Date: November 21, 2007 By: Lonnie Haughton Subject: Significance and Proper Use of Pin-Style Moisture Meters

Transcription

1 318 Harrison Street, Suite 103 Oakland, CA (510) FAX (510) Date: November 21, 2007 By: Lonnie Haughton Subject: Significance and Proper Use of Pin-Style Moisture Meters Hand-held meters provide a rapid means of sampling moisture content of wood-based materials during and after processing to maintain quality assurance and compliance with standards. However, these measurements are inferential, that is, electrical parameters are measured and compared against a calibration curve to obtain an indirect measure of moisture content. The electrical measurements are influenced by actual moisture content, a number of other wood variables, environmental conditions, geometry of the measuring probe, and design of the meter. The maximum accuracy can only be obtained by an awareness of the effect of each parameter on the meter output and correction of readings as specified by these test methods. ASTM D 4444 (Standard Test Methods for Use and Calibration of Hand-Held Moisture Meters) Handheld Pin-Style Moisture Meters Manufactured by Delmhorst ( Principals. Architects & General Contractors.. Richard Avelar, AIA Founder Michael Gilmore, G.C. Ralph McDaniel, AIA Ken Kosloff, G.C. Daniel Chekene, AIA Gary M. Garcia, AIA David M. Field, AIA Steve Penland, G.C. Joel Agnello, AIA Timothy Stokes, G.C. Eric Archuletta, G.C. Ed Vazquez, G.C. Joseph G. Garcia, AIA Lonnie Haughton, G.C. Greg Cole, G.C. Gregg dehaan, G.C.

2 Page 2 of 5 Pin-style conductance (or resistance ) meters measure ionic conductance between points of applied direct-current electrical voltage. The scales for these handheld meters are calibrated to provide moisture content readings (oven-dry mass basis) for a particular wood species and specific reference temperature used to calibrate the meter. Depending on the wood species being tested, the range of moisture content (MC) that can be detected accurately by such meters lies between a minimum of 6%-7% MC and a maximum of 25%-27% MC, which represents the typical fiber saturation point for most wood species. While the scales for most moisture meters do provide readings that extend above this 25%-27% MC fiber saturation limit, the higher readings only allow qualitative evaluation of how much water is contained within the test specimens and do not imply quantitative accuracy or reliability of the measurements. Other factors that can introduce some degree of error into the meter readings are: Temperature conductance meters are calibrated to a specific temperature (e.g., 25ºC or 68ºF). For other temperature conditions, small MC corrections, as published by the meter manufacturer, are necessary for close accuracy; however, for common forensic purposes the qualitative readings produced by unadjusted meters are sufficiently accurate to give a useful understanding of existing conditions. (Exception: moisture metering of partially frozen materials can produce widely varying results of little usefulness.) Species conductance meters are calibrated to a specific wood species. For other species, small MC corrections, as published by the meter manufacturer, are necessary for close accuracy; however, for common forensic purposes the qualitative readings produced by unadjusted meters are sufficiently accurate to give a useful understanding of existing conditions in wood members. Chemicals Wood products that have been treated with preservatives, fire retardants, or dimensional stabilization agents may give abnormal readings (usually high). Adhesives The adhesives or binders added to reconstituted wood products such as oriented strandboard (OSB) may cause abnormally high readings; therefore, repeated measurements should be taken to produce an average reading that has increased qualitative value. Non-insulated pins The use of non-insulated pins will bias the reading toward the highest moisture content in contact with the pins. Pin orientation ASTM D 4444 states, Take all readings with the pins aligned so that the current flow is parallel to the grain. (Note: review of recent independent testing indicates that pin orientation is less critical for accuracy than once believed.) ASTM D 4444 also notes that: For a single conductance meter reading corrected for temperature and species, the 95% confidence interval for predicting the true moisture content between the electrode tips is approximately 7% relative error (+ 0.5 to 2% MC), depending on species. In other words, even when the meter is precisely calibrated for temperature and species, a single reading of 22.0% MC represents an actual (95% confidence level) moisture content somewhere in the range of 20.5% to 23.5%.

