Firewood moisture meters, and "wet basis" vs "dry basis"

[Stubby]

2 hours ago, Ontario Firewood Resource said:

Cutting shorter lengths helps it dry faster. It seems like in the UK, you guys cut shorter logs than in North America. we cut 16 inch as a standard for load amount calculation and 16 inch is what most wood stoves can fit. Some are made to fit 20 inch or more, some are small and only fit 12 inch. If you deal with wood long enough, you wont need a moisture meter. If I'm loading a delivery, I sometimes see oak logs or other species that haven't full dried yet because theyre still heavy, unlike the rest of the logs. Kiln-dried wood is not better than naturally seasoned woo. Its too dry. I had a customer who got a load of kiln dried for 33% less than the average firewood cost and he called me to order wood after he ran out because he learned what is better

I agree about kiln dried not being as good as air dried . Kiln dried knocks the guts out of the wood .  If I was offered 20% kiln dried and 20% air dried I would sooner have the air dried . 

[Paul in the woods]

I've made several amendments to the proposed sticky to include a link to woodsure, add some calcs, explain a little about the meters and add some examples. I'll post up a new thread later and ask for it to be stickied.

 

Firewood moisture content

 

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 but even the ones now 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%

Edited February 28, 2021 by Paul in the woods
Latest updates

[Vedhoggar]

53 minutes ago, Paul in the woods said:

I've made a small amendment to the proposed sticky to include a link to woodsure. I'll post up a new thread later and ask for it to be stickied.

 

Firewood moisture content

 

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 wet basis but wood for other uses such as construction may be measured on dry basis. Many wood moisture meters do not state what basis they use and are likely to measure on a dry basis. The new law does not mention what basis is used but as it is relating to fuel it can be assumed the moisture content should be 20% wet basis.
 
If you have a cheap moisture meter which measures on a dry basis it is worth noting 25% dry basis = 20% wet basis.
 
Some useful calculations

Wet basis moisture content = weight water / ( weight dry wood + weight water) * 100

Dry basis moisture content = (weight water / dry weight) * 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

As many householders probably struggle with basic maths why not spell it out as:

weight of wet log minus weight of dry log divided by weight of dry log x 100 = dry weight expressed as a percentage; for wet weight expressed as a percentage just divide by wet weight instead of dry weight. Probably a good idea to also include a simple, but realistic, example of what is likely to be measured.

[Paul in the woods]

I've amended my post with the latest comments. I've included a bit more about the meters, added some calculations and added an example.

 

[Ontario Firewood Resource]

On 28/02/2021 at 05:46, Paul in the woods said:

I've made several amendments to the proposed sticky to include a link to woodsure, add some calcs, explain a little about the meters and add some examples. I'll post up a new thread later and ask for it to be stickied.

 

Firewood moisture content

 

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 but even the ones now 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%

Good article