It’s all about the MC
By Peter Garrahan
Kiln operators often wonder which handheld moisture meter is better: pin or pinless. But for the majority of mills, the answer is both.
Every drying decision is influenced by the measured or perceived moisture content (MC) of the material at every stage of the operation. With that in mind, we can never have too much good-quality information. The objective of this article is to provide an overview of the various MC-sensing technologies available to assist kiln operators in doing a better job, both in terms of productivity and lumber quality.
There is a wide range of technologies available to assess the MC condition of wood at every stage of the operation. The information from these systems can be used in many ways: optimizing the drying process, mitigating problems or quality assurance.
Trouble arises when we collect inappropriate data or misinterpret data concerning MC. For instance, just because your moisture meter displays values to 1/10 of a per cent MC, it does not mean the individual reading is that precise. A bit of information regarding the merits and limitations of the various technologies may help when defining your needs.
Direct versus indirect MC measurement
By definition, MC in solid wood products is defined as the relative amount of water present in the wood, to the amount of wood. The formula is as follows:
The most accurate means of measuring MC is by conducting an oven-dry test and using the above formula. This is referred to as a “direct method” of MC determination because we are actually measuring the amount of water present by taking a weight of the sample before and after we dry it. “Indirect methods” are those where we measure a property of the wood affected by the presence of water and, from existing data, infer what the MC is. In many cases, this is an electrical property, but can also be done from physical measurements (such as green weight) and using wood properties to estimate the MC.
You may be able to estimate someone’s weight if given their height and waist size, but we know there would be a margin of error associated with that. The same situation exists for all indirect methods of MC determination.
Green MC determination
Knowledge of the initial MC and MC patterns can be used to modify or select the most appropriate drying schedule, or to segregate material into green MC categories for drying separately.
Many people will use handheld moisture meters to assess the green MC of material. Since all handheld moisture meters are designed for application on material below the fibre saturation point (FSP ≈ 25 - 30% MC), these meters have a very definite limitation. If the material has a mix of green and semi-dry material, handheld meters might be useful in identifying the drier material or getting a rough idea of the amount of semi-dry material in the load. Readings indicating a board is above the FSP should only be considered as an indication the piece is still wet. Differentiating between pieces at 40 per cent MC versus 50 or 60 is not possible. These differentiations become even more complicated when working under winter conditions, as the electrical properties of ice versus liquid water are quite different.
There are a number of in-line green MC detectors on the market and most of these have been designed to facilitate sorting the green material into different MC/drying categories. In this application, the absolute MC of the piece may not be as important as the relative difference between pieces. If the objective is to sort material into “high initial MC” versus “low initial MC” categories, we are not too concerned about a small margin of error.
In-line, green MC detectors include those that rely on an electrical property of the wood (usually a non-invasive dielectric sensor) as well as weight-based systems. Weight-based systems can either operate in a “heavy” versus a “light” manner or use some properties of the wood, such as specific gravity, to estimate the MC similar to the human weight analogy mentioned previously. With either type of instrument - as long as we don’t go beyond the limitations of the equipment - we can achieve a separation that will potentially result in improved productivity, better final MC uniformity, and/or reduced drying degrade.
Though there are no systems available to reliably sort material into very small MC categories - such as 30 to 40 versus 40 to 50 - we don’t usually require such a precise separation to achieve the benefits mentioned above.
If you cannot justify installing a green MC monitoring system, the next best thing is to closely monitor the species mix and history of material being supplied to the kiln. A good kiln operator can modify schedules if, for example, they know they have a mix of old and fresh material going into the kiln.
In-kiln MC determination
In the same manner that green MC information is useful, MC monitoring while the material is drying can be used to our advantage. It allows the kiln operator to make decisions such as when to go to more aggressive drying conditions and, most importantly, when to stop drying and remove the material. For softwood dimension lumber, the ability to shut the kiln down more precisely will provide tremendous benefits, from shortening the drying time, to reducing over-drying and the associated increase in drying degrade.
