Moisture content: measurement and effect on flow behaviour
Even small deviations in moisture can change the flowability of a bulk solid considerably. We therefore recommend determining the moisture for every sample whose flow properties are to be compared.
You receive moisture and solids content, relative to the moist sample and to the dry mass – and, compared with yield locus and wall friction, the effect of moisture on the flow behaviour.
Parameters from the moisture determination
The initial mass m0 and the dry mass mdry give moisture and solids content – relative to the moist sample or to the dry mass.
| Parameter | Calculation | Use |
|---|---|---|
| Moisture | %M = 100 · (m0 − mdry) / m0 | comparing samples, batches and deliveries |
| Solids content | %S = 100 · mdry / m0 | dry substance of the sample |
| Moisture on a dry basis | %Mdb = 100 · (m0 − mdry) / mdry | setting a target moisture, dryer design |
| Solids on a dry basis | %Sdb = 100 · m0 / mdry | conversion between moist and dry mass |
Carrying out the moisture determination
The moisture is measured by the oven-drying method in a drying oven – usually at 105 °C, depending on the sample material – or with a halogen moisture analyser. On request we adjust samples to a desired moisture.
- Weighing The sample is weighed into a sample dish – the initial mass.
- Drying The moisture is driven off in the drying oven or halogen moisture analyser.
- Constant mass Drying continues until the mass no longer changes – the dry mass.
- Evaluation Initial and dry mass give moisture and solids content.
Oven-drying method and halogen moisture analyser
With the oven-drying method, the sample dries in a drying oven – usually at 105 °C – until constant mass. The halogen moisture analyser weighs the sample continuously during drying and records the drying curve; the measurement is considerably faster. Each measurement is carried out at least in duplicate; if the measurements extend over several days or the values deviate unusually strongly from each other, we repeat them.
Test report and customer portal
The test report contains initial and dry mass, mass loss, moisture and solids content – relative to the moist sample and to the dry mass – as well as the drying curve. In the customer portal, different measurements can be set side by side and compared.
Effect of moisture on flow behaviour
Mechanisms
With increasing moisture, liquid bridges form between the particles. Their capillary forces increase the adhesive forces and thus cohesion and unconfined compressive strength; the bulk solid becomes more cohesive. Moisture also changes electrostatic effects and the friction between the particles. How strongly it acts depends on particle size, shape, surface structure and the distribution of the moisture in the sample.
Comparison at different moisture contents
For the comparison, a second sample is measured: either you prepare the sample with the desired moisture, or you give us a target moisture, which we set in the laboratory. The yield locus and, if required, the wall friction are then measured.
Moisture in operation
If the moisture fluctuates in the process or the bulk solid absorbs moisture during storage, the flow behaviour changes with it. Together with time consolidation, moisture is a frequent cause of caking, arches and ratholes after standstill.
Frequently asked questions about moisture content
- What is the difference between %M and moisture on a dry basis?
- The moisture %M relates the mass loss during drying to the initial mass of the moist sample, moisture on a dry basis relates it to the dry mass. At 12.5 %M, for example, this is about 14.3 % on a dry basis. The test report contains both values as well as the solids content.
- How is the effect of moisture on flowability investigated?
- This requires a second test series or sample: either you prepare the sample with the desired moisture, or you give us a target moisture, which we set in the laboratory. Both samples are then measured with yield locus and, if required, wall friction and set side by side.
- Why is the moisture determined together with the yield locus?
- Even small deviations in moisture can change the flowability considerably. With the moisture of the measured sample, results from different batches and deliveries can be classified correctly.
- How much sample material is needed?
- For the moisture determination 100 to 200 g are usually sufficient. If yield loci or wall friction are measured as well, the quantity needed for them is added.
- How long does the measurement take?
- Results are usually available about two weeks after sample receipt – depending on laboratory capacity and the number of samples. Time consolidation tests can extend the standard lead time accordingly.
Have the moisture content measured
Describe your bulk solid and the question – we prepare a non-binding quote for you, on request with a comparison at different moisture contents.
- Moisture and solids content, also on a dry basis
- Yield locus and wall friction at different moisture contents
- Results online in the customer portal and as a test report