A resistance probe and a capacitance meter disagreed by four points on the same large round bale. The probe read the core; the meter read a shell, and the shell was drier because that is what the sun does.
Two probes, four points apart on one bale
window HAY WINDOWwritten 2026-08-30responsible Windrow Field Manual (a named masthead, not a person)
[['Resistance probe', 'Centre of the flat face', '18.4%', 'Core moisture'], ['Capacitance meter', 'Pressed on the curved side', '22.1%', 'The shell, plus surface moisture'], ['Resistance probe, second bale', 'Centre', '17.9%', 'Core, confirming the first'], ['Oven test on a core sample', 'Laboratory', '18.1%', 'The reference method']]
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Why the gap is not error
The two instruments do not measure the same physical quantity. The resistance probe drives a current through the sample and reads how well it conducts, which depends on the water in the material between its electrodes. The capacitance meter measures how the material changes an electric field, which is dominated by whatever is nearest the plate. One is a volume measurement at a point; the other is a surface measurement over an area.
| Instrument | Physical principle | What it is best at | Where it misleads |
|---|---|---|---|
| Resistance probe | Conductivity of the sample between electrodes | Core moisture of a bale or a row | Dry pockets read as very dry, so a bad contact reads low |
| Capacitance meter | Dielectric change near the sensing plate | A fast relative reading along a row | Anything about the core |
| Oven or microwave | Mass lost on drying | Being the reference | Speed; it is a laboratory method |
What the manual took from it
The manual's rule after this measurement is simple: a single number from one instrument on one bale is not a moisture content, it is an instrument reading. Two instruments that disagree are giving you information about the bale's structure, and the disagreement is worth more than either number alone.
This position costs something. It means the manual cannot print one clean threshold, and it means a reader who wants a single number has to take two readings to get it. The alternative is a number that is four points wrong on the bale that will heat.
The procedure the manual now uses
- Take a resistance reading at the core of three bales.
- Take a capacitance reading at the same three bales, on the side.
- If the two agree within about two points, bale. If they disagree by more, find out which part of the bale is wet before you decide.
- Write both readings down. The pair is the record.
Where the numbers come from
- S1Large Round Bale Storage (FSA-3131), University of Arkansas System Division of Agriculturesupports the storage practice the paired readings are being judged against
- S2Hay Storage Losses (FS1234), Rutgers New Jersey Agricultural Experiment Stationsupports the storage-loss figures that make the reading worth taking
- S3Baling Hay at the Right Moisture, Michigan State University Extensionsupports the baling-moisture relationship both instruments are read against
One link, one document. No home pages, no search results, no bare domains.
Put this page down Put this page down when you have two readings from two instruments and have written both down.