NPSL CSS 22-NI001_Gauge Block Interferometry System.pdf

CSS 22-NI001

CALIBRATION STANDARDS SPECIFICATION

PROCUREMENT PACKAGE

GAGE BLOCK INTERFEROMETRY

SYSTEM

DEPARTMENT OF THE NAVY

METROLOGY AND CALIBRATION PROGRAM

NOV 2022

DISTRIBUTION STATEMENT A: APPROVED FOR PUBLIC RELEASE, DISTRIBUTION IS UNLIMITED.

CALIBRATION STANDARDS SPECIFICATION

PROCUREMENT PACKAGE

GAGE BLOCK INTERFEROMETRY

SYSTEM

Prepared by:

Brad Patyk (Project Lead)

Navy Primary Standards Laboratory Metrology Calibration Engineering Branch

P.O. Box 357058 San Diego, CA 92135-7058

DEPARTMENT OF THE NAVY METROLOGY AND

CALIBRATION PROGRAM

Gage Block Interferometry System Specification CSS 22-NI001

Table of Contents

1 SCOPE

1.1 General

1.2 Usage

2 REQUIREMENTS

2.1 Equipment components

2.2 Design and construction

2.2.1 Principle of operation

2.2.2 Operator adjustments and controls

2.2.3 Internal cooling

2.2.4 Front panel display indicators

2.3 Digital interface

2.4 Electrical power sources and connections

2.4.1 Nominal and alternate power source

2.4.2 Electrical power connections

2.5 Enclosure physical characteristics

2.5.1 Mechanical stability

2.5.2 Enclosure requirement

2.6 Transit case

2.7 Environmental requirements

2.7.1 Temperature

2.7.2 Humidity

2.7.3 Particle count

2.7.4 Vibration

2.8 Safety

2.9 Marking and Identification

2.9.1 Marking

2.9.2 Equipment identification

2.10 Calibration interval

2.11 Reliability

2.12 Maintainability

2.12.1 Preventive maintenance

2.12.2 Maintenance provisions

1 SCOPE

1.1 General. This document specifies the minimum requirements for a Gage Block Interferometry System. For the purpose of this Calibration Standard Specification (CSS), the Gage Block Interferometry System shall be referred to as the GBIS. The system is intended for use at the Navy Primary Standards Laboratory (NPSL) to provide direct traceability to the SI base unit definition of the meter and act as reference to facilitate evaluation of standard dimensional length uncertainties.

The GBIS will be used specifically to calibrate and certify reference level gage blocks according to accuracy grades as defined by the ASME B89.1.9-2002 national standard for inch units. Gage blocks calibrated by the system will be evaluated for nominal length deviation, variation in length, and face surface flatness as defined by the ISO 3650 international standard.

The manufacturer shall submit a technical description or product literature for the equipment it is proposing, which clearly identifies the manufacturer, make and model. The offeror must demonstrate that its proposed equipment meets or exceeds each minimum requirement described in this solicitation by providing a citation to the relevant section of its technical description or product literature and passes all acceptance testing by the Navy for performance criteria outlined in this CSS.

1.2 Usage. US Navy civilian personnel specially trained in the dimensional measurement area at NPSL will be the sole authorized operators of the GBIS.

The system will be installed in a Type I laboratory environment as recommended by the ISA-TR52.00.01-2006 “Recommended Environments for Standards Laboratories”. The requirements of this CSS are tailored to the intended use of the equipment in the appropriate environment prescribed therein.

2 REQUIREMENTS

2.1 Equipment components. The components of the GBIS listed and described in this section are separated into two distinct categories: Test Measurement & Diagnostic Equipment (TMDE) and non-TMDE. Each specified item of TMDE is given an associated minimum accuracy specification that has been selected based on typical components used in known interferometer designs. Calibration of individual TMDE shall be accomplished by comparison to external standards traceable to an international standards organization. A statement of conformance, calibration and traceability shall be provided with each individual piece of the measurement instruments that makes up the GBIS. Each item of TMDE must be represented in the final uncertainty budget of the GBIS. It is acceptable for an item of TMDE to not meet minimum accuracy requirements provided that the resultant total expanded measurement uncertainty of the GBIS satisfies the limits described in section 2.2.1.8 Measurement Uncertainty.

TMDE

Chilled Mirror Hygrometer/Humidity Sensor Remotely monitored from external electronics console.

