Summary and conclusions

Afprøvning af fugtmålere til brænde - Projekt under tilskudsordningen til miljøeffektiv brændefyringsteknologi

Firewood should be dry. A moisture meter may be helpful

If your firewood is too wet, you are a nuisance to your neighbours and environment, and you run the risk of soot and tar accumulating in your chimney, giving rise to bad odours or even chimney fires.

However it may be difficult to judge whether your firewood is sufficiently dry. This study reports on testing of eight different brands of moisture meters that may help you in the task.

None of the meters are very accurate but unless you are very experienced, they are still better than the feel and the naked eye. Accuracy of the meters relates to their price but even the cheapest may offer some help.

Introduction

Fuelwood: The single most important source of renewable energy in Denmark

According to official figures more than 600 000 domestic stoves are in use in Denmark. The total energy output of the stoves is estimated at 27 PetaJoule annually, which is slightly more than the total output of wind power.

Even though the stoves to a large extent make use of a renewable resource that might not be utilized (wood from shelterbelts, gardens, roadside plantings etc.) the extensive use of firewood is not without problems: The stoves may contribute significantly to air pollution if used incorrectly.

Especially the moisture content of the firewood is a matter of concern. Too moist firewood gives rise to smoke, tar and soot particles during combustion, and the energy efficiency of the stove remains too low.

A possible solution to the problem is to buy your firewood well in advance of the winter and leave it protected from rain for a year or more. But for many consumers this is not feasible due to lack of time or space. Very often the firewood is bought shortly before use, and the consumer may face difficulties in determining whether the wood is dry enough for use.

Estimating moisture content by the feel of the wood or bare eyesight is very inaccurate. However, a number of portable, electronic moisture meters are on the market at prices ranging from €10 to €400. This small study, conducted by the Forest & Landscape Centre at the University of Copenhagen, aims at evaluating the usefulness of such electronic moisture meters.

The study

Different moisture meters and different wood species

Eight different types of electronic moisture meters were bought from shops and at the Internet marketplace.

Simultaneously, firewood was produced from seven different wood species:

Beech (Fagus)
Oak (Quercus)
Ash (Fraxinus)
Sycamore maple (Acer)
Lime (Tilia)
Birch (Betula)
Norway spruce (Picea)

After felling in September the wood billets were split once and left to natural drying until July the year after. At intervals samples were taken to be measured by the electronic meters and for exact moisture determination in the laboratory.

Main conclusions

Electronic moisture meters work – in general

Wood moisture estimation without any aid but feeling and eyesight is a difficult task for the un-experienced. Electronic moisture meters may be a good help, even though they are not very accurate.

Out of the eight meter types bought for this study, seven performed according to the objective, but with a varying degree of accuracy. None of the meters were reliable with moisture content exceeding 25%. Even below this level of moisture, a single measurement deviated on average 3 percent from the correct value. This general figure covers the fact that the results from poorest meter strayed more than 5 percent from the correct value whereas the best (and most expensive) one was only 2 percent off. Fortunately, most of the meters performed at their best in “the critical range” around 15-20 percent moisture content.

To obtain reasonably accurate results, it is important to read and follow the instruction pamphlet coming with the device, and additionally take two precautions:

Perform measurements on several different pieces of wood – do not rely on a single measurement.
Cleave the piece of firewood immediately prior to measuring, and perform the measurement on the freshly exposed surface.

The Environmental Protection Agency of Denmark recommends that firewood with higher moisture content than 18% (percent of total mass) should never be used in domestic stoves.

Some important results

Accuracy relates to price

Seven out of the eight meters were functional, but a relation exists between price and performance: The more affordable meters were the less accurate and the very cheapest (~€10) suffered from some instability over parts of the measuring range. However, even with this device it was in most cases possible to discern whether the firewood was too wet or ready for use. The best meter types bear a price tag of €100 upwards.

Two different principles apply to quoting the moisture content of a piece of wood: dry weight basis or total weight (wet) basis. The former is used in sawmilling, engineering and woodworking, while the latter applies to wood for energy. Moisture meters are calibrated to either dry or wet basis but in many cases this information does not show in the instruction manual for the device. Fortunately, in the “dry end” of the measuring range the difference is small enough to allow dry-basis meters being used for detecting the moisture content of firewood. The reading will be somewhat higher than the moisture content on total weight basis, but the only risk you run will be putting a too strenuous demand on the dryness of the firewood.

There is no simple relationship between wood species and the accuracy of the measurements. There is a tendency for the moisture content of the heavy wood species (beech and oak) to be under-estimated, especially when the moisture content was high. Also the moisture content of spruce is in general under-estimated. Some of the meters are able to perform reasonably well in the high end of moisture range when used on light weight species (lime and spruce).