Using Resistance Moisture Meters

8/18/2019 Using Resistance Moisture Meters

1/6

USING RESISTANCE MOISTURE METERS

Alan Dlugasch Sales Customer ServiceDelmhorst Instrument Co

Towaco New Jersey

Introduction

I would like to begin by thanking the m emb ers of the We stern Dry Kiln Association forinviting someone from Delmhorst Instruments to speak today. I am aw are that most of youare professionals in this industry and hav e been working w ith kilns and drying lumb er fora long time. As a result some o f what I say to you today you may already know or feel iskind of basic information. I can assure you that from the m any phone calls that we receiveat Delmhorst, there are still many people working in this industry and also related

industries, who know w hat a mo isture meter is but are not sure of what, why and how itworks. M y goal today is to try and m ake some of these questions about m oisture metersclearer and better understood by you.

irst Some General Information about Moisture Meters

One of the most dramatic changes in the lumber and woo d products processing industriesover the last decade has been the increased emphasis on quality. The accelerateddem and for high quality lumb er products has probably never been greater. Everybod y,both dom estically and abroad is deman ding a higher quality product at the right mo isturecontent.

The go al of your drying operation should be to p roduce a consistent, high quality product,at a profit, that meets or exceeds custom er requirements. The m oisture meter is a toolthat, when used with proper techniques can help you do your p art in this process. Somewood experts have found that up to % of problems they find are typically related tomoisture content. Either the person drying the lum ber did not know the MC requ irementsof the customer o r the p roblem can be traced to faulty drying practices.

Since m oisture content is a key to qu ality, moisture meters, already in widespread use,have b ecome an essential tool in the sawmill, dry kiln, planer mill and right on through tothe finishing rooms in furniture plants. We emphasize the word tool because moisturemeters are just that. Meters in use today hav e man y valuable and obvious advantages,especially when used with the knowledge of how w ater moves in the wood. T hey alsohave some limitations. Meters, like most tools, are most effective when the userunderstands their capabilities.

The Moisture Meter as a Measuring Tool

There are two basic families of meters on the market today: the resistance or pin meter ,which is the focus of this presentation, and the capacitance or pin-less meter , alsoknow n as the electromagn etic wave meter. Both type s are an indirect, electronic methodof measuring a prop erty of the wood. The ov en test is still the primary method by w hichall electrical systems are calibrated.

Pin-less or capacitance m eters operate on the relation betw een the dielectric prop ertiesand the m oisture content of the wood. W hen the m eter is placed on the board surface,an electric field penetrates into the board anywh ere from Y2 to 1 . The m oisture content

Western Dry Kiln Association

May 2004


2/6

nearest the electrode or surface mo isture has the greatest effect on the reading. T his isdue to the fact that the sensor is closest to mo isture at this location and the signal tendsto weaken the farther it travels. The result is a reading that reflects a biased ave rage MC ,independen t of moisture distribution. Pin-less meters are conv enient to use. They w orkwell on boards of known uniform MC. They can be useful when spot checking for wetpockets, and in sorting ap plications. Pin-less m eters can not discriminate between surfaceand core MC and therefore do not provide information on distribution throughout the

thickness.

The ov en test is the most accurate way to measure M C if executed properly and underideal conditions. T he sam ple size, the quality of the equipm ent and the technician's skillall play a role in the end result. The ov en test can produce accuracy to within 1/10 o f 1%on a single sam ple, but this method has limited value in practice.

Why Should You Use a Meter

First The User Gets Immediate Results. The oven test can take up to 24 hours to

com plete. When checking a load, the user can go right to the slower drying quarter-sawnboards for a quick determination of the overall MC condition.Second There Is less Labor Involved. There is no need to cut and weigh samplespecimens.Third You H ave a Larger Sam ple. This makes it possible to use the whole board to takemany readings which give a more significant average and an indication of the uniformityor non-uniformity throughou t a board and of the entire load.Fourth You Can Locate Wet Spots. Much lumber is ripped into strips for flooring andfurniture so it becomes desirable to know the MC of a ripp ing, rather than the averagecross section of the whole board.

And the Fifth Reason to Use a Meter Is They Are Easy to Use. They are fast, accurateand corrections are made easily through the m eter's firmware.

All considered, the resistance m oisture meter can be the m ost practical and effective toolto measure M C, especially in applications where a m oisture gradient, the difference in MCbetween the inner and outer portion of a board, m ay be present.

Next Some Important Information about the Pin Meter

The Pin M eters Principle of Op eration – Pin m eters indicate M C as a % of the oven-dry

weight of the wood. They use the relationship between the DC resistance and themoisture content of the wood. T he m eter uses the wood as an element in the circuit, bydriving a set of pins or electrodes into it. This method w orks well because m oistureis an excellent conductor of electricity and dry woo d is an effective insulator.

The relationship betwe en the electrical resistance of the wood and its mo isture contentis not in a one to one p roportion and here lies the advan tage: for a small change in MC,there is a tremend ous change in resistance. For examp le, at 8% MC in Douglas Fir, theresistance is approximately 5000 megohms, while at 10% the resistance is only 600megohms.

