What a meter reports
Moisture content (MC) is the mass of water in a piece of wood divided by the mass of the same wood with all the water dried out, its ovendry mass, given as a percentage. Because the divisor is dry wood, not wet wood, MC can be over 100% in green lumber. Below about 30%, the fiber saturation point, wood shrinks as it dries and swells as it takes water back. Above it, the cell walls are full and only free water in the cell cavities changes.
Equilibrium moisture content (EMC) is a different thing: the MC wood would settle at if it stayed long enough in air of a given temperature and relative humidity. A meter tells you where a board is now. EMC tells you where it is heading. How much the board moves on the way is a third question, answered from the species' shrinkage, and the wood movement calculator works it out. Its helpers also compute EMC from the air and MC from an ovendry sample.
The oven-dry test is the reference
The Handbook calls oven-drying (ASTM D4442) the most universally accepted way to find MC. Cut a full cross-section about 1 in long, at least 20 in from the end of the board. Weigh it, dry it at 101–105 °C (214–221 °F), and keep weighing every few hours until the mass stops falling, which usually takes 12 to 48 hours. Then weigh it again. It is slow and destroys the sample, and woods with volatile extractives read slightly high, but it measures the water directly. An electrical meter (ASTM D4444) is fast and does no harm to the board, and the Handbook calls it less precise. A meter reading is an estimate checked against this test, not the other way round.
Pin and pinless meters
| Feature | Pin (resistance) meter | Pinless (dielectric) meter |
|---|---|---|
| What it senses | Direct-current conductance between two pins, which rises with moisture | Capacitance or power loss in a field from a plate held on the surface |
| Where it reads | Uninsulated pins: the wettest wood along their length. Insulated pins: only at the bare tips | A zone under the plate whose sensitivity falls quickly with depth, so surface layers count most |
| Depth set by | How far the pins go in | The meter's design, stated by its maker for that model |
| Marks on the board | Two small holes per reading | None |
| Species setting | Species corrections supplied with the meter | A density (specific gravity) adjustment |
| Temperature | Corrected, by chart or built into the meter | Newer meters correct internally, per the Handbook |
| Reliable range (Handbook) | About 6–30% MC | About 4–30% MC |
From the Wood Handbook, Chapter 13. Neither type is the accurate one. Each is accurate only when used within its range, corrected and read at the right depth.
How deep a reading goes
A drying board is usually wetter in the middle than at the faces. The Handbook's rule for pins is to drive them to between a fifth and a quarter of the thickness, where the MC is close to the board's average. With uninsulated pins the reading also picks up any wetter wood nearer the surface. A face that has been rained on or wiped wet then reads high, and insulated pins avoid that. Pin length is not reading depth: a pin pushed halfway in reads halfway. Pinless depth is fixed by the meter's design, and only its maker can say what it is.
| Meter | Type | Depth, as stated | For thicker wood, the maker says |
|---|---|---|---|
| Delmhorst J-2000 | Pin | Built-in uninsulated pins penetrate at most 5/16 in; for wood up to 1-1/2 to 2 in thick | Use the hammer electrode with insulated pins, for wood up to 5–6 in |
| General MMD4E | Pin | Pins 9 mm (0.35 in) long; no reading depth stated | Not stated |
| Lignomat mini-Ligno MD | Pin | Integral pins measure up to 7/16 in deep | Measure end grain on a fresh cut across the board, or use a depth electrode |
| Wagner Orion 910 | Pinless | 3/4 in | Not stated |
| Wagner Orion 930 and 950 | Pinless | 1/4 in or 3/4 in, switchable | Over 0.75 in: read both faces in 3/4 in mode and average |
Each maker's figures for its own meter. Delmhorst's 'up to 1-1/2 to 2 in' and the 1/5 to 1/4 rule agree: 5/16 in is about a fifth of 1-1/2 in. For nominal and actual board thicknesses, see the lumber dimensions chart.
Note— The Orion 930's thickness figures
Wagner's manual gives a minimum wood size of 2.0 × 2.5 × 0.75 in. It also says wood thinner than 0.75 in reads low in 3/4 in mode and advises the 1/4 in mode under 0.50 in. Read the 0.75 in minimum as belonging to the deep mode. In either mode the sensor plate must be fully covered, with a 1 in air gap under the board and no hand beneath it.
Species correction belongs to the meter
Wood species conduct electricity differently and differ in density, so each meter is calibrated against a reference and corrected for others. The Handbook says to apply the corrections supplied with the instrument, and the makers do it in different ways:
- Delmhorst bases all its calibrations on Douglas-fir and selects other woods by species code. The J-2000 manual's feature list says 48 species, but its chart lists 50 codes, two of them a brand of OSB. Check the chart in the manual for your meter.
- Wagner's Orion meters take a specific-gravity setting from 0.20 to 1.00, with 0.50 as the default. Wagner keeps a species database for finding the value.
- Delmhorst's own pinless meter is calibrated at a specific gravity of 0.46 on a green-volume, ovendry-weight basis. A number from one maker's density scale is not a setting for another maker's meter.
