Moisture content measurements - logs

[openspaceman]

Out of interest do those of you drying at high temperatures in 24 hours find it only dries the outside or have you got around that?

 

We had too many issues with drying that way so we've gone to the other extreme and dry at 30-35 degrees over 3-4 weeks.

 

And in reply to woodworks, on our own log burner we'll burn anything up to 30, we just stack it by the side first. 15 is what I would like but we still see it as a luxury.

 

This depends on the size of the log, as the outside dries it becomes a poorer conductor, so the heat doesn't get to the inside to provide the energy needed to vaporise the moisture, what tends to happen is the cell water loses its viscosity as the log initially heats up, this then migrates rapidly through the grain and evaporates. The remaining "bound" water is associated with the actual cell structure by weak bonding forces and it is as this is vaporised that the log shrinks. This water is that at and below the fibre saturation point, about 25%mc. This is the water that takes time, and a bit more energy than the latent heat of vaporisation (AKA enthalpy of vaporisation about 0.75kwWh/kg) as the bonds have some energy requirement to break and this is reflected in the temperature being above the local boiling point.

 

So if you look at my graphs on the other thread you will see the initial drying can be very rapid, then it tails off to an equilibrium of around 15% in ambient air if not rewetted.

 

So the main answer is yes but I check weigh for the last hour or so to see if the log is still losing mass.

 

Energetically there is not much loss with burning wood at 30C, the thing is it is a bit more difficult to burn it smokelessly (which represents an energy loss as well as polluting with particulates).

[gdh]

This depends on the size of the log, as the outside dries it becomes a poorer conductor, so the heat doesn't get to the inside to provide the energy needed to vaporise the moisture, what tends to happen is the cell water loses its viscosity as the log initially heats up, this then migrates rapidly through the grain and evaporates. The remaining "bound" water is associated with the actual cell structure by weak bonding forces and it is as this is vaporised that the log shrinks. This water is that at and below the fibre saturation point, about 25%mc. This is the water that takes time, and a bit more energy than the latent heat of vaporisation (AKA enthalpy of vaporisation about 0.75kwWh/kg) as the bonds have some energy requirement to break and this is reflected in the temperature being above the local boiling point.

 

So if you look at my graphs on the other thread you will see the initial drying can be very rapid, then it tails off to an equilibrium of around 15% in ambient air if not rewetted.

 

So the main answer is yes but I check weigh for the last hour or so to see if the log is still losing mass.

 

Energetically there is not much loss with burning wood at 30C, the thing is it is a bit more difficult to burn it smokelessly (which represents an energy loss as well as polluting with particulates).

 

 

Thanks, I'll take a look at the graphs to.

[Alycidon]

Yes many are poor quality, been stacked outside uncovered for several months.

 

So would you expect the end-grain measurement to be less than a newly exposed centre measurement?

 

And what about the bark - seems to take a lot of drying out?

 

TIA

 

End grain measurements would depend on how the log has been stored. If exposed it will probably be higher than the center.

 

If the log has not been properly kiln dried then I would expect an end grain measurement to generally be lower than than the center of the log.

 

A

[openspaceman]

Thanks, I'll take a look at the graphs to.

 

NB my graphs are on a wet basis, i.e. the moisture content is expressed as the percent of water in the whole green weight.

 

I don't have a moisture meter but believe they are mostly designed for measuring joinery, which is generally expressed as the percentage of water compared with the oven dry weight. In my earlier post where I said 30C I meant 30% mc as measured by the meter

 

So if such a meter indicates 30% mc Dry Weight Basis then it will be equivalent to 23% in my graphs.

 

From what Cornish Wood Burner has said previously I would not expect such a meter to give good results above the fibre saturation point. See below, in round terms fibre saturation point is in the light blue region, pink is the sort area to aim for and black is dry enough to reabsorb moisture in ambient air. Burning wood in the dark blue region is likely to sizzle. A simple test is to try and suck through the end grain.

 

wet basis dry basis

60.00% 150.00%

55.00% 122.22%

50.00% 100.00%

45.00% 81.82%

40.00% 66.67%

35.00% 53.85%

30.00% 42.86%

25.00% 33.33%

23.08% 30.00%

20.00% 25.00%

15.00% 17.65%

10.00% 11.11%

5.00% 5.26%

0.00% 0.00%

[Logged On]

I've got a Stihl moisture meter. Brought some Larch home for burning over weekend, but it just doesn't seem to want to burn.

 

Yet the MM read the logs at anywhere between 8 and 16%??

 

Is Larch not a good wood to burn?

[RealEstate]

I've got a Stihl moisture meter. Brought some Larch home for burning over weekend, but it just doesn't seem to want to burn.

Yet the MM read the logs at anywhere between 8 and 16%??

Is Larch not a good wood to burn?

Needs to be well-seasoned.

A guide to which wood burns best in your wood burning stove

[cornish wood burner]

I've got a Stihl moisture meter. Brought some Larch home for burning over weekend, but it just doesn't seem to want to burn.

 

Yet the MM read the logs at anywhere between 8 and 16%??

 

Is Larch not a good wood to burn?

This is exactly the point Andrew and I are making. Just because your meter is saying 16% how do you know it's right.

I have burnt thousands of tonnes of larch in our wood chip boiler with no problems. Some short ends have found their way into my Rayburn again no problem. Simple enough to check your meter.

[Logged On]

Needs to be well-seasoned.

A guide to which wood burns best in your wood burning stove

 

So even after it's been through a kiln drier, it could still have a 'hidden' high moisture content??

[openspaceman]

So even after it's been through a kiln drier, it could still have a 'hidden' high moisture content??

 

I would doubt it, Have you split any?

 

It may be that conditions in the fire are wrong or the draw is poor.

 

You may simply need to have more surface area by splitting stuff much smaller.

[fonzy]

Nothing wrong with larch as long as it's under 25% moisture content (wet basis). Burn tonnes of it a year in ours and customers burners.