Timber Dimension Measuring Devices—Public Consultation Document

Timber Dimension Measuring Devices–Public Consultation Document (PDF, 120 KB, 15 pages)

Table of Content

Purpose

This document will serve as the basis of the Timber Dimension Measuring Device (TDMD) consultations. The intent is to develop a set of requirements or specifications (currently referred to as Terms and Conditions by Measurement Canada) relevant to these devices so that they may be approved for use in trade as per the Weights and Measures Act.

This is a consultation paper only and any errors or omissions in this document will not change the applicable requirements for the measuring machines.

Responsibilities

Measurement Canada (MC) will be responsible for development of legal requirements for the approval of TDMDs, approval test procedures and subsequent field test procedures. These will be developed ensuring that all existing and new legal requirements are addressed and the final measurement values are appropriate for the intended application, in that they meet the needs of all stakeholders involved in the measurement process. MC will also develop the requirements for suitable physical test standards required to inspect the measuring devices.

The CSA workgroup and various provincial authorities will be responsible for identifying the appropriate method and application for use of the approved measured parameters in relation to the provincial legislation. The CSA workgroup on timber scanners will be responsible for ensuring that the needs of the stakeholders are addressed in the development of requirements for this device type.

MC will not specify details regarding the number and location of measurements that must be taken to accurately derive timber volume information. This will be left to the CSA and the provincial authorities to decide and document. MC will ensure that any measurement results are accurate within applicable limits of error (LOE) using procedures that have been developed and documented as part of this project.

The manufacturers of the devices will be responsible for any modifications required to the design, composition and construction to meet the approval requirements should they wish to have their devices approved for use in trade. It is expected that manufacturers will play a key role in the development of the requirements to ensure that they are consistent with the capabilities of the available equipment.

The users of the devices will be responsible for ensuring that their installation and use is as per the manufacturers' requirements, that the measurement remains accurate as required by the legal requirements applicable to them and that all other legal requirements are being met at all times while the devices are being used in trade. User groups are expected to ensure that their needs are met throughout the development of requirements for this device type.

General

The Weights and Measures Act requires that any device that is to be used in trade be approved and inspected before being placed into service. Devices that are not intended to be used in trade are required to be marked appropriately to ensure that it is clear they are not legal for trade. The only exception to the approval and inspection requirements are for devices which are specifically exempted from these requirements under the Weights and Measures Regulations (sections 3‑11). The use of unapproved or uninspected devices is an offence under the Weights and Measures Act.

The Weights and Measures Act requires all approved measuring devices to measure in recognized units of measure Footnote 1 and to be calibrated in relation to a relevant local standard Footnote 2. The final measurement output is required to be in terms of actual measured properties. For this reason, Measurement Canada will not accept volumes based on theoretical formulas which assume physical properties of the object being measured (Smalian's formula, etc.) and will not allow for dimensional adjustments to the measured properties based on theoretical corrections.

Existing Requirements

An exhaustive search was made to see if there are existing suitable requirements in place elsewhere in the world. Very little in the way of formal documented procedures or requirements was found. One manufacturer did provide some reference material from Germany Footnote 3 and Austria Footnote 4. Limited information was found from Australia.

Summary of German requirements

  • Germany has provisional regulations which are based on Austrian regulations.
  • The calibration limit of error for diameter in Germany appears to be ±2.5 mm for the arithmetic average of 10 diameter measurements and ±1 cm for any individual measurement.
  • Diameter is measured at the ½ log length and is considered to be the average of the two smallest measured diameters at this point.
  • The calibration limit of error for length in Germany appears to be ±1% of the measured value but not less than 5 cm.
  • Components that may affect calibration must be protected by sealing.
  • Calibration is valid for two years and is performed by the Federal State.
  • Calibration limits of error are twice the limits of error specified above after the device is in service for 1 year.
  • Approval is performed by the PTB Footnote 5.
  • The software as approved by the PTB is the only software that may be used and must be protected against change Footnote 6.