- Lignomat's mini-Ligno MD has two wood-group settings rather than species.
- General's MMD4E has no species correction. General suggests building your own reference readings against known samples.
Temperature
Resistance rises as wood gets colder, so a pin meter calibrated at room temperature reads cold wood low and warm wood high. The Handbook gives about 0.5% MC per 10 °F (5.6 °C) for resistance meters: add it below the calibration temperature and subtract it above. That is a general figure, not a substitute for the maker's own. The J-2000 compensates internally and needs correcting only outside 50–90 °F (10–32 °C), by Delmhorst's table. Wagner states that its pinless readings are virtually unaffected by wood temperature and asks users to contact it about very hot or frozen wood; that is the maker's claim, not tested here.
What a meter can show and what it can measure
| Meter | Wood range shown by the maker | Handbook reliable range for its type |
|---|---|---|
| Delmhorst J-2000 | 6–40% (J-2000/X: 6–60%); below 6% shows negative, above 40% shows 999 | About 6–30% |
| General MMD4E | 5–50% | About 6–30% |
| Lignomat mini-Ligno MD | 6–25% | About 6–30% |
| Wagner Orion 910, 930, 950 | 4.0–32.0%, varying slightly with the species setting | About 4–30% |
The Handbook says precision falls near the ends of the range and readings above about 30% are qualitative: a reading of 45% means wet, not 45%. Not a ranking.
Resolution is not accuracy. The Orion meters display MC to 0.1%, and Wagner states no plus-or-minus figure. General states '±2%' for the MMD4E without saying for which wood, range or temperature. The Handbook's own statement is narrower and more useful. Used properly on lumber dried to a uniform MC below fiber saturation, the average of several corrected readings should fall within 1% of the true average. That figure is for an average of corrected readings, not for any single one.
Calibration, checks and self-tests
| Term | What it is | Example from a maker |
|---|---|---|
| Factory calibration | The maker's built-in conversion from the electrical reading to MC for a reference wood | Delmhorst: Douglas-fir is the basis for all its calibrations |
| Calibration check | A known resistance or reference the meter should read within a tolerance | J-2000 check key: 12% ±0.2. MMD4E: 18.3% ±1% on the cap's resistor in wood mode |
| Check block | A separate reference block bought or supplied for the meter | Lignomat sells a test block for the mini-Ligno MD |
| User calibration | The owner resets the meter against a reference | Orion meters: recalibrated by the user on the serial-matched On-Demand Calibrator |
| Self-test | The meter checks its own electronics automatically | mini-Ligno MD: an automatic calibration check before each measurement |
A failed J-2000 check usually means a low battery, Delmhorst says; if a fresh battery does not fix it, the meter goes back to Delmhorst, since the owner cannot recalibrate it. None of these tells you the MC of a board. Only the oven-dry test does that.
Equipment notePaid link
Moisture meter
General Tools MMD4E
The pin meter whose cap check is in the table above: in wood mode it should read 18.3% ±1% on the cap's resistor.
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Modes that are not wood moisture content
- Relative mode. Wagner's REL mode gives a comparative number for materials the meter is not calibrated for. It is not MC.
- Building-materials mode. The MMD4E's BLDG scale (1.5–33%) is a separate calibration for other materials. A wood reading taken in it is not wood MC.
- EMC display. The Orion 950 calculates EMC from the air's temperature and humidity. It shows where wood in that room is heading, not what the board under the meter holds now.
Taking a reading you can use
Set the meter for the wood
Choose the species code, specific gravity or wood group your meter's maker gives. Check the battery, and run the maker's calibration check where there is one.
Prepare the spot
Pick clear wood away from knots and defects. For a pinless meter, wipe off surface water and let the face dry a few minutes, as Wagner directs.
Read at the right depth
Push pins in the direction the maker says (for the J-2000, in line with the grain) to a fifth to a quarter of the thickness where they reach. Use a pinless meter flat on the face in the mode that suits the thickness, with the air gap below.
Correct the number
Apply the maker's species and temperature corrections if the meter does not apply them itself.
Take several and average
Read several boards and several places on each. The Handbook's 1% statement is about an average of corrected readings.
Compare with the target
Set the result against the moisture content the wood should be at for where it will be used, not a universal number.
There is no single good MC. The Handbook's recommended MC for interior work depends on the climate where the piece will live, set out in choosing lumber. For how a board's width changes between two MCs, use the wood movement calculator. The wood weight calculator gives a board's weight at the MC you measured.
Before trusting a reading
- Meter set for the species or density, by its own maker's scale
- Battery good and the maker's check passed
- Reading taken at a known depth, in clear wood
- Temperature correction applied, or not needed for this meter
- Reading inside the meter's range and below about 30%
- Several readings averaged, not one
The rest of this series
- Moisture meter safety: sharp pins, batteries, hot and wet wood, and keeping a meter reading true.
- Choosing a moisture meter: six meters set out by their makers' specifications.
- Wood itself: the lumber guide and wood grain, which explains the ring orientation movement depends on.