Accuracy: ± 0.2 °C (Dew Point measurement)

Platinum Resistance Thermometers/Resistance Thermometry Bridge Three PT100 reference thermometers compatible with 100 ohm terminated resistance bridge. Remotely monitored from external electronics console.

Accuracy: ± 0.01 °C

Barometric Pressure Sensor

Remotely monitored from external electronics console.

Accuracy: ± 0.15 mbar

Carbon Dioxide Analyzer Remotely monitored from external electronics console Accuracy: ± 1% of scale concentration

Charge-coupled Device (CCD) Camera Image sensing capable of discriminating fringe spacing distance where a fringe is defined as the half wavelength of the light source.

Accuracy: ± 0.01 of a fringe

Non-TMDE The following list of non-TMDE components are examples of items typically represented in known interferometer designs. Specific non-TMDE items may be added or removed under the discretion that the overall resultant design of the GBIS meets the description and requirements outlined in section 2.2 Design and Construction.

Optical Lenses and Mirrors Beam Splitter Frequency Stabilized Helium-Neon Laser Source Environmental Chamber Electronics Cabinet Gage Surface Platen (Material: Steel and Chrome Carbide) Phase-stepping Module Computer/Software Fiber optic coupling Air Flotation Table

2.2 Design and construction. The test equipment shall be designed and constructed to permit compliance with performance, measurement uncertainty, and all other applicable requirements as defined in this CSS.

2.2.1 Principle of operation. Calibration of reference grade gage blocks is accomplished using an optical interference principle where dimensional traceability is provided through the wavelength of light. A high coherence light source is directed through an interferometer to project fringe patterns onto a gage block length wrung to a high polished surface. The fringe spacing is related to the wavelength of the light source and is captured by a high sensitivity camera that produces images called interferograms to be analyzed by computer algorithms. The monitored environmental conditions at the exact moment the interferograms are taken are used to calculate the air refractive index utilized by the computer algorithm to determine the nominal length of the gage block. A minimum of two distinct monochromatic wavelengths are used in conjunction with an exact fringe method calculation in order to reduce ambiguity in determining the true nominal length of a gage block. A detailed breakdown for design characteristics of individual components is described in sections 2.2.1.1 Interferometer through 2.2.1.8 Measurement Uncertainty.

2.2.1.1 Interferometer. The interferometer type shall be of a Twyman-Green design or equivalent modified Michelson type design utilizing a frequency stabilized helium-neon laser as the light source.

2.2.1.2 Environmental chamber. An enclosed chamber shall provide a measuring environment that houses the interferometer components, environmental sensors, CCD camera, rotating platen, and accommodates gage block lengths up to 12” in the vertical orientation. The chamber shall be constructed of sufficient material to provide a stable temperature at the gauging point to within ± 0.1 °C. Access to all internally housed components shall be provided by an insulated door. A staging area shall be made available for acclimation of at minimum 12 gage blocks awaiting calibration. The chamber shall be isolated from vibration through air flotation pads or a floating table. If a temporal phase-shifting technique is used to sequentially produce interferograms for the system, displacement is limited to no more than 1 µin. for frequencies up to 100 Hz, and acceleration to no more than 0.001 g for frequencies over 100 Hz (Recommended by Ferahian and Ward as referenced in the ISA TR52.00.01-2006 technical report on environments for standards laboratories). If the GBIS is designed as a dynamic interferometer that uses simultaneous phase-shifting, displacement is limited to 10 µin. from

0.1 Hz to 30 Hz and acceleration is limited to 0.001 g from 30 Hz to 200 Hz.

2.2.1.3 Gage surface platen. The gauging surface of the GBIS shall be a highly polished removable platen driven by automatized motor control. The platen shall be able to fit up to 12 separate gage blocks wrung to its surface and support block shapes of rectangular, square (hoke style), and round form. The platen shall be not less than 0.45 in. thick and it shall have a wringing face with flatness deviations of less than 1 µin over diameters of

1.6 in. Four total platens composed of gage block matching material shall be provided; two Steel and two Chrome-Carbide.

2.2.1.4 Electronics cabinet. An electronics cabinet shall provide housing for all necessary power supplies as well as the instrumentation that monitors environmental sensors for temperature, atmospheric pressure, relative humidity, and CO2 concentration. The electronic instrumentation shall be able to interface with the computer software program described in section

2.2.1.6 Computer and Installed Software as well as the power supplies for the GBIS system.

2.2.1.5 Image processing. A CCD camera shall be incorporated into the interferometer design as the detector component. The CCD camera will capture the projected fringe pattern on the gage block as interferograms.