Summary of Austrian requirements (Official Calibration Gazette No. 7/1984)

  • Diameter is taken at the ½ length of the log. The diameters must be taken in an area of not more than 20 cm centered on the log center.
  • Diameter is defined as two consecutive measured normal individual diameters taken within 20 cm of each other or three individual diameters taken at an angle of 60 degrees to one another (details are unclear).
  • Diameter measurements must be taken with a resolution of 5 mm or better.
  • The arithmetic average diameters may be left unrounded or may be rounded down to the nearest whole centimetre.
  • Length measurements must be taken with a resolution of 1 cm or better.
  • Length measurements are rounded down to the nearest specified length step and are then referred to as the "graduated length". Length over this step is referred to as "excess length".
  • Diameter acceptance limits of error are ±10 mm for any individual measurement and ±2.5 mm for the arithmetic average of 10-20 individual measurements.
  • Length acceptance limits of error are ±1% of the measured length but not less than 5 cm.
  • In-service limits of error are 1.5 × acceptance limits of error.
  • Austrian terminology:
    • Operational limit of error is equivalent to Canadian in-service LOE.
    • Calibration limit of error is equivalent to Canadian acceptance LOE.

Summary of Australian requirements

  • An Australian approval document for timber scanning was discovered that appears to be a one of a kind approval and little information as to the test procedures used was available.

Scanning Technology

Most modern timber scanners appear to work on a very similar principle. The measuring device is fitted with several scanner heads. The log to be measured is passed through the device, scans are taken at regular intervals along the length of the log and a computer is used to create a three dimensional model of the object. The length of the log is typically determined using a separate transducer from the scanner heads. Logs are typically transported using transport chains fitted with flights. The flight supports the log above the chain allowing the scanner to see as much of the outer circumference of the log as possible. In many cases, the entire circumference of the log cannot be scanned and the machine's measurement algorithm is used to fill in the missing information. In a similar fashion, when the scan detects a flight, the algorithm is used to remove this undesirable information from the log scan data. It should be noted that there are other scanner types, including a device whose scanner head moves over a stationary log.

Scanner heads are often fitted with lasers that project a laser beam into the measurement field. When the laser beam hits an object, such as the log to be measured, the laser projection draws a line on the object. The scanner head also includes a camera used to detect the laser line projected onto the object to be measured. Since the distance from the laser to the camera is known, the distance to any point on the line can be calculated (limited by the resolution of the camera and the field of view covered by the laser line). This information is used to calculate a three-dimensional segment of the log as seen by the scanner head. As each head can only see a portion of the circumference of the log, multiple heads are required to scan the entire circumference of the log. Scanners generally have a minimum of three heads but larger scanners may have more.

The alignment of the heads is critical to ensure that the scan represents the outline of the log in a single plane. If the alignment is off, each head will be scanning a different portion of the log and the resulting diameter will be in error. Manufacturers may provide calibration fixtures intended to help align the scanner heads to ensure that they are capable of producing accurate results. These fixtures do not calibrate the device for accurate measurement. Calibration is often done using static physical standards. These standards may be made from a variety of manufactured materials or may be actual log samples. It has been confirmed that scanners are better able to see certain colours and texture; black surfaces for example, may be difficult to scan. Ambient light levels may also affect the ability of the scanner to accurately scan the object. Most scanners are fitted into a protective housing that may serve to control ambient light levels. Scans are typically taken every 12.5 mm along the length of the log (depending on the speed of measurement). It is not necessary to use every scan to make the final measurement. In some cases, multiple scans may be averaged, or otherwise weighted to provide a measurement at a larger interval (e.g., 10 cm) along the length using proprietary algorithms developed by the device manufacturers.

Scanners are subject to interference from dust and dirt present in the scanning area as well as build up of debris on the scanner heads. Vibration is also a significant factor that may affect accurate measurement.

The resolution of the scan, which is the number of measurements taken around the circumference of the log, depends on several factors, including the diameter of the log, the distance from the scanner head to the log, the resolution of the cameras used in the scanner heads and the speed of the measurement process. The scanner heads are usually self‑contained, internally calibrated transducers that are purchased from third party suppliers. For the purposes of approval and inspection, these heads will be considered as the measurement transducers. Data is typically sent digitally from the scanner heads to the primary process computer on an internal network or bus (e.g., Ethernet, CANbus, Profibus, Modbus or similar) and includes the X, Y coordinates for each scanned point on the circumference of the log as well as the transverse location of the measurement. Proprietary software turns this information into multiple cross sections and ultimately a three-dimensional image of the log.

Scanners may operate at speeds of 18 – 73 metres per minute. Typical speeds are in the 45 metres per minute range.

