Gravimetric methods

Authors: Hans-Jürgen Schwarz

back to Moisture Measurement

Abstract

The gravimetric method is the most common method for determining the material moisture content. Here, the different possibilities for drying samples are presented on an introductory level.

Introduction

Figure 1: Diagram of drying samples in a drying cabinet, Graphics: Fa. Sartorius GmbH

Figure 2: Diagram of drying samples with microwave radiation, Graphics: Fa. Sartorius GmbH

Figure 3: Diagram of drying samples with infrared radiation, Grafik: Fa. Sartorius GmbH

The gravimetric method [Nagel:2002]Title: Thermogravimetrische Materialfeuchtebestimmung
Author: Nagel, Horst
is a destructive method. Different samples are taken from an object or building, i.e. in the shape of a drill core sample. The sample is weighed, then dried to constant weight and weighed again. The difference in mass denotes the moisture content of the sample. The gravimetric Darr method is standardized internationally and serves as a reference for other measuring methods.

Figure 4: There is always a difference between the measured and the real material moisture, Grafik: Fa. Sartorius GmbH

The accuracy of the method depends on a number of factors. Firstly, at collection of the sample a change in moisture content may take place, e.g. due to heating during drilling. The sample has to be packed absolutely airtight directly after collection, because even at room temperature significant losses occur (see Fig. 4). The amount of material needed for a sample depends on the homogeneity of the material. The minimum lies at around 1 g for fine aggregate samples.

Heating inside a cabinet dryer is the most common drying method (see Fig. 1). The drying temperature for most materials is 105 °C. It is a prerequisite that no chemical change of the material occurs at this temperature. Gypsum, for example, needs a drying temperature of 40°C, because it looses some of its chemically bound water at 45°C and is then converted into ß-hemihydrate. The relative humidity (RH) inside the drying chamber must be kept at sufficiently low levels. If ambient air is used for the exchange, even at low drying temperatures the high RH in summer may produce significant residual moisture in the sample and cause measurement errors.

Figure 5: Automatic IR moisture determination, Foto: Fa. Sartorius GmbH

A shortening of the drying time can be achieved by microwave drying (see Fig. 2), because practically the whole sample is brought up to drying temperature within a very short period of time. However, it requires some experience to meter the microwave power properly.

A falsification of the results can occur due to hygroscopic water absorption in between the two weighings. When working carefully, an accuracy of ± 0.2 - 0.5% can be achieved.

Furthermore, the drying of the sample using infrared radiation is possible (see Fig. 3). This is an option for many devices that are commercially available (see Fig. 5).

Instead of drying through temperature increase, samples can also be dried in the vacuum. For this purpose the sample is placed into a desiccator, which has been evacuated. The vapor pressure is lowered using e.g. hygroscopic salts (phosphor pentoxide) or silica gel.

Main disadvantage of the gravimetric method: it is destructive and it is unsuitable for in situ application.

Literature

[Nagel:2002]

Nagel, Horst (2002): Thermogravimetrische Materialfeuchtebestimmung, Verlag Moderne Industrie

--SLeithaeuser 00:15, 28 January 2013 (CET)

Calciumsulfatehemihydrate exists in two modifications: α - and β-hemihydrate. β-hemihydrate is formed in dry media, when gypsum is heated over 40°C, while α –hemihydrates is formed in aqueous media when heated over 97.2°C.

relative humidity

see exsiccator