Multiple interferograms per gage block shall be produced by a phase-shifting method. There shall be two acceptable phase-shifting methods and only one is required. A temporal phase-shifting technique may be accomplished through mechanical displacement of the reference mirror that creates a 90° phase shift between images, resulting in a total of 5 interferograms obtained sequentially. A simultaneous phase-shifting technique may be accomplished through the use of quarter wave plates and birefringent mask to simultaneously cast 4 interferograms shifted 90° from each other at the detector array. The CCD camera shall be able to transmit interferogram images to the computer software program described in section 2.2.1.6 Computer and Installed Software.

2.2.1.6 Computer and installed software. A computer for the GBIS shall be provided in a desktop or laptop format. The operating system of the computer shall be Microsoft Windows 10 Professional or newer. The computer shall come pre-installed with a software program written to evaluate gage block characteristics examined through the interferometer.

The software shall also be provided on a physical format compatible with Windows operating systems. The computer shall be capable of accepting output readings from all environmental measurements monitored at the electronics cabinet to include air temperature, work piece temperature, ambient pressure, relative humidity, and CO2 concentration. The software shall be capable of using the provided environmental readings to calculate the air refractive index inside the environmental chamber. The computer shall be capable of accepting interferogram images from the CCD camera.

The software shall use the interferogram fringe spacing distance along with the air refractive index value to evaluate nominal length characteristics of the block under test. The software shall also analyze interferogram patterns to evaluate flatness and variation in length characteristics of the gage block face surface. The software shall be capable of expressing calculated values in metric and inch units. The software shall provide templates to record the history of block calibration values and determine certification according to the ASME B89.1.9-2002 American national standard.

2.2.1.7 High coherence light source. The light source for the interferometer shall be at minimum two monochromatic wavelengths provided by frequency stabilized Helium-Neon lasers. The internal mirror He-Ne tubes shall be ‘hard sealed’. The lasers shall be coupled to the interferometer through fiber-optic cabling to isolate heat sources from the measurement chamber.

Radiations of the frequency standards shall be as recommended by the International Committee for Weights and Measures (CIPM) for the practical realization of the meter. Refer to International Report “Practical realization of the definition of the metre, including recommended radiations of other optical frequency standards (2001)” published in Metrologia 40 (2003) pp 103-133. The values of the frequency f and of the vacuum wavelength λ should be related exactly by the relation λf = c0, with c0 = 299 792 458 m/s.

For a red wavelength with radiation from absorbing molecule 127I2, a16 or f component, R(127) 11-5 transition:

f = 473 612 353 604 kHz λ = 632 991 212.58 fm Relative standard uncertainty of 1 x 10-10

For a green wavelength with radiation from absorbing molecule 127I2, a9 component, R(12) 26-0 transition:

f = 551 579 482.97 MHz λ = 543 516 33.1 fm Relative standard uncertainty of 2.5 x 10-10 For a yellow wavelength with radiation from absorbing molecule 127I2, a1 component, P(62) 17-1 transition:

f = 520 206 808.4 MHz λ = 576 294 760.4 fm Relative standard uncertainty of 4 x 10-10

2.2.1.8 Measurement uncertainty. The GBIS shall provide a total expanded measurement uncertainty that meets the OIML R 30 Level 1 recommendation of ± (0.8 + 0.2 x L) µin. where L represents gage block length expressed in units of inches. It is acknowledged that at the time of publication for this specification, the OIML R 30 recommendation has been withdrawn by the OIML and that adoption of the recommendation is based upon meeting stipulated measurement decision risk limits outlined in the OPNAVINST 3960.16B. The historical OIML R 30 recommendation shall be deemed appropriate until an approved national or international standard has been established for the uncertainty of laser interferometers used to verify end standard lengths. The manufacturer shall clearly provide an uncertainty budget and calculation that list out in detail all the factors contributing to the uncertainty magnitude. The measurement uncertainty shall be given following the procedures in the ISO/IEC JCGM 100:2008 Guide to Uncertainty in Measurement (GUM). The combined standard uncertainty of the GBIS, using a coverage factor of k = 2 to provide a 95% confidence level, shall be based on the NIST Technical Note 1297 along with the uncertainty components as listed below:

• Laser short-term stability

• Fringe estimation

• Wringing film

• Roughness

• Flatness and parallelism

• Phase-change

• Optics flatness (including platen)

• Obliquity

• Source Size

• Sensor to air temperature difference

• Sensor to gage block temperature difference

• Variation of temperature

• Thermal expansion coefficient variations

• Resolution of instruments

• Drift of laser and environmental sensors

• Calibration of laser and environmental sensors

• Edléns equation

• ITS 90 (Material) interpolation

• ITS 90 (Air) interpolation

• Magnus equation

• Self-heating of air PRT

• CO2 concentration

2.2.2 Operator adjustments and controls. Operator adjustments and controls shall be as specified below in 2.2.2.1 Operator adjustments and 2.2.2.2 Operator controls.