Speed measurements are often taken using shaft encoders on the feed chains. It is critical that the log not move in relation to the chain and that the encoder is rotating positively with respect to the chain and drive shaft. Shaft encoders are set for a certain number of pulses per revolution (ppr) and this information is critical to accurate log length measurements. Changing the encoder type or model can have a significant effect on length measurements. It is also critical that in-feed and out-feed chains from the scanners operate at the same relative speed in order not to cause slippage (this is only an issue if these are separate chains).

The shaft encoder (or other applicable length transducers) and associated setup parameters will be subject to metrological control and must be sealed in an approved system. Length data is sent to the main computer. The proprietary software incorporates this data with the scan data and calculates the log shape, length and volume. This software is primarily used to determine other non metrological information such as setting cutting profiles. This information is not considered metrological and will be ignored for purposes of approving and inspecting the scanner.

Vibration and other non desirable movement of the log being measured appear to be an issue. Manufacturers use various methods to filter out unwanted vibrations from the measured values. Approved TDMDs must be installed so as to ensure the log is not subject to external influences which may cause it to vibrate or otherwise move (e.g., roll, turn, slide, etc.) excessively. A clear feed chain of at least one log length on both the in-feed and out-feed chains may therefore be required. It is recognized that this could be problematic for some intended applications.

Further information on scanning technology is available at:

Workgroup

The CSA Timber Scanning Workgroup is comprised of representatives from the provincial and federal governments, industry associations, research groups, device manufacturers and end users. The Timber Scanning Workgroup is a subgroup of the CSA TC on Scaling of Primary Forest Products (A359TC).

Definitions

The following definitions apply to this discussion paper only and do not necessarily apply elsewhere.

d

or "actual scale interval" means the value expressed in units of length, area or volume, that corresponds to the difference between two consecutively indicated values.

device

means a weight, weighing machine, static measure or measuring machine including any equipment and accessories attached to or used in conjunction with the device that have or can have an effect on the accuracy of the device.

diameter

for the purposes of this document, the diameter of a non circular log is considered to be equal to the circumference of the log divided by Pi (π) (π = 3.14159…).

interval (increment)

see d

length

for the purposes of this document, the length of a log is considered to be the length through the geometric centre of the ends and following the profile of the log.

limit of error (LOE)

represents the maximum acceptable difference between the displayed measurement value and the true measurement value.

local standard

means any physical test standard designated by the Minister under section 13 of the Weights and Measures Act.

measuring machine

means any machine that measures length, area, volume or capacity, temperature or time and has a moving or movable part that has or can have an effect on its accuracy.

minimum measured

means the smallest measurement that may be made by a TDMD and still value be considered as a valid and legal measurement.

static linear measure

means a certified and traceable length measure (tape or rule) that does not have a moving or movable part that has or can have an effect on the accuracy of the measure.

trade

means the selling, purchasing, exchanging, consigning, leasing or providing of any commodity, right, facility or service on the basis of measure and includes the business of providing facilities for measuring.

Executive Summary ()

This summary outlines the decisions the CSA Timber Scanning Workgroup made by consensus and after preliminary consultation with the CSA Scaling of Primary Forest Products Workgroup. The information contained within this document will form the basis of the technical and legal requirements for MC evaluation, approval and inspection of timber scanning devices, hereafter referred to as Timber Dimension Measuring Devices (TDMDs).

All TDMDs must comply with the applicable provisions of the Weights and Measures Act and Regulations and other requirements, including relevant specifications or terms and conditions. Specific requirements for TDMDs will likely take the form of specifications in a document entitled Terms and Conditions Relating to Timber Dimension Measuring Devices. TDMDs which incorporate features covered by existing terms and conditions must meet these requirements as well (e.g., metrological software, audit trails, etc.).

Each model, type or design of TDMD will be required to be submitted to MC for approval evaluation against the TDMD requirements. If successful, a Conditional Notice of Approval (NOA) will be issued to the applicant for that model of device. The NOA will indicate any specific applicable requirements, exemptions or restrictions over and above the general requirements for the device type. After a TDMD design is approved, each device is required to undergo an initial inspection before being placed in service. The initial inspection will ensure the device meets all of the specified requirements, including accurate measurement. Once installed and in use, each TDMD is subject to random, targeted or periodic subsequent inspections. TDMDs will be evaluated against the requirements using a yet-to-be-developed set of standard test procedures (STPs).

