Filter cake moisture is only comparable when the reporting basis and measurement method are known. A result of 10% moisture on a wet basis describes a different water-to-solids ratio from 10% on a dry basis. Confusing them can distort a filter performance review, a wet-tonnage calculation, or a comparison between laboratory reports.
For mineral filtration, record the moisture value together with its basis, sample location, time, and test method. These details help determine whether a change came from the filter or from the way its cake was measured.
Wet-basis moisture divides water mass by the total wet sample mass. Dry-basis moisture divides water mass by dry-solids mass. Because the denominators differ, the same cake has a higher numerical moisture percentage on a dry basis.
For example, a cake containing 10 kg of water and 90 kg of dry solids has 10% wet-basis moisture and approximately 11.11% dry-basis moisture. Both describe the same material.
METTLER TOLEDO's moisture determination overview distinguishes moisture relative to original wet weight from moisture relative to final dry weight. The latter is also called ATRO moisture in some industries.
Use sample masses after subtracting the empty container mass. For the following calculations, assume the measured mass loss represents water and the drying procedure leaves the dry solids unchanged.
Wet-basis moisture (%) = (Wet sample mass − Dry sample mass) / Wet sample mass × 100
Dry-basis moisture (%) = (Wet sample mass − Dry sample mass) / Dry sample mass × 100
Suppose a container weighs 40 g. The container and wet cake weigh 240 g. After drying, the container and sample weigh 220 g.
Wet sample mass: 240 − 40 = 200 g.
Dry sample mass: 220 − 40 = 180 g.
Mass lost: 200 − 180 = 20 g.
Wet-basis moisture: 20 / 200 × 100 = 10%.
Dry-basis moisture: 20 / 180 × 100 ≈ 11.11%.
Using 240 g as the wet sample mass would include the container and understate the result. Preserve the original weighing records so the calculation can be checked.
Let W be the wet-basis percentage and D the dry-basis percentage. Enter percentage numbers such as 10, rather than fractions such as 0.10, in these equations:
D = 100 × W / (100 − W)
W = 100 × D / (100 + D)
| Wet-basis moisture | Equivalent dry-basis moisture |
|---|---|
| 8.00% | 8.70% |
| 10.00% | 11.11% |
| 15.00% | 17.65% |
| 20.00% | 25.00% |
These are mathematical conversions, not recommended moisture targets. Select the process target from the actual material and its downstream requirements.
A report showing 11.1% and another showing 10.0% may therefore agree within rounding. Check their bases before changing disc speed, vacuum, or another operating setting.
When moisture is reported on a wet basis:
Dry tonnes = Wet tonnes × (1 − W / 100)
Wet tonnes = Dry tonnes / (1 − W / 100)
For example, 50 wet tonnes per hour at 12% wet-basis moisture contains 44 dry tonnes per hour and 6 tonnes per hour of water.
If the same 12% were reported on a dry basis, the dry-solids rate would instead be 50 / 1.12, or approximately 44.64 tonnes per hour. That difference comes entirely from the reporting convention.
When using dry-solids rate to assess filtration area, keep the basis consistent with the vacuum disc filter capacity calculation.
Reducing wet-basis moisture from 12% to 10% is a decrease of two percentage points. Relative to the original moisture percentage, it is a reduction of about 16.7%.
Neither figure directly states how many tonnes of water were removed unless production is also defined. At a fixed 100 dry tonnes per hour:
Cake at 12% wet-basis moisture contains about 13.64 tonnes per hour of water.
Cake at 10% wet-basis moisture contains about 11.11 tonnes per hour of water.
The difference is about 2.53 tonnes per hour of water.
These illustrative calculations assume the same dry-solids production in both cases. Comparing periods with different production rates requires a mass balance.
An oven or halogen moisture analyzer measures a change in mass during drying. Other volatile substances can contribute to that loss. The result depends on how the material behaves under the chosen conditions.
METTLER TOLEDO explains that gravimetric methods measure loss on drying, while Karl Fischer methods are specific to water. Selecting a suitable method still depends on the sample and the purpose of the measurement.
For a mineral cake containing process reagents or heat-sensitive constituents, ask the laboratory what the reported value represents. Record the method identification, temperature or program, endpoint, and reporting basis. Do not substitute a generic drying temperature for the method agreed for that material.
A rapid analyzer can provide useful process feedback, but its readings should be compared with the agreed reference method before they are used interchangeably.
Split representative cake into matched portions, preserve them consistently, and test across the moisture range encountered in production. Examine both the difference between methods and the repeatability of each method. Agreement on a single dry sample does not establish agreement on wetter cake.
METTLER TOLEDO's guide to moisture analysis identifies sample preparation and method development against oven results as parts of reliable analyzer use. Project acceptance limits should be agreed with the laboratory and the parties using the result.
Cake sampled immediately after filter discharge may differ from cake sampled after a conveyor, storage bin, or outdoor stockpile. Evaporation, drainage, water addition, and mixing can change the result after filtration.
Define whether the measurement represents filter discharge or the product delivered downstream. Record the location and time, preserve the sample promptly, and avoid selecting only visibly dry pieces.
Also distinguish a local sample from a production average. A single piece taken from one side of a chute may identify uneven cake, but it should not automatically represent the entire machine.
An arithmetic average of moisture percentages gives each result equal weight. That may be inappropriate when the corresponding wet masses differ.
Consider two batches: 100 wet tonnes at 10% moisture and 20 wet tonnes at 20% moisture. Together they contain 14 tonnes of water in 120 wet tonnes. Their combined wet-basis moisture is approximately 11.67%, rather than the unweighted average of 15%.
For a production-period result, combine the water and total wet masses or use an equivalent wet-mass-weighted calculation. State when the available sampling cannot support such an average.
A practical record contains sample identification, equipment, sampling location, collection time, wet and dry masses, container mass, method, reporting basis, and any handling delay. Include operating conditions when the result will be used to evaluate the filter.
To discuss a moisture target with Tongzhiren Filtration, send the laboratory report and process conditions. A defined measurement basis helps connect the target to a meaningful filtration assessment.
No. Ten percent wet basis equals approximately 11.11% dry basis. Ten percent dry basis equals approximately 9.09% wet basis.
Yes. It exceeds 100% when water mass exceeds dry-solids mass. Wet-basis moisture remains below 100% for a sample containing both water and dry solids.
Possible reasons include different reporting bases, drying methods, sample locations, handling, or sample composition. Compare those details before attributing the difference to filtration performance.
Yes. State the basis, measurement method, sampling point, and approach to averaging so that the requirement can be interpreted consistently.