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T&D > Programs Areas > Inventory & Monitoring > Tree Decay Detection Program Areas
Evaluation of Decay Detection Equipment in Standing Trees
Bob Monk, Project Leader

The following report was prepared to compile information on the various types of equipment capable of detecting decay in standing trees. The project proposed to San Dimas Technology and Development Center (SDTDC) requested testing and comparison of the Arborsonic Decay Detector, Fractometer, Digital MicroProbe, increment borers, Picus Sonic Tomograph, Portable Compression Meter, Resistograph, Shigometer, Stress Wave Timer and Elastometer.

The report includes a list of equipment available, estimated cost, a brief description by type, some advantages and limitations, and comparisons where information is available.

The Forest Product Lab has completed testing of some of the equipment and published reports detailing their findings. The following link, Stress Wave How To, opens a document explaining the techniques to use and interpret stress wave detection tomography. The link, Stress Wave Study, opens the report of a study done with stress wave tomography to determine the strength and stiffness of standing trees.



The link, Micro-Drill Study, is to a study using the microdrill resistance technique to determine decay in bridge members. While this is not a standing tree study, the results and discussion are applicable to standing trees. The link, Bridge Member Study, is a study of several stress wave tools to determine the condition of bridge members.




A project was proposed to test and compare several of the devices that are able, to some degree, to detect decay in trees. Decay in trees is directly related to the hazards that they present. However, detection of decay does not necessarily mean that a tree is hazardous. The identified devices can be used to determine what decay is present. It is still necessary to use professional experience and sound judgment to decide if a tree actually poses a hazard. General guides or "rules of thumb" can also be used, such as Guidance notes from the Minnesota Department of Natural Resources and the USDA Forest Service, 1996 that suggests a 25 mm ring of sound wood is required for every 150 mm of stem diameter at any point on the stem. If the proportion of decayed wood to sound wood exceeds this level then action may need to be taken to minimize the hazard posed by the tree (Lawday and Hodges, 2000).

Existing Information

There is a large amount of information available about individual pieces or types of equipment. There is also some information comparing some of the devices. Some information is in formal reports and some is more anecdotal, some is by the manufacturers of the equipment (need to be careful not to put too much weight on their claims). So far no report has been found that compares all of the equipment. Much of the work has been done in Europe (England and Germany mainly) and that is where much of the equipment is manufactured.

The USDA Forest Service publication Urban Tree Risk Management: A Community Guide to Program Design and Implementation is available at the following website:

The manual was designed to preserve public safety and improve the health of urban forests by assisting communities in the design, adoption, and implementation of a tree risk management program.

Chapter 3, Section 7, entitled Tree Risk Inspections and Use of Specialized Diagnostic Tools includes information on decay detection devices commonly used in the United States.

Decay Detection Equipment

Besides those mentioned, there were several other devices identified when doing a search for information. Most of these are similar to the ones mentioned in the project proposal. The equipment suggested for evaluation, and others, are identified in Table 1. This list is by no means meant to be inclusive of all equipment available. More sophisticated devices such as X-ray, gamma ray tomography, magnetic resonance imaging, and thermal imaging are limited in use because of cost and practicality of field use. Some of these latter devices may have more practicality with evaluation of logs rather than standing trees.

Testing all of the pieces of equipment available would be very expensive. The equipment can be grouped into types and a representative of each type could be compared with each of the other types. The types of equipment are ultrasonic, stress wave timer, microdrill, electrical resistance, mechanical, visual, and manual. Table 1 lists several pieces of equipment grouped by type. For comparison purposes, Table 2 provides estimated cost of the equipment.

Table 1—Tree decay detection equipment.
Type Name Manufactuerer
Ultrasonic Arborsonic Decay Detector Fujikura Europe, England
James "V" Meter James Instruments, Chicago, IL
Sylvatest Sandes SA, Switzerland
FAKOPP Ultrasonic Timer FAKOPP Ent., Hungary
Picus Sonic Tomograph Fujikura Europe, England
Stress Wave Timer Metriguard Model 239A Metriguard, Pullman, WA
FAKOPP Microsecond Timer FAKOPP Ent., Hungary
FAKOPP 2D Microsecond Timer FAKOPP Ent., Hungary
IML Impulse Hammer IML, Germany
Microdrill SIBTEC Digital microProbe Sibert Technology, England
IML Resistograph IML, Germany
Electrical Resistance Shigometer Osmose Wood Preserving, Buffalo, NY
Mechanical Fractometer I and II IML Germany
Portable Compression Meter N/A
Visual Increment Borer Several
Manual Plastic Mallet Various