Timber Dimensional Measuring Devices

Installation and Use

Only stationary TDMDs will be considered for approval. Stationary in this context means that the device is designed to be setup, configured and calibrated for use at a single location. The device may be moved from one location to another, provided that appropriate measures are taken to ensure it works properly at the new location. Specifically excluded from evaluation for approval are systems that measure logs by way of a device mounted on a mobile piece of equipment such as a harvester or processor. These devices are referred to as on-board measuring systems and are not covered by these requirements. On‑board weighing systems are currently covered by other existing legislation and, unless approved for use in trade, they must not be used in a trade capacity.

There will be no restriction as to where a TDMD may be installed as long as it is able to perform accurately within the applicable LOE and is accessible for inspection purposes. The applicable LOE will be established as part of this project. Typical installations may include the in-feed to a mill or a stand alone device in a log sort yard. Portable systems designed to be used in a stationary manner may be considered for approval if they meet the requirements of the TDMD specifications.

Physical Standards and Test Objects

Physical standards of known and certified dimensions shall be used to assess TDMD performance whenever possible. These standards will be constructed of a material with optical and physical properties similar to those of a log to ensure that they may be used in a TDMD without causing a bias to the measurement results. As it is likely that it will not be possible to evaluate all aspects of the TDMD using only physical standards, local test objects may have to be created. Local test objects will consist of selected logs or similar objects available on-site. The logs will be selected for straightness and uniformity to ensure that the measurements made are as accurate as possible. A means to assess the uncertainty in measurement will be explored to ensure that the applicable LOE is expanded appropriately to allow for a true evaluation of the device. Much work is still required in the area of the development of physical test standards.

Measured Parameters

TDMDs measure the length and diameter of a log. Logs will be measured as presented, that is, the overall length and the overall diameters will be measured at identifiable locations along the stem with no allowances made for bark, snow, ice, etc.

Length measurement results must reflect the true length taken through the approximate geometric centre from one end to the other end of the log. Logs with spikes (extra wood) and missing wood on the ends should normally not be presented to the TDMD, but if they are, the machine must still calculate the length from centre to centre. Therefore, unless the spikes or missing pieces are large enough to reach the centre of the log, they will not affect the overall length measurement. Length measurements shall follow the sweep of the log if any is present. Logs, if used as test objects to assess the TDMD performance, will be selected to minimize any sweep or other deviation from perfectly straight.

Diameter measurements will be representative of the actual scanned diameter and will include bark if the log has not been debarked. Any required allowances for bark will be made subsequent to the actual trade measurement, or alternatively, measurement may take place after debarking. If snow, ice or debris build-up is a concern, steps must be taken to prepare the log before scanning takes place. TDMD must not make tare allowances for such undesirable material. As many diameter measurements may be taken as required; however, each measurement location must be specified as an offset from the leading edge of the log and each such measurement must be accurate within applicable LOEs.

When testing a TDMD for performance, it is likely that a log or log facsimile (test object) will be used as a physical standard. In order to use a log, its dimensions must be found and compared to a relevant local standard, in this case, a certified linear measure. Since a log is not a true circle, and therefore, the concept of diameter is not neatly defined, the reported diameter of the log is to be determined as circumference divided by pi (π), where π is defined as the ratio of a circle's circumference (C) divided by its diameter Footnote 7 (2r). This gives the best approximation of the true average diameter of the log.

Therefore, π = [C÷2r]= 3.14159… or 2r = d = C÷Π

Circumference is not only easier to measure and with better repeatability than trying to measure diameter directly, it also eliminates the need to determine where and how many diameters are to be taken on the ends of the log. Circumference should only be taken at a location that ensures the entire surface of the log is directly measured. In order to ensure that an accurate circumference is found, the log should be selected to ensure that it maintains a convex profile at all points around the circumference. Care must be taken to make certain that the linear measure used does not span across low sections of the log, but rather measures the true circumference as accurately as possible.

Logs are typically not true straight-sided frustums of a cone; they take various other forms depending on the species, age and other factors. Of particular note is the tendency for a tree to flare out at ground level. This flare is referred to as butt flare and is more significant in some species than in others. With many current provincially authorized volume formulas, butt flare results in skewed volume calculations if not addressed. This problem may be addressed in several different ways; however, a flared butt is to be measured directly by the TDMD the same as for any other diameter measurement. If butt flare allowances are to be made, they must be calculated after the actual trade measurement using multiple diameter measurements and a formula to be specified by the provincial authorities. The TDMD must not project log taper or otherwise provide theoretical diameters at the butt end.