Sources
Moisture-content science, meter behaviour, reliable ranges and the accuracy statement are the Wood Handbook's. Every range, depth, correction and check value for a named meter is that maker's own and applies only to that meter; correction values are never carried from one maker's meter to another's. Where a maker's documents disagree with each other or with the Handbook, both are shown. No meter was tested.
- 1.
Chapter 13: Drying and Control of Moisture Content and Dimensional Changes. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Moisture content on an ovendry basis (Eq. 13–1); ASTM D4442 oven-drying as the most universally accepted method, slow and slightly high with volatile extractives; specimens at least 500 mm (20 in) from the ends, dried at 101–105 °C until weight stops changing between 3-hour weighings, usually 12 to 48 hours; electrical meters (ASTM D4444) as rapid but less precise and generally limited to below about 30%; conductance (resistance) and dielectric (capacitance or power-loss) types; uninsulated pins sensing the highest moisture content along their length, insulated pins sensing at their tips, pins driven to 1/5 to 1/4 of the thickness for the average with a normal gradient, a rewetted surface making uninsulated pins read high; a dielectric meter's field decreasing rapidly with depth so surface layers dominate; species corrections supplied with the instrument; about 0.5% per 10 °F (5.6 °C) temperature correction for resistance meters, newer dielectric meters correcting temperature internally but needing a specific-gravity adjustment; reliable ranges of about 6–30% (resistance) and 4–30% (dielectric) with readings above 30% qualitative; the average of corrected readings on lumber dried to a uniform moisture content below fiber saturation within 1% of the true average; recommended moisture content at installation (Table 13–2).
- 2.
Chapter 4: Moisture Relations and Physical Properties of Wood. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Moisture content as water mass over ovendry mass (Eq. 4–2); fiber saturation at about 30% moisture content, below which wood shrinks and swells; equilibrium moisture content set by the temperature and relative humidity of the air (Table 4–2).
- 3.
J-2000 Owner's Manual (510INS-0003, Rev. 11/18), as hosted by the University of Pennsylvania. Delmhorst Instrument Co., 2018.
Used for: The J-2000 pin meter: 6–40% on wood (6–60% on the J-2000/X), below 6% shown as a negative number and above 40% as 999; species codes with Douglas-fir as the default and the basis of all Delmhorst calibrations; temperature correction needed outside 50–90 °F; a check key that should show 12% ±0.2, a failed check usually meaning a low battery, otherwise returned to Delmhorst; pin settings for uninsulated and insulated pins, insulated pins reading lower above 10%; pins parallel to the grain at full penetration; built-in uninsulated pins going 5/16 in deep, for wood up to 1-1/2 to 2 in thick, and a hammer electrode with insulated pins for thicker stock; averaging up to 100 readings. The feature list says 48 species and the species chart lists 50.
- 4.
Correction Tables & Species Look Up. Delmhorst Instrument Co..
Used for: Delmhorst's species tables based on Douglas-fir; a temperature table for meters without built-in correction, with none needed from 50 to 90 °F; an insulated-pin correction table (a reading of 10 corrected to 10.6, 20 to 21.4); its pinless meter calibrated at a specific gravity of 0.46 on a green-volume, ovendry-weight basis, reading higher on denser wood.
- 5.
Orion 910 Moisture Meter Operating Manual. Wagner Meters.
Used for: The Orion 910 pinless meter: 4.0–32.0% with 0.1% resolution, reading to 3/4 in; a specific-gravity setting from 0.20 to 1.00 (default 0.50); minimum wood 2.0 × 2.5 × 0.75 in with the sensor fully covered and a 1 in air gap beneath; a relative mode for other materials; user recalibration on its serial-matched On-Demand Calibrator.
- 6.
Orion 930 Moisture Meter Operating Manual. Wagner Meters.
Used for: The Orion 930: 1/4 in and 3/4 in depth modes; wood under 0.75 in underestimated in 3/4 in mode, 1/4 in mode advised under 0.50 in; both faces read and averaged on thicker wood; the same range, resolution, species setting, minimum size, air gap and calibrator; surface water wiped off and knots avoided; Wagner's statement that readings are virtually unaffected by wood temperature.
- 7.
Orion 950 Moisture Meter Operating Manual. Wagner Meters.
Used for: The Orion 950: the 930's two depth modes plus air temperature, relative humidity, dew point and a meter-calculated equilibrium moisture content (0–40.7%) from the air.
- 8.
MMD4E Pin-Type LCD Moisture Meter Manual (as hosted by a distributor). General Tools & Instruments.
Used for: The MMD4E pin meter: two 9 mm (0.35 in) pins; 5–50% in wood mode and 1.5–33% in a separate building-materials mode; 'measuring accuracy ±2%' stated without conditions; a calibration check against the cap's built-in resistor that should read 18.3% ±1% in wood mode; no species correction.
- 9.
mini-Ligno MD. Lignomat USA.
Used for: The mini-Ligno MD pin meter: 6–25% in 1% steps; two wood-group corrections; integral pins measuring up to 7/16 in deep, and for thicker wood a fresh cut across the board measured on the end grain or a separate depth electrode; an automatic calibration check before each measurement and a test block sold separately.
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