2.2.2.1 Operator adjustments. The equipment shall include built-in adjustments or compensating devices for any function or parameter that would exceed the specified tolerance within the calibration interval. Under all specified service conditions the variation of any adjustment or control shall produce the desired control action to bring the equipment within tolerance.

2.2.2.2 Operator controls. Operating controls and indicators shall be readily accessible to the operator. The readings presented on controls and indicators shall be available to the operator without the need for operator conversion or calculation. The GBIS shall provide a pendant control to facilitate manual orientation of the motorized platen.

2.2.3 Internal cooling. The equipment shall be capable of operating without interruption for at least 24 hours at the designated temperature range as specified in section 2.7.1 Temperature without overheating, losing power, or otherwise exhibiting failure.

2.2.3.1 Cooling devices. Any temperature control devices used in the equipment shall be an integral part of the equipment and not degrade any aspect of performance or safety.

2.2.4 Front panel display indicators. Front panel/control panel display indicators designed to present change-of-state information to the operator shall be distinguishable at a distance of 1 meter in a maximum ambient light level of 100 foot-candles (fc) and a minimum light level of 1 fc. All display indicators shall be discernible at viewing angles from 0 degrees to 45 degrees in all directions.

2.3 Digital interface. The user of the GBIS shall be able to interface with the operation of the interferometer system through a software program installed onprovided laptop or desktop computer. The user shall be able to initiate the measurement process through controls provided within the software program.

The user shall be able to input necessary parameters for each measurement to include nominal length of block, thermal expansion coefficient, measurement units, gage type, serialized identification, and types of measurement to be performed. The software interface shall provide views of the interferograms produced from the interferometer system. The interface shall display real time readouts from environmental monitoring systems as well as fringe fraction values for each color wavelength determined from interferogram analysis. The software shall display measurement results to include deviation from nominal length, flatness of measuring face, and variation in length. The software shall also support generation of measurement uncertainties and test results as a certificate document in both Microsoft “Word” and “Excel” compatible formats.

Raw data of results shall be exportable in .csv, .xlsm, and .xlsx format. Primary power control and other non-measurement functions need not be controllable through the digital interface.

2.4 Electrical power sources and connections. The electrical power sources and connections shall be as specified below in 2.4.1 Electrical power source and

2.4.2 Electrical power connections.

2.4.1 Nominal and alternate power source. The equipment shall operate at the nominal values for voltage and frequency as specified in a and b below. An alternate power source is not required.

a. The nominal voltage shall be 120 V or 240 V (±10%).

b. The nominal frequencies shall be single phase 50 Hz and 60 Hz (±5%)

2.4.1.1 Interruption of power source. Voltage or frequency interruptions shall not cause damage to the equipment. A power source interruption is when the voltage falls in the range of 0 to 70% of nominal or frequency falls in the range of 0 to 90% of nominal. Equipment performance is not required during interruption of the power source. After power interruptions of any duration, the equipment shall automatically resume operation at either the last operating condition before interruption, or a default power-up condition.

2.4.1.2 Battery restrictions. For Navy applications, the use of lithium and mercury batteries is restricted and shall not be used in equipment. An exception is made for small lithium batteries used for memory backup or portable laptop style computers. A label containing the following caution and information shall be affixed to the outside of the unit as applicable for lithium batteries:

2.4.2 Electrical power connections. The requirements specified in 2.4.2.1 Input power cable through 2.4.2.4 Indicators shall apply to the equipment. The equipment shall operate with a ground at the primary power.

2.4.2.1 Input power cable. Input power cables shall be a detachable power cable provided by the manufacturer.

THIS UNIT CONTAINS LITHIUM BATTERIES

Battery Manufacturer:

Battery Part Number:

2.4.2.2 Input power switch. The operator shall be able to energize the equipment via an input power switch. A standby capability may be incorporated to activate and deactivate the equipment. If such a capability is provided, suitable warnings shall be placed in the operating and service manuals and on the equipment to warn the operator of possible shock hazards during servicing, due to partially activated power supplies.

