How does a moisture analyser work? Measuring moisture content with a moisture balance
A moisture analyser or moisture balance combines an accurate balance with a heating system. The instrument determines how much mass a sample loses during a controlled drying process. This enables fast moisture checks in the laboratory and in production, but a reliable result starts with a good method.
How does a moisture analyser work?
Most laboratory moisture analysers work on a thermogravimetric loss-on-drying principle:
- the empty sample pan is tared;
- the sample is weighed in;
- a heating element heats the sample according to the chosen programme;
- the balance continuously measures the decrease in mass;
- the instrument stops on time, on a mass-change criterion or according to another set end condition;
- the result is displayed as, for example, moisture percentage or dry matter percentage.
Important: in essence the instrument measures mass loss during the chosen drying process. If other volatile components disappear besides water, or the product decomposes, this can affect the result.
Moisture percentage, dry matter and different calculations
Depending on the instrument, results can be calculated on different bases. Common ones are moisture percentage relative to the initial mass and dry matter percentage after drying.
Only compare results when the same calculation method is used. Two methods can use the same weights yet report different percentages if the calculation basis differs.
A good moisture measurement is above all a good method
For reproducible measurements you need to specify more than just a temperature. A working method preferably includes:
- sampling and homogenisation;
- sample quantity;
- type of sample pan or any glass fibre filter;
- distribution and layer thickness;
- drying temperature and heating profile;
- end criterion;
- calculation method;
- acceptance limits;
- any comparison with a reference method.
When the result is used for quality decisions, method validation, or at least a comparison with an accepted reference method, is advisable.
Sample preparation: often more important than extra decimal places
A moisture analyser only measures the small sample on the pan. If that sample is not representative of the batch, a technically perfect measurement can still lead to the wrong conclusion.
- Homogenise the product where possible and permitted.
- Work quickly with samples that absorb or lose moisture from the air.
- Spread the sample thinly and evenly over the pan.
- Avoid layers that are too thick, which dry on the outside while the core stays wet.
- Use a suitable sample weight: enough to be representative, but not so much that drying becomes unnecessarily slow or uneven.
Which drying temperature should you choose?
The best temperature depends on the product. A higher temperature does not automatically make the test better.
| Too low | Well chosen | Too high |
|---|---|---|
| Long drying process, possible residual moisture | Sufficiently fast and reproducible drying | Risk of decomposition, crust formation, spattering or loss of other volatile substances |
For unknown products the method is usually developed step by step: comparing several temperatures and sample sizes, checking reproducibility and comparing the result with the desired reference.
When does a moisture analyser stop?
Depending on the model, the drying process can stop after a fixed time or automatically when the change in mass has become small enough over a certain period. A switch-off criterion that is too early can leave residual moisture; one that is too strict can lengthen the measuring time unnecessarily.
For routine measurements, a reproducible end criterion is often more important than the shortest possible measuring time.
Moisture analyser versus drying oven
| Moisture analyser | Drying oven / reference method | |
|---|---|---|
| Speed | Often minutes | Often longer |
| Sample quantity | Relatively small | Can be larger |
| Operation | Weighing and drying in one instrument | Several steps and weighings |
| Use | Fast routine and process control | Often as the established reference method |
A moisture analyser is therefore not automatically a one-to-one replacement for every drying oven method. For comparable results the fast method must be tailored to the product and validated where necessary.
Common mistakes in moisture determination
- using too much sample;
- putting the sample in a heap instead of a thin layer;
- choosing the temperature for speed rather than product behaviour;
- using the same method for very different products;
- leaving a hygroscopic sample exposed for too long;
- comparing results calculated on a different basis;
- checking the weighing function but never verifying the heating performance;
- not using a fixed sampling procedure.
Which moisture analyser do you need?
When choosing, look at more than just readability:
- maximum sample mass and weighing resolution;
- display resolution of the moisture result;
- temperature range and type of heating;
- programmable drying profiles;
- memory for methods and results;
- USB, RS-232 or network connection;
- GLP/GMP reporting where required;
- ease of cleaning and availability of sample pans and filters.
View the current moisture analysers from KERN, OHAUS and Adam and compare the models on range, precision and functions.
Checking and calibration
The moisture measurement depends on both the weighing part and the drying process. Check the balance according to your quality system with suitable weights, and for temperature checks follow the manufacturer's instructions and tools. Weighing Experts also provides calibration and maintenance of weighing equipment.
Frequently asked questions about moisture analysers
How does a moisture analyser measure the moisture percentage?
A moisture analyser weighs the sample, heats it according to a set programme and follows the change in mass. The mass loss is displayed as a moisture or dry matter result according to the chosen calculation method.
Is a moisture analyser the same as a drying oven?
No. Both can be based on mass loss on drying, but the sample quantity, heating method, time and method differ. A fast moisture analyser method must therefore be developed for the product and, where necessary, validated against a reference method.
Why should a sample be spread thinly and evenly?
An even layer ensures predictable heat transfer and moisture evaporation. Thick or uneven samples often dry more slowly and less reproducibly.
Which drying temperature should I use?
That depends on the product. Too low a temperature can lead to incomplete drying, while too high a temperature can cause decomposition, burning or loss of other volatile components.
Does a moisture analyser need to be calibrated?
For reliable results, both the weighing function and, where relevant according to the quality system and the manufacturer, the temperature performance should be checked periodically.
Which moisture analyser suits your product?
Tell us which product you measure, what moisture range you expect, how many samples are tested per day and which reference method you currently use. With that we can recommend a model far more precisely.