Firewood Moisture Content

[Paul in the woods]

The Air Quality (Domestic Solid Fuels Standards) (England) Regulations coming into force soon stop small quantities of firewood from being supplied where the moisture content is above 20%. For details see: https://woodsure.co.uk/new-regulations-confirmed/

 

Moisture content of a piece of wood is defined in two ways:
 
Wet basis which is a measure of the water in a log, expressed as the weight of water as a percentage of the wet/unseasoned log. (i.e. wood and water)
 
Dry basis which is a measure of the water in a log, expressed as the weight of water as a percentage of the completely dry log. (i.e. just the wood)
 

Firewood is measured on a wet basis and although the new law does not mention what basis is used it is relating to fuel so it can be assumed the moisture content should be 20% wet basis.

 

One of the most common ways of measuring the moisture content is by using a pin moisture meter. These meters were originally sold for measuring materials used in construction where moisture is likely to be measured on a dry basis. Many wood moisture meters do not state what basis they use and even the ones aimed at the firewood market are likely to measure on a dry basis.

 

If you have a cheap moisture meter which measures on a dry basis it is worth noting 25% dry basis = 20% wet basis.
 

Further information and useful calculations:

 

Wet basis moisture content 

= (weight of water / weight of wet wood) * 100

= (weight of water / (weight of dry wood + weight of water)) * 100

= ((weight of wet wood - weight of dry wood) / (weight of wet wood)) * 100

 

Dry basis moisture content

= (weight of water / weight of dry wood) * 100

= (weight of water / (weight of wet wood - weight of water)) * 100

= ((weight of wet wood - weight of dry wood) / weight of dry wood) * 100

 

You can convert from dry basis to wet basis using the following formula:

Wet basis moisture content = (100 * Dry basis moisture content) / ( 100 + Dry basis moisture content)

 

Further information, including how to test the moisture content by drying test pieces of wood in an oven, can be found in the following link. Note, this method could be used to confirm what basis a moisture meter uses to measure: https://www.forestresearch.gov.uk/documents/1979/FR_BEC_Testing_Moisture_Content_Simple_method_2011.pdf

 

Example

If we take a freshly cut wet log with a wet weight of 2000g and we remove all the moisture until we are left with 1200g of dry wood we have removed 800g of water. So:

 

Wet basis the moisture would be = (weight of water / weight of wet wood) * 100 = (800 / 2000) * 100 = 40%

 

Dry basis the moisture would be = (weight of water / weight of dry wood ) * 100 = ( 800 / 1200 ) =  66.7%

 

You can convert from dry basis to wet basis using the following formula:  Mwet = (100 * Mdry) / (100 + Mdry)

 

In the above example is (100 * 66.7) / (100 + 66.7) = 40%

[Logburner]

Thank you, Paul in the Woods.  Really helpful.  It's amazing if moisture meters don't tell you which basis they are using.

 

Also there seem to be wildly different readings for different moisture meters as I mentioned in my entry last Thursday on Moisture Meters and Kiln Dried Wood.  I wondered if you could add any comments on the different readings I was getting.

 

Thanks

 

Logburner (Shez)

[AndyChalmers]

Im lost in the calculations.  So basically anything my stihl moisture meter says is 25% or less is ok to sell.  So I can carry a peice of paper to explain the differences etc if a customer asks which in essence explains why its ok to have 25% reading on general moisture meters when the rules say 20%.  The letter can explain everything and calculations to prove im not winding them up.  Sorted.

   Spanner in the works.  My own felled timber is always dry throughout however I've just bought in some kiln dried oak, 0% dry moisture on the outside and 30% in the middle.  Average would be below 25% at a guess but what are the rules on this? 

[Paul in the woods]

10 minutes ago, AndyChalmers said:

So basically anything my stihl moisture meter says is 25% or less is ok to sell.  So I can carry a peice of paper to explain the differences etc if a customer asks which in essence explains why its ok to have 25% reading on general moisture meters when the rules say 20%.  The letter can explain everything and calculations to prove I'm not winding them up.  Sorted.

This is where I feel / hope Woodsure will step in to help. It's highly likely your meter is measuring on a dry basis so anything under 25% dry basis is under the magical 20% wet basis. However, the only way to know for sure is to ask Stihl. (It would be very helpful if anyone who does get an answer from their manufacturer posts up what they say).

 

I wrote the post to explain there is and will be confusion for everyone until things are clarified by Woodsure.

 

As for the kiln dried oak, what's your meter's minimum measurement? I think some will only read down to about 6% so the wood may not be zero on the outside. I think the only way you'll get an accurate measurement is to try the oven dry method.

[arboriculturist]

As I've said previously - if a MM cost lest than £ 250 - 300 it will measure on Dry Basis.

Woodsure wont be able to help you.

[Logburner]

AndyChalmers, your 0% on the outside and 30% on the inside is a little worrying, though as you say there would most likely be an overall moisture content below 25%.  I can see that drying on the outside and leaving rather too moist on the inside could be a real temptation for kiln-dried suppliers  -  greater throughput, lower cost etc, especially in a month such as February when most seem to have run out of their seasoned wood.  Some suppliers, are clearly very careful and have truly impressive descriptions on their websites, including the splitting of samples right across each batch to be sure that the internal moisture levels are low.

 

As we speak, I have my kitchen oven filled (well not 'filled'!) with my 'kiln dried ash that won't burn', testing for overall moisture percent in the manner recommended.

 

I see splits emerging that weren't there before, but have yet to do the second weighing.  It's a great learning process!

[Gav73]

   Spanner in the works.  My own felled timber is always dry throughout however I've just bought in some kiln dried oak, 0% dry moisture on the outside and 30% in the middle.  Average would be below 25% at a guess but what are the rules on this? 

So according to the video on the woodsure website, if you took a few of your kiln dried oak pieces, split them, measured the centre and then averaged the results (assuming they are all 30%) then they wouldn’t be within the rules.

But I’ve also read somewhere the measures would be the average reading of both ends and the centre of the split piece of firewood if woodsure were to conduct the test themselves. (Think there’s a thread on here somewhere that mentions it)

[Justme]

I think woodsure are fudging the specs/testing.

 

If you have a limit then the totality of the log needs to be under that limit.

 

So the log needs mincing, mixed, sampled & tested.

To be truly scientific / fair. 

[openspaceman]

28 minutes ago, Justme said:

If you have a limit then the totality of the log needs to be under that limit.

I'm not convinced of that in a single log but persuade me.

[Justme]

15 minutes ago, openspaceman said:

I'm not convinced of that in a single log but persuade me.

How else can it be fair? 

 

I am not saying all the log needs to be under the limit on spot tests.

 

I am saying that the actual average is under not just a few select tested spots. 

 

Think about how nutrition is tested

 

Blend total meal,sample and test.