lat. thermo – heat, gravi – weight, metry – methodThermogravimetric analysis – a process of determination of a substance massdecline as a result of heat-up. The sample is weighed before and after heating-up, thedifference is calculated in relation to initial weight or final weight (dry mass).Moisture in substancesThermogravimetric analysis includes all ingredients evaporating from substances duringheating-up, which results in weight decrease.In result of the above, determining of moisture content in substances is not equal watercontent. Beside water, moisture consists of all other volatile matter: fats, alcohol,aromas, organic dissolvent and other substances resultant as en effect of thermaldecomposition.Thermogravimetric analysis does not distinguish water from other volatile matters.Infrared radiation drying is more effective than traditional methods (e.g. in an oven) asthe radiation deeply penetrates the substance, which shortens drying time.10.1 Infrared radiation sourceADS series moisture analyser uses 2 halogen heaters (rated power 200W, l=118mm) inserial connection as a radiation source. The heaters emit also visible radiation, whichdoes not affect drying process.10.2 Infrared radiation drying descriptionSample drying is a result of absorption of infrared radiation, which results in sampletemperature increase and evaporation of volatile matters.Infrared radiation penetrates surface layers, the depth depends on penetrability of asample (different in various substances). Part of radiation is reflected by the samplesurface. Penetrated layers absorb the radiation and convert its energy into heat. Emittedheat propagates inside the sample. Effectiveness of the propagation depends onthermal conductivity of the sample. The better the conductivity, the faster drying processand volatile matter evaporation. During drying process sample parameters change, itsthermal conductivity decreases so there is a risk of burning the sample. Someparameters may be estimated “by sight”, e.g. smooth and light surfaces reflect radiationbetter. This must be taken into account when setting drying parameters.