Measuring Principle - METTLER TOLEDO
 

Measuring Principle

 The way in which the moisture is determined using the Halogen Moisture Analyzer is explained here. This includes the method of drying (heating the sample using thermal radiation) and the principle of the switch-off criterion.



 

The Halogen Moisture Analyzer


 
The Halogen Moisture Analyzer works along the lines of the thermogravimetric principle, i.e. the sample's start weight is recorded, then a halogen radiator dries it while an integrated balance continually records the sample weight. The total loss in weight is interpreted as the moisture content.

Drying with the halogen radiator is a further development of the infrared drying method. The heating element consists of a glass pipe filled with halogen gas. As the mass of the halogen radiator is very low compared with that of a conventional infrared radiator, the maximum heating output can be reached quickly and outstanding controllability is achieved. In combination with the gold-plated reflector, this ensures an optimum, even distribution of the thermal radiation over the entire sample surface. This is indispensable to achieve repeatable results.
  • Thermogravimetric principle


 

The drying process


 
In contrast to the traditional oven in which the sample is heated using convection and dried over a long period of time, the sample in the Halogen Moisture Analyzer absorbs the infrared radiation (thermal radiation) from the halogen lamp and, as a result, heats up very quickly.

Different substances have different absorption characteristics. These depend primarily on the color and the material. You should therefore ensure that the sample is homogeneous and of even granulation. Smooth and light surfaces usually reflect infrared radiation more, so less energy is absorbed and the sample warms up less. This means that the absorption characteristics of a sample influence the effective sample temperature.

You will be familiar with this absorption effect: If you stand in the sun wearing a black shirt, you will get much hotter than if you were wearing a white one. You should therefore select a slightly lower measurement temperature for a dark sample than for a light sample.
  • Absorption of IR radiation
  • Absorption of IR radiation


The switch-off criterion


The switch-off criterion (AK) determines the point at which measurement with the Halogen Moisture Analyzer is automatically ended and the result displayed. The Halogen Moisture Analyzer offers two different kinds of switch-off criteria: A time-controlled form of switching off or the decrease in weight per time unit. The integrated balance continually determines the loss in weight of the sample during drying. If the loss in weight (Δ g) falls below the prescribed figure over a certain time (Δ t), the drying process is terminated at this level of dryness and the result displayed. You can select from 5 pre-defined switch-off criteria.

  • AK 1 (1 mg/10s): Suited to quick trend measurements
  • AK 2 (1 mg/20s): Intermediate level
  • AK 3 (1 mg/50s): Standard setting, suited to most types of sample
  • AK 4 (1 mg/90s): Intermediate level
  • AK 5 (1 mg/140s): Suited to samples that dry slowly and have a low moisture content (e.g. plastics)

With the HR83 Halogen Moisture Analyzer, the time period and desired weight loss can be freely defined.
The switch-off criterion selected influences the measurement period and measurement accuracy. The drying process will be completed in the shortest time if AK 1 is selected, but the drying is then often not fully finished and repeatability is reduced. By selecting the switch-off criterion you optimize the duration of the measurement period against the required accuracyof the measurement result.