Introduction to Two Different Measurement Techniques for Moisture Measurement
It is important to define the moisture content before introducing various moisture measurement values. The moisture content is usually expressed as a percentage by weight of the total product (wet basis) or dry product (dry basis).
Moisture content on a wet basis:
M=100 x (wet weight dry weight)/wet weight
Moisture content on a dry basis:
M=100 x (wet dry weight)/dry weight
According to the above formula, the moisture content on a wet basis cannot exceed 100%. The moisture content on a dry basis may exceed 100% and is a non-linear function. The moisture content can be determined through various techniques. These can be divided into two main categories, primary and secondary measures.
Primary moisture technology typically extracts moisture directly from products and directly measures moisture content.
All major methods are destructive and time-consuming. The main method is executed offline, but it is usually very accurate. The sample size may not be sufficient to represent bulk products.
The common main method is weight loss, in which the sample is weighed, dried until there is no further weight loss, and then reweighed.
Other methods include Karl Fischer titration. The accuracy of all offline main methods depends on the accuracy of laboratory instruments and the skills of laboratory personnel.
Due to the offline method requiring obtaining product samples from the process, the sampling method requires providing consistent product samples for testing.
Secondary moisture technology measures the attributes of variables (moisture), rather than directly measuring variables. All continuous moisture meters use secondary measurement principles and require calibration based on primary reference techniques. They have the advantage of continuous or fast sampling measurements and can be used for real-time process monitoring and control.
Without continuous measurement capability, typical processes will be controlled by obtaining product samples and conducting laboratory analysis. These methods are time-consuming. By the time the results are obtained, the process may have undergone significant changes.
In a simple form, even without calibration, a continuous moisture analyzer will provide trend information between laboratory samples. In this form, the instrument is a useful setpoint controller that can adjust the process setpoint after each laboratory sample.
There are many online moisture measurement technologies available. Dielectric measurement and near-infrared reflectance have been proven to be accurate and reliable in many industries.