Table 2—Tree decay detection equipment costs.
Type Name Estimated Cost ($)
Ultrasonic Arborsonic Decay Detector 3,000
James "V" Meter 2,650
Sylvatest N/A
FAKOPP Ultrasonic Timer 2,580
Picus Sonic Tomograph
(12 sensors plus software)
Stress Wave Timer Metriguard Model 239A 5,375
FAKOPP Microsecond Timer 1,970
FAKOPP 2D Microsecond Timer  
6 channel 4,820
8 channel 6,130
16 channel 11,390
IML Impulse Hammer 2,000
Microdrill SIBTEC Digital microProbe  
Basic kit 7,100
Field printer 1,600
IML Resistograph F series
(300 to 500mm)
3,800 to 4,600
w/ electronic unit + 2,100
Electrical Resistance Shigometer 1700
Mechanical Fractometer I 1,000
Fractometer II 2,000
Fractometer Electric 4,000
Portable Compression Meter N/A
Visual Increment Borer 200 to 500
Manual Plastic Mallet ≈ 10

The following websites are for some of the manufacturers or suppliers of the equipment mentioned in this paper. Pictures of the equipment and other information are presented.

Fujikura Europe -
James Instruments -
Fakopp Ent. -
Sylvatest -
Metriguard -
Sibert Technology -
Osmose Wood Preserving -


These devices use a sound wave sent by a transmitter through the tree to a receiver. Because the sound wave travels fastest is the most solid wood any decay causes the speed of the signal to go slower. Time for the signal to reach the receiver is measured and displayed then this information is compared to the ideal transit time for the species and diameter of the stem. Where cavities are present the sound wave travels through the wood in a non-direct route. This signal takes longer.

The stress wave transmission time of healthy trees is species dependent. In general the times can be grouped into softwoods and hardwoods. As a rule of thumb, the baseline transmission time is 1,000 microseconds/meter (300 microseconds/foot) for softwoods and 670 microseconds/meter (200 microseconds/foot) for hardwoods. The following formulas are derived (where T is baseline transmission time and D is the diameter of the tree in meters or feet) (Wang and others, 2004)

Softwoods –
T = 1000D (microseconds per meter)
T = 300D (microseconds per foot)

Hardwoods –
T = 670D (microseconds per meter)
T = 200D (microseconds per foot)

Transmission times less than those generated from the formulas would indicate a sound tree. Transmission times greater would indicate a decayed tree. More precise transmission times can be determined in the field by measuring transmission times of sound trees of the same species.

This equipment gives a limited view if only one reading is used. To get a better indication of internal appearance, several readings can be taken. This could provide a limited two dimensional view which would depend on the number of readings taken. Some of these ultrasonic devices have only one receiver (such as the Arborsonic Decay Detector) thus requiring several individual readings. The Picus Sonic Tomograph can have a large number of sensors but is typically available with either 10 or 12. This device can generate color tomographic (two dimensional view) images on a computer screen on-site. These ultrasonic devices require contact with wood for proper readings. It is necessary to either remove some of the bark or penetrate the bark with a nail or screw.

Advantages – Multiple readings in various orientations can be used to define the extent and location of internal decay. Wang and others (2004) displayed information for determining a two dimensional view approximating the extent and location of decay. The Picus Sonic Tomograph uses multiple sensors (typically 10 to 14 are used) and software to provide onscreen tomographic display.

Limitations - The high frequencies dissipate over distance limiting use of these devices to trees approximately one meter in diameter (Dolwin and others, 1999). With many units, incorrect placement of the sensors can lead to incorrect readings. It is difficult to determine the early stages of decay.

Invasiveness – Requires contact with wood either by penetration of the bark or by removal of a bark plug.

Comparisons – When a pilot hole is used with the Sylvatest, readings at a given location are quite consistent and do not tend to be as operator sensitive as other techniques (Seavey and Larson, 2002). In a comparison of three tomographic techniques (electric, ultrasonic and georadar) Nicolotti and others, 2003 found that ultrasonic tomography demonstrated to be a very effective tool for the detection of internal decay, accurately locating the position of the anomalies and estimating their size, shape, and characteristics in terms of mechanical properties.

