Moisture Meter accuracy

[Darius Ferlas]

I'm looking for some wisdom fo the Creekers, collective and individual.

I got some free green cherry lumber a couple months ago so I decided to also build a small solar kiln, and then a mini Mini-Ligno E/D (http://www.rockler.com/product.cfm?page=18085&filter=moisture%20meter) a few weeks ago. The kiln is only partially finished (only the floor is insulated, no mechanical exhaust or inside air movement yet) but due to weather and storage issue I decided to put the lumber inside it. The highest recorded temp so far was 55(130F).

The MM is connected via two deck screw probes. They are driven about 1 inch deep, from an edge of one board at a 45% angle and are spaced at exactly the same distance as the meter's pins. The probes are inside a board which is about 1/3 from the floor of the kiln. A pair of wires is connected to the probes, with the other ends fed through the kiln's wall to the outside. I connect the outside ends to the pins to get readings.

At this point the lumber went down from 25% MC when I bought the MM to 14% as of yesterday. My drying rate spreadsheet tells me that the 8/4 is drying a little too fast, although I see no checking or other issues. AFAIK the rate should be maximum 2.8% for 8/4 cherry. I also noticed that the MC, while having the constant tendency to go down, fluctuates in this pattern recorded over the last few days:

18-17-16-16-18-17-16-16-15-15-14-13-15-15-14.

Considering this I have some questions:

1. does any of the SMC members have anything to say about the accuracy of this particular meter (it has 5 good reviews on Rockler)
2. Would the up and down fluctuations be indicative of some issue with the drying process.
3. Are the probes plus wire skewing my readings?
5. Would it be safe to take the lumber out of the kiln when it reaches 10% MC as I feel that going lower is almost pointless since the wood will reabsorb some moisture once it's out of the kiln?
6. Any other suggestions, tips and tricks?

Thanks

Darius

[Lee Schierer]

3. Are the probes plus wire skewing my readings?


I would suspect that the wire length and resistance would affect your readings to some degree. I would suggest taking a reading in the same piece of wood where you have your remote probes connected to see what the meter reads directly from the wood and then immediately taking a reading with your remote probes. If the readings are the same then the siring is not causing a difference. IF they are different, then you remote reading is likely not as accurate as the meter.

[John Coloccia]

These meters (by these I mean all meters except maybe high end meters) do a great job if distinguishing between green lumber and dry lumber, finding water damage, or knowing when something's hit equillibrium. If you really want to know an accurate MC, the oven method is better. I don't know how much I would trust any of the meters to give a reliable, linear reading from 10% to 30%, for example.

I just use mine as a guide. I know about what all the species I use read when they're at equilibrium, and I know what equilibrium is from my conditions, so I have a good idea of when it's there. Other than that, I don't really believe it enough to calculate a drying rate or anything like that.

Maybe others gave found them to be more accurate than I have. I'm only a sample size of one person with a couple of meters!

[Rod Sheridan]

Hi, the wire resistance won't affect the readings on the meter as the wood resistance is millions of times higher than the wire resistance.

Regards, Rod.

[Darius Ferlas]

Thanks for the replies.

Lee,

Using lumber known to be dry I compared the reading with with wire and directly by pins. Out of about 20 random tests only two pairs of comparisons were off by 0.1%. The readings of that wood were in the range between 8 and 9% MC.

John,

I'm not sure I really need super accuracy. Would you say then that a reading of 10% (once I reach it) would put me into a comfort zone?

I was kinda worried that perhaps a screw driven into wood creates a sort of an escape route for moisture along the length of the screw and thus the readings could be... well... screwed by values too large to ignore.

[Scott T Smith]

Darius, let me preface my response by stating that I operate a kiln with a remote monitoring system.

What you have proposed will not provide accurate readings (as described), but it is possible to make a simple modification that will address this.

There is typically a moisture gradient between the center of a board and the shell of the board. The faster the drying rate, the greater the delta. As an example, several days into the drying cycle the shell of the board may be at 35% moisture content, and the core of the board is at 37% MC. Part of a good drying process involves tracking the drying rate and gradient, and adjusting the rate to make sure that the gradient is not too large.

The apparatus that you described will only provide accurate moisture content readings of the shell of the board. In order to obtain accurate core readings, you will need to insulate the probes that you're driving into the boards, so that only the tip of the pin can take the reading. Commercial kiln probes have an epoxy paint coating on them, with only the tip of the probe having bare metal.

It's a good idea to use two sets of probes - a core probe and a shell probe. The core probes are driven to the core of the board (1/2" deep for 4/4, 7/8" deep for 8/4, etc. The pins for reading the shell MC's can be 1/4" long and not insulated.

The probes should be driven in from the face of the board, not from the edge.

If you're seeing less than a 1% delta between your core and shell readings from green down to dry, something is not right. Typically you should experience a 1 - 2% delta between the readings; less is better in terms of drying quality.

You will need to drive your pins into the board at the exact same spacing as the pins used on your meter (very important!), otherwise you will not receive a consistent or accurate reading. You mentioned this in your original post but I wanted to reinforce it's importance.

Something else that is important, moisture meters are not very accurate above the fiber saturation point of the wood, thus above 30% MC the meter is not as accurate as other methods.

As Rod indicated, the resistance of your probe wire should not be an issue.

Because each tree is different, each board is different from a cellular (and drying) perspective. I typically use six sets of probes during my kiln cycles (in six different boards in six different parts of the kiln), and there will be some variance across the readings during the drying cycle. Readings are measured and recorded every 15 minutes, which provides me with easy visibility to the drying curve. Some interesting observations: Quartersawn boards dry slower than flat sawn boards, and this is obvious during the drying cycle. Boards that have a varied grain pattern - ie one edge of the board is riftsawn and one edge is flatsawn - will not only dry at a slightly different rate, the drying-related shrinkage is different from edge to edge as well.

John is correct in that the "oven method" is the best way to measure moisture content during a kiln run. Meters that are made for kiln operators (such as the Delmhorst models) are an excellent choice for consistency and accuracy.

Regarding the drying rate, the primary damage to lumber during the drying process occurs from green down to 35%; however this damage is usually not revealed until the lumber is below 25%MC. Your original post indicates that your lumber was at 25%MC when it went into your kiln; below 25% the rate of drying is less important and you generally will not see degrade due to an elevated drying rate.

[Darius Ferlas]

Scott, thank you very much for the great tips.
It all makes sense to me, but it does get experience to actually know what works what doesn't. Luckily, you were kind enough to share yours.

Rainy day today and tomorrow here, so I will get on all the suggested improvements as soon as it clears.