TDMDs may also be designed to measure true volume of a log, but this aspect of use has not been fully discussed or developed at this point. Since no jurisdiction currently uses true volume calculations, this will be left as a future expansion of the requirements. While the calculation of true volume will not be pursued at this point, its use will not be explicitly excluded in the final document. The choice to allow or prohibit the use of true volume calculations will be left to the provincial authorities. No consideration will be made for the use of non-true volume calculations such as those obtained by most current provincially authorized formulas (Smalian's formula, etc.).

Performance Criteria

Measurement resolution or more commonly, interval size or d, will not be specified by the requirements, but rather left up to the manufacturer and device owner as a configurable parameter. The requirements will specify a relationship between the value of d and the use of the device. Although the requirements will not specify values of d that may be used, the absolute smallest and largest value of d may be specified in the Notice of Approval (NOA) issued by MC if the interval must be restricted in order to ensure accurate measurement. The value of d will also be noted on the most recent certificate of inspection and must not be changed without a new inspection to verify that the device continues to perform accurately. The interval size d must be a sealable parameter. Typically, the largest log size is physically limited by the TDMD design. The interval size d may be different for measurements of diameter than for measurements of length.

The minimum measured value for a TDMD will be specified as ten times the configured interval size or 10d. By relating the minimum measured value to the interval size, the manufacturer and end user are left with maximum freedom in selecting and configuring the TDMD to suit their needs. Minimum measured values apply to any measurement of diameter or length made by a TDMD. In order to measure smaller diameters or lengths, the device must be configured with a smaller interval d. The approved TDMD must be able to identify and reject a measurement which is smaller than the approved minimum measured value.

Example: if d = 1cm, then minimum measured value = 10d or 10 × 1 cm = 10 cm.

The LOE is the maximum acceptable difference between the displayed measurement value and the true measurement value. The true measurement value is the value represented by the physical standard or test object. The LOE applicable to the TDMD will be related to the interval size d. The applicable LOE for length is ±1d and the applicable LOE for diameter is also ±1d. This means the absolute value of the LOE changes depending on the configuration of the machine, specifically the values of d for length and diameter. By relating the LOE to the interval size the manufacturer and end user are left with maximum freedom in selecting and configuring the TDMD to meet their needs.

Example: if d = 1 cm then the LOE for any measurement from minimum measured value, 10d or 10 cm, to capacity = ± 1d or ±1 cm.

TDMDs must indicate measured values in approved and inspected units and to the approved and inspected resolution. All measurement values must be in exact mathematical agreement with displayed and printed indications. The approved indication must be clearly identified and is the only legal measurement value for trade purposes. All measurement values derived or calculated from this primary display must include the primary legal indication and any other information that may be required so that the derived value may be subsequently checked for validity. All measurement values must be stored and be accessible, upon demand, to an authorized inspector or any party to the transaction for a period of not less than 90 days from the date the measurement was made. Provincial authorities may have much longer storage requirements.

All measured values must be accompanied by the recognized unit of measure or an acceptable abbreviation. Schedule I and II of the Weights and Measures Act specify acceptable units of measurement and abbreviations. Further restrictions may be imposed by the NOA for the particular TDMD. No alternate units or abbreviations are acceptable.

Markings and Usage

TDMDs must be marked appropriately as required by the Terms and Conditions and the NOA for the device. Some of the items that may be required include the manufacturer's identification, a unique model number, an approval number, a minimum and maximum approved capacity and an interval size. Minimum and maximum operating speed must be marked if they affect device operation. Further markings may also be specified by the NOA or by the most recent inspection certificate. TDMDs that have been approved and initially inspected will be permanently marked with an initial inspection stamp by the MC inspector or a recognized technician and those that have been verified at time of subsequent inspection will be marked with an inspection sticker. Devices which fail any inspection will be marked and/or removed from service as required by the Weights and Measures Act. All unapproved devices will be marked as required by the Weights and Measures Act and Regulations.

All metrological aspects of the TDMD must be protected under a physical seal (e.g., wire and lead), or by an approved audit trail. The audit trail feature detects and logs any changes made to metrological and sealable parameters such as feed speed, transducer calibration, measurement intervals, etc. MC's Terms and Conditions for the Use of Metrological Audit Trails provides more information on the evaluation and approval of audit trails.