Stress Wave Timer

These devices are similar to the ultrasonic devices in that they measure the time it takes for sound to travel through the stem. The sound is a low frequency impulse generated by a special hammer that is used to tap one of the sensors and start a sound wave through the tree. There are variations of the makeup of the hammers and sensors. The FAKOPP 2D Microsecond timer has several receivers and can generate tomographic data/images which can be interpreted to generate a two dimensional "picture" of the stem. These devices also require contact with wood and use screws that are inserted through the bark a short distance into the wood.

Advantages - The lower frequencies used by these devices do not dissipate as quickly as the ultrasonics thus allowing us on larger diameter trees (Dolwin and others, 1999).

As with the ultrasonic devices multiple readings in various orientations can be used to define the extent and location of internal decay. The FAKOPP 2D Microsecond Timer uses multiple sensors (typical units are supplied with 6, 8 or 16 sensors) and software to provide onscreen tomographic display.

Limitations – As with the ultrasonics, sensor placement is important. It is difficult to determine the early stages of decay.

The ability to detect rot appears to be dependent upon the type of fungi present (Schwartze and others, 1995). Generally, brown rots appear more easily detected than white rots.

Stress wave timers may fail to detect certain kinds of decay, at least in the early stages (Dolwin and others, 1999).

Invasiveness – Usually requires shallow penetration of the wood.

Comparisons – The Metriguard unit is simple to use but care should be taken interpreting the results. It is somewhat sensitive to the level of the impact therefore selection of the proper gain is important (Seavey and Larson, 2002)


These devices use a drill (rechargeable) with a very small probe (approximately 2 to 3mm in diameter) and can drill up to one meter. Various length probes are available. A constant force is applied and measured. As the probe penetrates the wood the difference between harder and softer wood is measured and thus gives an indication of wood condition. This information is made available in the field on a paper graph or can be downloaded to a computer for data storage and presentation. Growth rings can be recorded where there is a difference between harder and softer wood. The information gained by using microdrills is limited to the area immediately adjacent to the probe penetration. It may require several borings to develop a picture of the inside of the stem.

Advantages – The Shigometer can detect early stages of decay that the Resistograph may not. See advantages of the Shigometer below. Use of these tools in combination can result in improved information.

Limitations – Microdrills cannot detect early to intermediate stages of decay (this may vary amongst fungi types).

Invasiveness - The hole, though smaller than other devices that bore a significant depth into the stem, may allow decay into the wound. Also boring through compartmentalized decay barriers may lead to spread of decay. Because of the size of the hole this may be of minimal concern.

Comparisons – The Sibert Decay Detecting Drill measures changes in the speed of penetration, at a constant forward pressure. The Resistograph measures changes in torque of a probe penetrating the wood at a constant speed (Dolwin and others, 1999).

The SIBTEC (Sibert) Digital microProbe (DmP) has a flexible probe that can be deflected, giving misleading information. The probe is blunt requiring less replacement than the Resistograph which has a sharp probe. The Resistograph probe is stiffer and can be snapped instead of being deflected (Dolwin and others, 1999). The DmP probes are of various length, up to 1000 mm. Using longer probes requires a probe support zip which can add several hundred dollars to the cost (up to approximately $1,000 for the 1000 mm support and probes). The Resistograph is available with several probe lengths up to 500 mm. Each probe length is a separate tool. The Resistograph has a slightly larger diameter (approximately 1 mm larger) probe than the DmP. The Resistograph can provide information on single growth rings. The Sibert DDD 200 (now SIBTEC) does not provide very detailed information about single growth rings (Nicolotti and Miglietta, 1998)

Electrical Resistance

The Shigometer (the Vitalometer is a similar product produced in France) measures electrical resistance by use of a probe with two wires on its tip. The probe is inserted into a 3/32 inch (2.38 mm) diameter hole drilled into the tree with a portable drill. This method can detect the early stages of decay that may not be evident during a visual inspection of wood samples). During decay, metal ions are released by damaged cells. This causes a decrease in resistance of the decaying wood compared with non-decaying wood. This method only provides information about conditions in the area immediately adjacent to the bore hole. It is a good idea to visually inspect the wood in the flutes of the drill bit. This may give additional clues about the soundness of the wood.