2.4.2.3 Excessive current drain protection. An automated safety mechanism (circuit breaker/fuse) shall be incorporated to automatically disconnect the equipment in the event of current being drawn that is in excess of the equipment rating. The mechanism shall be readily available to the operator. Safety breaker or fuse shall be implemented on the input power line side (minimum) with external access to the breaker or fuse.

2.4.2.4 Indicators. Visual indication shall be provided on the front side of the equipment to indicate when the equipment is energized.

2.5 Enclosure physical characteristics. The physical characteristics of the equipment enclosure shall be as specified in 2.5.1 through 2.5.2.4.

2.5.1 Mechanical stability. The equipment shall be designed to preclude tipping during normal handling and operation.

2.5.2 Enclosure requirement. The degree of protection provided by the enclosure shall allow the complete equipment to satisfy environmental requirements.

2.5.2.1 Accessory stowage. The enclosure shall have provisions for the stowage of all accessories. Stowed items shall be available without disturbing enclosure seams.

2.5.2.2 Corners. All corners shall be designed to preclude injury to personnel or damage to material. All corners shall be adequately reinforced to protect the instrument from damage during environmental conditions specified for the enclosure.

2.5.2.3 Connections and controls. All electrical connections (except power connections) and operator controls shall be on the front of the enclosure.

Electrical power connections shall be on the rear.

2.5.2.4 Enclosures, console cabinet. Environmental monitoring equipment shall be fitted with a console cabinet. The console cabinet enclosure shall provide protection for the contained equipment from the specified environmental conditions. Access to the interior of the enclosure shall be provided.

2.6 Transit case. The GBIS shall be provided transit cases for each He-Ne laser module. Each transit case shall be a pelican style outer casing with inner cushioning material for reduction of shock, vibration, concentrated forces, and abrasion during handling and shipment. The laser modules shall be form fitted to the interior cushioning without the need for any dunnage material. The outer shell of the transit cases shall have identification marking as outlined in section

2.9 Marking and Identification.

2.7 Environmental requirements. The GBIS environmental performance requirements shall be as specified below in 2.7.1 Temperature through 2.7.4 Vibration. It shall conform to the specified performance when subjected to the environmental conditions of this specification.

2.7.1 Temperature. The GBIS shall conform to the specified performance and accuracy while being operated in a controlled temperature of 20 °C ± 0.3 °C.

2.7.2 Humidity. The GBIS shall conform to the specified performance and accuracy for conditions where the relative humidity is 35 to 45 percent.

2.7.3 Particle count. The GBIS shall conform to the specified performance and accuracy for conditions where particle count is held below 4 x 105 for particles larger than 1 µm per cubic meter of room volume and below 2 x 106 for particles larger than 0.5 µm per cubic meter. No particles larger than 50 µm.

2.7.4 Vibration. The GBIS shall conform to the specified performance and accuracy for conditions where maximum acceleration is held to 0.001 g for frequencies of 30 to 200 Hz and maximum allowable displacement is 10 µin for frequencies 0.1 to 30 Hz.

2.8 Safety. Any equipment that uses lasers shall meet at a minimum Class 2 specification as defined in the ANSI Z136.1 standard for safe use of lasers.

Operation of the GBIS shall not subject the operator to prolonged eye contact with a laser beam that cannot be mitigated by blinking or averting their eyesight.

Normal operation and routine servicing of the GBIS shall not require disassembly of the laser module or removal of its power supply enclosure.

2.9 Marking and Identification. Marking and identification of test equipment shall be as specified below in 2.9.1 Marking through 2.9.2 Equipment identification.

2.9.1 Marking. Test equipment shall be marked in a manner that does not adversely affect safety or performance.

2.9.1.1 Warning markings. Warning markings shall warn of the location, the nature, and the extent of a hazard. Letters of warning markings shall be of clearly legible gothic capitals. Warning markings shall have high contrast between the letter and background colors. The markings shall be as permanent as normal life expectancy of the test equipment, and located as close as possible to the point of danger. Warning markings indicting circuits of more than 500 Vdc or Vrms shall read:

DANGER HIGH VOLTAGE [maximum applicable voltage] VOLTS

2.9.1.2 Panel markings and processes. Panel markings shall enable the operator to identify the functions and use of all items requiring operator intervention.

