If two instruments read the same sample differently, neither is necessarily faulty – they are most likely measuring two different physical quantities. The catalogue holds five distinct measuring principles; below is where each one gives the right answer.
Why is “method” a question at all?
No portable instrument weighs water directly. Each measures something else that changes with water – resistance, dielectric constant, infrared reflectance – and converts it to a percentage through a calibration curve. The commodity selected on the instrument (wheat, rice or flour) therefore determines the result directly.
The one exception in the catalogue is the loss-on-drying instruments, which really do evaporate the water and weigh the difference.
Electrical resistance
The sample is pressed between two electrodes; resistance falls as water rises. In the catalogue the PRv-930, Riceter FV-202, HB-610, HX-700 and the HK-300 series work on this principle.
Its strength is simplicity and field durability. The HB-610 is the typical example of the class: it measures inside bales and sacks, without disturbing the sample. Its weakness is that resistance is sensitive to moisture distribution and temperature – on a bale with a dry surface and a wet core the reading can mislead.
Capacitance / dielectric
The dielectric constant of water is far above that of grain or wood; the instrument places the sample inside a capacitor and measures that difference. In the catalogue the PM450 and PM650 work at 50 MHz, the HM-540 (wood) and HI-520-2 (concrete and mortar) at 20 MHz.
The advantage is that it sees the whole sample volume and is less distracted by surface moisture than the resistance method. The figure given for the PM650 – “standard error of 0.5% or less against the drying method” – shows the level this class reaches in practice, and at the same time names the reference it is compared against.
NIR – near infrared
Water absorbs specific wavelengths; the instrument measures light reflected from the surface and derives moisture from it. In the catalogue the KB-30 works this way.
It needs no contact, destroys no sample and suits continuous in-line measurement. In return it sees the surface only; colour, particle size and surface texture all affect the result.
Heating and weighing (drying method)
The sample is weighed, dried with heat and weighed again. The difference is water. In the catalogue the FD-660, FD-720 and FD-800 work this way, with ranges given as 0–100% (wet basis) and 0–500% (dry basis).
The reference method is oven drying; the FD series works on the same principle, heating and weighing the sample in a shorter time. The trade-off is that the sample is destroyed. Measure fast on the line, and verify regularly in the laboratory with a drying instrument.
Wet basis or dry basis?
The two separate ranges on the FD series are no accident. Wet basis is water as a share of total weight and cannot exceed 100%. Dry basis is water as a share of dry matter weight and can comfortably pass 100% – which is why the upper limit reads 500%.
For one sample both numbers are correct, but they are not the same number. When buyer and seller speak on different bases, the disagreement comes from the definition.