Larsson, Bengtsson, and Gustafsson (2004) described a less invasive method using resistivity. The method provides an indication of the presence of decay but not the location in the stem. Also measurements of resistance are moisture and temperature dependent so that time of the year and location of site (wet or dry) can affect individual tree readings. Thus determining decay from an individual reading would be difficult. The readings for an individual tree need to be compared to other readings taken under similar conditions. Decay reduces wood resistivity. Measuring a large sample of trees under similar conditions could give an indication of which trees may have decay. This method would not be very useful for individual tree evaluation but may be useful during stand examinations when trying to determine how to treat a stand that may have significant tree decay that is not visible. It may also be useful to determine to limits of spread of root rot centers.

Advantages – Release of metal ions during the early stages of decay makes equipment such as the Shigometer able to detect decay in the early stages.

The Shigometer/Vitalometer can be used in combination with Resistograph to provide more accurate identification of the internal condition of roots and stems. Because the same hole created by the Resistograph is used for taking Shigometer/Vitalometer readings, wounding to the tree is minimized (Moore, 1999).

Limitations – Electrical resistance is an indirect measure of decay presence. Information is limited by probe length (8 and 12 inches available).

Invasiveness - The hole, though smaller that the increment borer, may allow decay into the wound. Also boring through compartmentalized decay barriers may lead to spread of decay.


The Fractometer and the portable compression meter are two devices that are different in the way they evaluate the presence of decay.

The Fractometer comes in two types. An increment core is taken from the tree (it is very important that the core is taken with a sharp increment borer). The Fractometer I gives a reading on a scale of the stiffness and breaking strength of the sample core. The Fracometer II also provides a measure of the compressive strength. These values are compared to the Fractometer table which provides information on many species. It may be necessary to develop tables for tree species not covered by the tables (much of the work done with the Fractometer is from Europe, particularly Germany). There is an electronic version available of the Fractometer I where measurement is controlled electronically.

The portable compression meter is a long, thin steel probe with a slightly enlarged tip that is inserted into a hole drilled into the tree. The probe is pushed and is activated by a self-firing, spring loaded punch. The number of punch thrusts is counted for each increment the shaft moves into the wood. The diameter of the hole drilled is critical to the operation of this device. It needs to be slightly smaller than the probe tip but not so small as to restrict movement of the probe. The number of thrusts required for any given increment of the probe can give an indication of the condition of the wood compared to other areas along the length of the hole being tested.

With both of these tools the information gained only relates to the area immediately adjacent to the area bored. As with the microdrills several borings may be necessary to infer a representation of the interior of the tree. Since these tools make a larger hole than the microdrill there is more potential for damage.

Advantages - Both are relatively small devices and easy to carry. The Fractometer has proved to be a sensitive technique for measurement of wood strength for various types of fungi (Schwartze and others, 1995).

Limitations – The increment borer used for extracting the core for use with the Fractometer must be kept sharp and clean. A damaged core may affect the readings taken or at least make them suspect. Some species strength data is provided with the instrument. It may be necessary to develop strength data for local species. Because wood properties can vary within a given tree and between different individuals of the same species, the published Fractometer values must be used with much caution (Matheny et al. 1999). The portable compression meter is depth limited. It is also somewhat time consuming.

Invasiveness – Removal of the core used with the Fractometer creates a hole (approximately 9 mm) that may allow decay into the wound. Also boring through compartmentalized decay barriers may lead to spread of decay. The hole made for use of the portable compression meter is smaller (4 to 4.5 mm) but creates the same concerns.

Visual and Manual

Though they are fairly basic we should not forget the increment borer and the mallet. Tapping on a tree with a mallet (hard plastic recommended), while listening to the sound can sometimes indicate if there is a cavity or dead wood present. Examining the core taken with an increment borer may help determine in there is decaying wood present. This may be indicated by discoloration or softer wood. These are simple techniques but that may be all the information that is desired.

A portable drill with a long thin bit can be used by an expert to quickly determine if decay is present. They can use resistance to drilling, discoloration of wood in the flutes of the drill and possibly even their sense of smell to evaluate a tree for presence of decay.

Advantages – Quick, easy to transport.

Limitations – As with all the equipment, they require experience and knowledge of decay.

Invasiveness – The hole (approximately 9 mm) may allow decay into the wound. Also boring through compartmentalized decay barriers may lead to spread of decay.

What Equipment to Use?

The equipment selected for use, in large part, would probably be influenced by the answers that are needed, cost of equipment, time available, risk of causing additional (or initial injury), and ease of transport. If all you need to know is if decay is present in a tree it may require much less expensive equipment than knowing the extent and location in the tree. A combination of equipment may be less invasive than using just one piece of equipment.