The P in Meters Range – T hey work be st in the range of the fiber saturation point (25-30% ) down to about 6% . Fiber saturation is the MC at which the cell walls are saturatedwith bound water but no free water remains in the cell cavities. As you know a woodcell will not shrink un til it reaches fiber saturation, wh en large chang es in man y ph ysicaland m echanical properties of wood begin to take place.

We stern Dry Kiln Association

May 2004


3/6

It is difficult to make accurate, repeatable readings below 6% because of the highresistance in very dry wood. Due to the presence of free moisture , readings above 30%are only qualitative indications that the wood is gaining or losing moisture over time.These readings are v alid if, at elevated wo od temp eratures, the temperature-correctedreading falls below 30%.

Wh at is the Accuracy of a Pin Meter?: T he electrical accuracy of the meters is +/- 0 .2 ofthe indicated reading. The secondary calibration is better than 0.5% between 6 and 12%MC , better than 1.0% at 20% and better than 2% at fiber saturation point. The user shouldcheck the accu racy of his m eter periodically either through an internal calibration checkif the meter has this feature, or with an external resistance standard supplied by themanufacturer.

Origin of the Original Calibration of the Pin Meter: The original USDA calibration wasmade on a very large number of rather thin samples of Douglas-fir, which wereconditioned at various levels of temp erature and relative hum idity to achieve uniformmoisture distribution. The electrode used was fitted with four non-insulated pins thatpenetrated a m aximum of 5/16 t . These op timum laboratory conditions are certainly notoften found in the field. Here is where a big pa rt of the challenge com es into play whenusing a pin m eter. You as the kiln operator m ust develop the skills and kn owledge in theuse of a moisture meter to obtain the best MC readings.

Primary Factors That Affect the Moisture Meter Readings Besides the MC

First, The Mo isture Gradient and Th ickness – This is m ost critical during drying. If the MCof a board could be guaranteed to be un iform throughout, thickness would not be a factorin accuracy. But because of the norma l variations and gradients that exist when dryinglumber, we m ust consider thickness and pin pen etration in order for the reading to be anaccurate estimate of the average M C. A no rmal gradient develops from the w etter coreto the drier surface. Two or three quick readings at different levels of penetration, with pinsthat have insulated shanks, will give a goo d idea of the gradient and how the wood isdrying. Any changes in these shell and core readings are a true indication of a changein MC . O ne of the m ain challenges of the kiln operator is to relieve the board stresses bycarefully bringing these readings close together by the end of the charge.

Wh ere is the best place to take readings on a board? Start with three readings: one ateach end, about 12-18 in from the end, and one in the center of the board. All readings

should be taken in the m iddle 1/3 r portion. Dep ending on the variation among the initialreadings, you may have to take more in order to have reliable info on the board'suniformity.

How far do we drive the pins into the wood? The famous 1/5 t rule states that theaverage moisture content of a 12 inch board section is located at 1/4 to 1 15 t the thicknesson boards that are d ried below the fiber saturation point, even at the core. It 's easy toprove this rule. Simp ly drive an electrode with insulated pins into a sm all sample at 1/4increments and average the four readings. Do an ov en test on the samp le. The resultshould be within A of 1% of the meter's average.

When ma king hot checks in the kiln following these rules will produce a better and moreaccurate reading. First, when samp ling hot lumb er a deep er pinning depth p rovides abetter estimate of final MC . As a result, be sure to drive p ins as close to the center of theboard as possible. Remem ber the 115 t rule does not apply to edge readings. Second,remem ber on thick lumber or when a significant moisture gradient is present, the pinningdepth is even m ore critical.


8

May 2004


4/6

Next, The Effect of Wood Temperature – After MC and gradient, temperature has themost imp act on the accuracy of the meter readings. As the wood temperature increases,its electrical resistance decreases and the indicated M C rises. The lower the temp erature,the lower the indicated MC. M eters are generally calibrated at 70°F. W e usually app ly acorrection if the wood tem perature is outside of the 50°F-90°F range. This correction ismad e through the digital meter's firmw are. If you are using an analog m eter, a chart orslide rule is supplied by the meter manufacturer.

When taking a moisture meter from room temperature into a hot kiln remember thefollowing:

One -If the m eter is colder than the dew p t. temperature of the warm air; themo isture will condense on the cold equipm ent.

Two-T he condensation m ay cause an erratic reading. Three-Low MC 's cannot be measured until the moisture is evap orated from the

meter, it may take several hours to ev aporate. nd four, the recomm endation is don't take the meters into a hot kiln and exp ectto take accurate readings, unless the meter is warmed to approximately thetemp erature of the kiln.

The E ffect of Sp ecies – Because the electrical characteristics of different woo d spe ciesvary, all species read differently at given m oisture content. A correction is required whenusing the m eter on sp ecies other than the calibration standard - Doug las-fir. Correctionfactors for other species are either programm ed individually in to the m eter or, can befound in a chart that is supplied with the meter. G roup sp ecies corrections, while not asaccurate as individual ones, are u seful, especially in cases where it m ay be d ifficult toidentify specific species for marketing purpo ses. The SP F correction is a good exam ple.