2.9.2 Equipment identification. All equipment, excluding small accessory items contained within a reusable pouch or container, shall be identified with the items specified in a through d.

a. Manufacturer name

b. Equipment model number with installed options

c. Equipment nomenclature

d. Serial number

2.9.2.1 Identification plate. An equipment identification plate shall be affixed to any equipment enclosure that serves as a transit case. The identification plate shall contain the information provided below in 2.9.2.1.1 Identification plate.

The identification plate shall be permanently affixed.

2.9.2.1.1 Identification plate. The identification plate shall contain the information in a through f.

a. Configuration model number

b. Configuration nomenclature

c. Configuration CAGE

d. Component nomenclature

e. Component part number

f. Component serial number

2.9.2.1.2 Identification plate, supplemental. When accessories are required to meet the requirements of the CSS and the combination is given a part or model number designation that differs from that of the main unit, there shall be a supplemental identification plate containing the information in a and b. The plate shall be placed on the accessory container, as applicable. Whether or not a configuration model designation is assigned to the accessory or group of accessories, all major accessories shall be marked as in c:

a. Model configuration number: (Model number assigned to the equipment configuration).

b. Consists of: [part number of all components (excluding cables and adapters) comprising the equipment configuration].

c. Part of (or p/o): (Model number assigned to the equipment configuration, or main unit model number, as applicable), s/n:

(serial number assigned to the equipment comprising the set or the serial number of the primary instrument if a separate s/n is not assigned to the set).

2.9.2.1.3 Identification plate shall be corrosion resistant material.

2.9.2.1.4 Identification plate, marking. The plate background color shall be black.

It shall have a minimum character size of 1/8 inch. All characters shall be permanent using best commercial practices for laser engraving, etching, or photosensitive marking of material.

2.9.2.2 Reusable pouch or container. If a reusable pouch or container is provided, containers shall be marked “accessories for use with” (test equipment nomenclature or model number), or, “maintenance aids for use with” (test equipment nomenclature or model number), as applicable.

2.9.2.3 Item unique identification (IUID). A two dimensional Unique Identification (UID) data matrix conforming to MIL-STD-130 shall be included on the Identification Plate.

2.9.2.3.1 IUID data elements. The following is required per DFARS clause 252.211-7003, Paragraph 4(e):

• UID Type

• Issuing Agency Code

• UID Data Element

• Enterprise ID

• Original Part Number

• Serial Number

• Quantity Shipped

• Unit of Measure

• Description

• Unit Acquisition Cost

• Current Part Number Effective Date

• Current Part Number

2.10 Calibration interval. All frequency stabilized He-Ne laser modules shall be able to meet and maintain all performance specifications as indicated for a minimum of one year.

2.11 Reliability. All components of the GBIS to include laser modules when routinely calibrated according to their prescribed interval shall be capable of providing full specification for 1500 hours of continuous operation once a year, for a minimum lifetime of ten years. The manufacturer shall provide a summary of maintenance or warranty records to establish the validity of the statistical assertion. Records of the actual equipment model shall be used, unless the Government authorizes the substitution of records from a similar model, due to the lack of sufficient supporting documentation on the offered equipment. The manufacturer shall be able to reasonably establish the claim that the offered equipment shall provide reliability similar to the model used to provide substitute records.

2.12 Maintainability. Maintainability shall be as specified in 2.12.1 through 2.12.2.

2.12.1 Preventive maintenance. Preventive maintenance shall not require more than 15 minutes per a 30-day period. Preventive maintenance shall not require breaking of the equipment seams where calibration seals would normally be placed.

2.12.2 Maintenance provisions. Equipment shall be as specified in a through d to facilitate maintenance:

a. Accessibility for maintenance. The equipment shall be designed to ensure that maintenance access can be accomplished utilizing ordinary tools.

The equipment shall be constructed so that no damage to any component shall occur and no permanent distortion to any structural member shall be caused during maintenance and calibration.

b. Maintenance and calibration aids. When specified, special adapters, special tools, and patch cables required for maintenance or calibration shall be provided. They shall be identified as a set with unique part number and national stock number. Stowage shall be in a reusable pouch or container.

c. Accessibility for repair. The equipment shall be designed so that subassemblies and chassis components can be removed without removing other hardwired subassemblies or components.

d. Accessibility for adjustment. The equipment design shall permit adjustments to be made without removing any component or subassembly.

2.2.4 Front panel display indicators