The newest generation of in-kiln MC detectors uses a dielectric principle with probes placed through the sticker openings in bundles distributed throughout the kiln. There has not been a lot of independent research conducted to verify the accuracy and/or range of this equipment. However, there are currently at least three manufacturers of this type of equipment, and they have become popular and accepted in the softwood lumber drying sector.
As with any new technology, it takes some time for mill personnel to get comfortable with it and achieve the maximum benefit. Taking the time to carefully correlate the readings from the in-kiln device, to readings taken later (i.e. at the planer mill), is crucial in order to define the targets for a particular kiln and product.
Some kiln controllers are equipped with DC-resistance probes placed in individual boards scattered around the load. Although DC-resistance is not accurate at high MCs, it can be used to help make drying schedule changes towards the end of the cycle, and to assist with determining the shutdown point.
Handheld moisture meters
Handheld moisture meters have been in existence longer than any other type of indirect MC-sensing technology so we know more about how they perform under a wide range of operating conditions and wood species. Handheld meters are broken into two main categories. The first is widely known as DC-resistance and involves driving pins into the wood. The second category is broadly referred to as dielectric meters, and since they do not require probes, are often called “pinless” meters.
Both handheld meter types are affected by wood properties and wood temperature. In pinless meters, it is the specific gravity (or density) of the wood that impacts the meter’s response. In DC-resistance meters, there is a species adjustment, which is due to a combination of factors including chemistry of the wood.
Readings on both meters are affected by wood temperature with the dielectric readings being affected to a lesser amount. Temperature corrections for both meter types become more important when obtaining readings over a wide range of operating conditions. There are a number of manufacturers for both meter types. (Technical data is available from independent sources such as FPInnovations and the U.S. Forest Products Laboratory.)
In general, handheld meters are considered to be accurate within +/- one to two per cent MC within their intended operating range. The principles under which they operate dictate the operating range is from a low of about five to six per cent to a high of about 25 per cent.
Handheld meters can be used in the kiln to conduct a “hot check” to determine when to shut the kiln down. This is less popular these days as it requires shutting the kiln down to cool if off enough for the operator to enter. Handheld meters are more widely used after the material comes out of the kiln to test MC in the yard or planer mill. Regardless of the environment, it is important to use correction factors to adjust for the components mentioned above.
In-line moisture meters
There are several manufacturers of in-line moisture meters all operating on the dielectric principle. This is not to infer they are all the same, as the term dielectric describes a broad range of electrical properties. In-line meters are typically installed at the planer mill where they can be used to scan every piece of lumber entering that part of the operation. As with handheld dielectric meters, the readings are affected by wood density and temperature. Since planer mills are typically processing one product for an extended period, both of these adjustments do not need to be made on a board-by-board basis.
In-line meters scan every board and the readings are taken when the product is relatively stable. Given proper application, maintenance and monitoring, the readings taken with these types of meters can be the most comprehensive and accurate of any indirect MC measurements. One of the main advantages from applying this type of meter is to use it to correlate other MC-sensing systems including hot checks at the kiln and in-kiln MC sensors. The more obvious application for in-line meters is as part of a QC program to assure all material leaving the mills meets the MC targets for that particular product and customer.
The main advantage of any indirect method for MC sensing is the ability to collect data on a large number of pieces from the material being tested. Although individual readings may have a certain margin of error associated with them, we can overcome this limitation by focusing on the statistics for a group. The table below shows a hypothetical situation to demonstrate this point. In this case, the average for the 10 meter readings is very close to the average oven-dry MC, even though individual meter readings had errors of over two per cent MC.
There are many technologies and ample supporting information available these days to help assess the MC condition of wood at various stages. It is worth reviewing from time-to-time, to see if you should be taking advantage of more of these technologies to achieve improvements in your drying operations.