3 Page 3 of 5 In short, at all times, single field use of a pin-type moisture meter introduces potential bias of + 2% MC, or greater. (Similar bias occurs when non-pin capacitance meters are used.) Therefore, where practical, the following measures should be taken to provide increased confidence in the useful accuracy of the field measurements: 1. Take repeated measurements at close locations and average the results (i.e., take the mean of the meter readings). 2. Where available, use insulated pins. 3. Where non-insulated pins are used, avoid obvious areas of surface moisture. 4. Where appropriate, identify the wood species being tested and make the species correction published by the manufacturer. 5. Where appropriate, identify temperature conditions and make the correction published by the manufacturer. 6. Where practical, orient the pins in the direction of the wood grain. By following these measures, the meter readings can have excellent qualitative usefulness in understanding existing conditions at wood framing and sheathing, particularly for comparative identification of dry vs. wet areas at the project, but still should not be considered to precisely represent actual moisture content values. If close accuracy is required, extract samples for oven dry testing in conformance with ASTM D 4442 (Standard Test Methods for Direct Moisture Content Measurement of Wood and Wood-Base Materials). Moisture Content Measurements of Gypsum Board Even the multipurpose (wood & gypsum) handheld pin-type moisture meters commonly used for building envelope forensics are calibrated for a particular wood species. Meter readings of electrical resistance of other building products (e.g., gypsum sheathing) are derived from generic linear calculations of electrical resistance (i.e., 20% MC in wood is approximately equivalent to 1% MC in gypsum); however, these qualitative results must be used with caution due to the inherent variability of the gypsum board product. Independent testing in Europe by Nordtest ( confirms the variable moisture content of differing gypsum boards that register identical MC readings on handheld pin-type resistance meters: Tests have shown that different types of gypsum boards have differing moisture contents when exposed to a specific climate condition (ºC, RH). If, on the other hand, instruments measuring the electrical resistance on the materials, calibrated on wood or on fixed electrical resistance, are applied, the same results are by and large obtained for the different gypsum materials conditions under the same temperature and same relative humidity conditions

4 Page 4 of 5 These concerns of measurement variability are confirmed by Jerry A. Walker, executive director of the Gypsum Association: A moisture meter must be calibrated for the specific material being tested if it is to function properly and the trained operator must adhere strictly to the instructions if the information is to be considered valid. For example, it is generally recognized that a moisture meter calibrated to measure the moisture content of a specific species of wood will not necessarily provide an accurate reading for other species of wood, and especially a completely different material such as gypsum board. 2 This conclusion should not be interpreted to mean the results of pin-type moisture metering of gypsum board have no qualitative usefulness for comparative evaluation of dry vs. wet conditions during forensic investigations. It also is valuable to recognize that even if a standard pin-type meter does not have a setting for gypsum the readings of electrical resistance produced by this wood meter when used in gypsum have the same qualitative usefulness (within the limitations discussed above) for comparative evaluation of dry vs. wet conditions during forensic investigations. In other words, the wood and the multipurpose pin-type moisture meters provide an identical function a simple measurement of electrical resistance when used in gypsum. Both meters assume that the tested material is a wood species. The only difference is that the multipurpose meter when informed by the operator that the material is gypsum has the capability of translating the same measurement of electrical resistance into an approximate calculation of the actual moisture content of the gypsum. For either meter, the use of the standard wood setting for comparative forensic evaluation is acceptable and can provide qualitative results that are relatively useful within the constraints discussed above. Summary Discussion In summary, handheld moisture meters are valuable forensic tools that can produce reasonably accurate and useful readings, but for no type of handheld unit can the results of electrical resistance metering of any porous building material be considered a true representation of actual moisture content. The higher the meter reading, the greater the likelihood for variance from true conditions, even when the unit is calibrated for the correct species and temperature. The potential usefulness of these qualitative results can be improved by averaging repeated measurements and by implementing calibration measures published by the manufacturer; however, in cases where quantitative precision is required, oven dry laboratory testing must be carried out. 2 Jerry Walker, The Moisture in Gypsum, Walls & Ceiling magazine, November 2002

5 Page 5 of 5 Despite the uncertain quantitative accuracy in these meter readings, in most forensic investigations we do not need to know the precise moisture content of an inspected material in order to gain useful understanding of existing conditions. For example, if most sections of the exposed OSB sheathing at a project register ~18% MC, but some of the sheathing areas register ~24% MC, then we can conclude with confidence that these areas have experienced excess water infiltration (in liquid and/or vapor form) conditions not occurring at the comparatively dry areas. Or, if typical clean, bright wood stud framing exposed at a project registers ~15% MC, but some obviously water-stained studs exhibit a range of MC measurements from ~20% to more than 40%, it often can be reasonably assumed (subject to further investigation) that the higher readings are located nearer to the source(s) of water infiltration into the wall assembly. *****