Approval and Examination

Approved TDMDs will be expected to measure accurately in their intended environments for extended periods of time. In order to ensure that this is the case, they will be subject to approval testing to evaluate overall design, initial inspection to evaluate initial installation and calibration and subsequent testing to evaluate ongoing performance. The devices will be expected to perform accurately at all times. In the event that they fail, they must not continue to indicate a measured value, or any other value that may be construed as a measured value. Alternatively, the machine may fail to a non-measurement Footnote 8 state. At no point should the device make inaccurate measurements due to a foreseeable condition. Some of the items to be addressed by approval and inspection include the following:

  • TDMDs must have means to ensure that they are ready for measuring after a cold start and that they maintain their ready state throughout the expected operation cycles. If for any reason this ready state is lost, the device must sense the problem and take appropriate steps to ensure that inaccurate measurements are not made.
  • TDMDs must be able to perform accurately when subjected to any reasonably expected environmental condition including power failure or disruption, ambient temperature and humidity extremes, vibrations and noise as well as dusty or dirty conditions.
  • If power is lost during a measurement cycle, TDMDs must preserve interim measured values or identify the unmeasured log which would have to be removed from the system and remeasured.
  • If TDMDs allow for stop, start or reverse movement of the log during a measuring cycle, they must continue to measure accurately or reject the measurement, identify the log and require that a new measurement be made.
  • After installation, access to TDMDs must be maintained for purposes of calibration and inspection. All hazardous conditions that exist in or near the device must be able to be locked out if required for inspection and calibration. This includes, but is not limited to, high powered lasers, X-rays or other unsafe emitting devices used for scanning as well as chains, conveyors or other systems used to move the log through the scanner or to move the scanner over/under the log. TDMDs must operate with the same calibration factors during inspection and calibration as those used during normal operation.
  • The loss of any sensing element, either due to sensor failure or blocked or dirty camera or scanner windows must not result in inaccurate measurements. TDMDs must continue to measure accurately or reject the measurement, identify the log and require a new measurement be made.
  • TDMDs will be subject to environmental influence factor testing. This includes, but is not limited to, power supply fluctuations, operating temperatures, electromagnetic and radio frequency interference, ambient light level changes, etc.

TDMDs will be subjected to evaluation using yet to be developed STPs. STPs are used for both approval evaluation and for field testing of a device. Many of these STPs will be the same for both approval evaluation and field inspection; however, there may be a need to have individual STPs which are specific to one or the other. STPs will be designed to ensure that each of the specified requirements may be adequately assessed and to ensure that the measurement results obtained from a TDMD are accurate within the applicable LOE. The development of STPs will begin with development of the Terms and Conditions and will likely continue on through early field testing. Due to the relatively unique measurement methods employed by a TDMD, it is anticipated that actual field testing will be required to finalize many of the procedures.

Contact Information

For more information on this document, please contact:

Luciano Burtini
Senior Program Officer, Gravimetric
Measurement Canada, Industry Canada
151 Tunney's Pasture Driveway,
Ottawa ON K1A 0C9
luciano.burtini@ic.gc.ca
Telephone: 250-862-6557
Facsimile: 250-712-4215
Government of Canada

Errors, omissions or other required changes should be reported to the author for correction.

Footnotes

Footnote 1

Weights and Measures Act, section 7. Only units as defined in Schedule I (metric) or II (Cdn) may be used in trade.

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Footnote 2

The term "relevant local standard" is used by MC for a physical test standard designated by the Minister under authority of the Weights and Measures Act and is used to certify a measuring device.

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Footnote 3

Zulassung Eichung Germany en final.rtf provided by Siegfried Hindinger from MiCROTEC Industrieautomation GmbH (English translation), .

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Footnote 4

Amtsblatt Eichwesen Austria en final.rtf provided by Siegfried Hindinger from MiCROTEC Industrieautomation GmbH (English translation), .

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Footnote 5

Physikalisch-Technische Bundesanstalt (PTB) is the national metrology institute of Germany.

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Footnote 6

The document is not clear on how software integrity is ensured.

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Footnote 7

Diameter has been denoted by 2r (2 × radius) to prevent the ambiguous use of the more common d which is used to denote intervals in this document.

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Footnote 8

In this case, the machine may continue to operate for other non metrological purposes as long as it does not produce data which may be considered as legal for trade measurements.

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