Experience with tree decay and use of the equipment plays a significant role in the evaluation to determine the extent of decay in standing trees. Use of expensive, high-tech equipment cannot offset inexperienced users. Proper interpretation of the results is vital. Conversely, low-tech equipment used by a highly experienced individual may provide the information needed.

Table 3 on the following page provides a brief comparison of the various types of equipment mentioned in this paper.

The Elastometer mentioned in the original proposal is not a decay detection device. This piece of equipment can be used to measure the potential for tree failure under bending pressure. Most of the work done with this equipment appears to be done in Germany. This equipment could be evaluated but can not be compared to the decay detection devices. If evaluation of this equipment is necessary it is suggested that it be done in a separate project.

Table 3—Equipment comparison by type.
Type Ooutput Display Depth Limitations* Decay Stages Detected Tomographic Capabilities Invasiveness
Ultrasonic LCD for most units. Picus is capable of 2 dimensional color images. Up to approximately one meter. May detect earlier stages of decay if calibrated locally by species. Picus Sonic Tomograph, others - no Requires contact with wood - penetration of bark or removal of bark plug.
Stress Wave Timer LCD for most units. FAKOPP 2D capable of 2 dimensional computer screen display in field. No diameter limit Do not detect early stages of decay, especially some species of decay. FAKOPP 2D , others - no Requires contact with wood – penetration of bark.
Microdrill Field generated paper graphs. Optional computer display available. Limited by length of probe. Some models up to one meter. Do not detect early to intermediate stages of decay, especially some species of decay. No 2 or 3 mm hole to depth of probe.
Electrical Resistance LCD 8 and 12 inch probes available. Can detect early stages of decay. No 2.38 mm hole to depth of probe.
Mechanical Gauge for Fractometer. Electronic version available with LCD. Increment borers up to at least 28 inches are available. Fractometer may detect early to intermediate stages of decay. No 9 mm hole to depth of borer.
Visual N/A Same Cannot detect early stages of decay. No Same
Manual N/A N/A Cannot detect location of decay. No Non invasive

* Probe limitations may not be as much of a concern if the operator if trying to determine how much sound wood is available during hazard evaluation. If it is necessary to get a complete picture of the tree interior the probe length could limit this ability.

Literature Cited

Dolwin, J.A.; Lonsdale, D.; and Barnett, J. 1999. Detection of Decay in Trees. Arboriculture Journal. 23: 139-149

Larsson, B.; Bengtsson, B.; and Gustafsson, M. 2004. Nondestructive Detection of Decay in Living Trees. Tree Physiology. 24: 853-858

Lawday, G.; and Hodges, P.A. 2000. The Analytical Use of Stress Waves for the Detection of Decay in Standing Trees. Forestry. 73(5): 447-456

Matheny et al. 1999. J of Arboriculture 25(1):18-23. [Fractometer]

Moore, W. 1999. The Combined Use of the Resistograph and the Shigometer for the Accurate Mapping and Diagnosis of the Internal Condition of woody Support Organs of Trees. Arboriculture Journal. 23: 273-287

Nicolotti, G.; and Miglietta, P. 1998. Using High-Technology Instruments to Assess Defects in Trees. Journal of Arboriculture. 24(6): 297-302

Nicolotti, G.; Socco, L.V.; Martinis, R.; Godio, A.; and Sambuelli, L. 2003. Application and Comparison of Three Tomographic Techniques for Detection of Decay in Trees. Journal of Arboriculture. 29(2): 66-77

Schwartze, F.W.M.R.; Lonsdale, D.; and Mattheck, C. 1995. Detectability of Wood Decay caused by Ustulina deusta in Comparison With Other Tree-Decay Fungi. European Journal of Forest Pathgology. 25: 327-341

Seavey, R.; and Larson, T. 2002. Inspection of Timber Bridges. Minnesota Department of Transportation Technical Report MN/RC-2002-34. St. Paul, MN. 43 p.

Wang, X.; Divos, F.; Pilon, C.; Brashaw, B.K.; Ross, R.J.; and Pellerin, R.F. 2004. Assessment of Decay in Standing Timber Using Stress Wave Timing Nondestructive Evaluation Tools: A Guide for Use and Interpretation. Gen. Tech. Rep. FPL-GTR-147. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 11 p.

Costello and Quarles. 1999. J. of Arboriculture 25(6):311-317. [Resistograph]
Costello and Peterson. 1989. J. of Arboriculture 15(8):185-188. [Shigometer]