This group correction was adopted because of the difficulty to distinguish visuallybetween Spruce and Balsam Fir lumber. The group SPF correction is strongly impa ctedby the correction for Spruce which makes up the largest portion of this mixture andhapp ens to have the largest correction factors .

You m ay com e across a species for which a correction is simply not available. Dependingon the level of accuracy required, take most readings below 1 0% at face value as thecorrection is usually small. But you can also use the m eter to establish the EM C of thespecies in a particular environment. Knowing the actual MC itself is not always asimportant as allowing the wood to attain its EMC level to w here it won't change dimension,

crack or split. The equilibrium moisture content is num erically equa l to the MC that woodwill eventually achieve when exp osed to a given hum idity

What is meant by 2-Pin/ 4-Pin Electrode? – As stated earlier, the original calibration wasdeve loped with a 4-p in electrode whose pins were not insulated. Reference to a 4-p incorrection means 2 or 4 pins that are not insulated; the 2-pin correction refers to 2insulated pins. M ost measurem ents today are either taken with 2 insulated pins that arefitted to a ham mer-type electrode and driven into the board, or with 2 non-insulated pinsthat are built into the top of the hand-held meter. Th e insulated pins read lower than thethan the non-insulated ones. M ost meters can m ake the necessary corrections through

firmware.

The E ffect of Grain Direction - The electrode pins m ust be oriented parallel to the grainsince this is how the original calibration was made. The resistance of the wood is greateracross the grain, and a lower reading will result if the pins are driven into the board acrossthe grain. This difference is slight at 10% and below, but around 20 % , the reading can beas much as 2% lower.

W estern Dry Kiln Association

9

May 2004


5/6

Basic Care of Your Meter – The meter is a precision instrument and so somemaintenance is necessary to keep it in good working condition. Keep the batteries freshand charged, store the meter in its case and in a clean, dry room. Tighten the pins'retainers to insure good contact, and replace pins whose insulation has worn. Have itfactory checked at least every 2-3 years.

The K nowledgeable User – M oisture m eters are simp le to use and with the electronicsavailable to us today, will becom e ev en easier. It is still up to the user to m aximize themeter.for best accuracy. Check the ca libration. Input the prop er settings for temp erature,species, and pins. Align the pins parallel to the grain. Be aware of possible surfacemoisture. Drive insulated pins to the desired depth, non-insulated pins to their fullpenetration.

The Meter as a Data cquisition Tool

While the basic measuring principles have not changed much o ver the years, the bellsand whistles available with meters certainly have. These changes are probably most

significant in the area of data storage and reporting. The use of statistics and p redictivetechniques with moisture meters is really a topic that requires its own time slot.I'll touch on this area briefly since it is such an integral part of meters in use today. We canuse the m eter to collect and process a large number o f data in a very short time. Eitheras a stand-alone device, or in combination with a PC, the m eter allows us to obtain moreand more information on a population of boards and to process it efficiently, therebyincreasing the value o f the equipm ent.

The m eter does all of the number crunching but the user must know how to p roperly useand interpret the basic statistical functions that some meters offer. For example, the

estimated average of a board or load is mo st useful when comp ared against standarddeviation of the same batch. T he standard deviation is a statistical param eter which givesthe user, in one single figure, an indication of the range of readings w hich make u p theave rage. The low er the standard deviation, the closer all readings are to the average,indicating a uniform distribution of M C througho ut the batch or load. Two loads o f lumbermay h ave the sam e average, but if the values for standard deviation of each load are notclose, it may b e an indication that one load is far more uniform in MC than the other. Keepthis in m ind if, for ex. comp aring kiln pe rformance or ev aluating a new drying schedule.

On ce the average and standard deviation are known, one can estimate the average of an

entire pop ulation of boards or p redict whether a charge mee ts a specific standard. Thiscan be made within a certain degree of confidence by using the confidence intervalfunction. Its values are between 0.0 and 1.0 and again, a low num ber would indicate amore uniform distribution of mo isture.

The use of a data collection type meter, a PC, and a spreadsh eet program certainly makeus all smarter by the num bers and can serve as a good m anagem ent tool. Rather thanfall to the initial tendency of ov er processing and analyzing com puter generated num bersand reports, we can make best use of this capability by using this information inconjunction with the other basic elements of the wood/moisture relationship.


10

May 2004


6/6

Conclusion

The resistance moisture meter is a proven, indispensable tool in all phases of lumberproduction and wood processing. It is a precision instrument that provides fast, accuratereadings. Optimize its effectiveness by following the manufacturer s procedures, know themeter s capabilities, as well as its limitations, and use it with your knowledge andexperience.

Thank you for your attention and I hope that some of this information about moisturemeters will prove useful in your goal to produce the best possible product.

Western Dry Kiln Association

May 2004

Documents

Using Resistance Moisture Meters