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that a weight meet the adjustment tolerances established for Classes A,

M, or S-1 (as described in Circulars 3 and 547, Volume III, of NBS Handbook 77*) prior to submission. Normally weights are available from manufacturers, many of whom can furnish directly documentation suitable for meeting quality assurance contracts and requirements.

(b) Individual weights, or sets of weights, in the range of 30 kilograms to 1 milligram or 50 pounds to 1 micropound in decimal decimal subdivisions, which are designated as reference standards, must be of design, material, and surface finish comparable to but not necessarily limited to present Classes A, B, M, S, or S-1. Design, material, and surface finish of large mass standards (over 50 lb to 50,000 1b) must be compatible with the intended usage. For these large mass standards, an adjustment with reference to a nominal or desired value can be included as a part of the calibration procedure.

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(c) The values of true mass (and an apparent mass correction) included in the report will be determined by using computed volumes based the manufacturer's statement of density of the material, on the density computed from measured volumes, or, in the absence of this information, estimated density values. The apparent mass corrections are computed for 20 °C with reference to Normal Brass (density 8.4 g/cm3 at 0 °C, volume coefficient of of expansion 0.000054/C°) in an ideal air density of 1.2 mg/cm 3 Apparent mass corrections to any other basis can be furnished if requested.

(d) For periodic recalibrations of reference mass standards, the user need measure only differences between weights or groups of weights within a set and compare them with computed differences. As long as the agreement is within allowable limits, the values can be considered constant within the precision of the comparison process. Mass standards which are submitted to the Bureau for recalibration frequently are tested in this manner. If these tests indicate that no significant changes have occurred, a continuation report so stating and referring to the previous NBS Report of Calibration will be issued.

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212.311z Calibration of reference mass standards or supporting services described in TN 288.

212.312 Reference standards of volumetric apparatus.

(a) The procedure used almost universally for testing glass volumetric apparatus is to weigh the amount of distilled water contained or delivered with reference to the graduations marked on the instrument, the volume being computed from the density of the water (for tables see NBS Circular 19, available on request). The quality of the markings and the care exercised in reading or setting the liquid level are major factors in test calibration and usage. Normally the Bureau will accept instruments for calibration which have volumes in the range 1 mm to 1 gallon and which conform essentially to requirements contained in NBS Circular 602, "Testing of Glass Volumetric Apparatus,"* Federal Procurement Specifications NNN-B00789 (Buret, straight, precision), NNN-F-00395a (Pipet, volumetric). NNNP-00350a (Pipet, measuring), or NNN-F-00289a (Flask, volumetric), if such instruments are to be used as reference or transfer standards.

213.210 General.

(a) Direct inquiries to, and mark shipments for the attention Pressure Measurements Section, MET A153, National Bureau of

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ards, Washington, DC 20234. Telephone: 301-921-2177.

(b) Fees for the services described in this schedule have not fixed and charges will be made for actual costs incurred which usually not exceed $1,000.

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11a Deadweight piston gages other than controlled clearance gages (also called deadweight gage testers and pressure balances). Determination of effective area of the piston by comparison with a pressure standard, up to five test points at room temperature, 20 to 25 °C. Pressure fluid may be air, up to 600 psi, petroleum oil up to 60,000 psi, or synthetic lubricant (dioctyl sebacate) up to 120,000 psi. Determination of effective area requires previous weighing of piston assembly and the use of calibrated weights. If the Bureau has weights to fit, the gage may be calibrated using NBS weights. Controlled clearance piston gages. Determination of effective jacket pressure for zero clearance, and variation of effective area with jecket pressure. Test is made on an assembly of piston, cylinder, and jacket, not to be disassembled later. Determination of effective area requires previous weighing of piston assembly and the use of calibrated weights. If the Bureau has weights to fit, the gage may be calibrated using NBS weights. Inquiry should be made as to the parts to be submitted.

11b

11c

11d

11e

area,

Barometers, Fortin or similar types, range 28 to 31 in. Hg, tube bore 0.5 in. or greater, calibration at room temperature (20 to 25 °C) and atmospheric pressure.

Manometers, manually operated, with tube bore 0.5 in. or greater, in which both mercury surfaces are observed. Range to 100 in. Hg, calibration test at room temperature (20 to 25 °C) up to ten test points.

Manometers, with tube bore 0.5 in. or greater, in which both mercury surfaces are observed, and of design which permits calibration in terms of length, temperature, etc. Calibration of scale and thermometer, performance verified by comparison with pressure standard.

11z Special measurements not covered by this schedule (for example, calibration of pressure gages of high precision and stability).

213.220 General.

Direct inquiries to, and mark shipments for the attention of: Vacuum Measurements Section, MET A153, National Bureau of Standards, Washington, DC 20234. Telephone: 301-921-2196.

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213.221a

213.221z

*

Vacuum gages, in range 1 to 1,000 millitorr, of high stability such as diaphragm and quartz Bourdon tube types. Test at room temperature (20 to 25 PC).

Special measurements not covered by this schedule.

Vibration Measurements

213.230 General.

Direct inquiries to, and mark shipments

for the attention of:

Vibration Measurements Section, EM - 103, National Bureau of Standards, Washington, DC 20234. Telephone: 301-921-2533.

213.231 Vibration pickups.

(a) This applies to the calibration of velocity and acceleration types of vibration pickups. A Report of Calibration will be issued.

(b) Each piezoelectric acceleration pickup which is used with a cathode-follower or amplifier shall be accompanied by the cathode-follower or amplifier, the power supply if other than batteries, and the output and connecting cables.

(c) When instruments submitted are found to be unsuitable for test or unreliable, a charge will be made to cover the cost of the work done. For each test frequency in excess of 10, an additional charge equal to one-tenth of the fee will be made.

*McLeod gages are not accepted by NBS for calibration because use of a McLeod gage as a reference instrument depends on dimensional and volumetric measurements of the instrument made prior to assembly and on use of appropriate corrections, particularly capillary depression fluctuations, determined periodically during use. Thus, a calibration by comparison is considered to be of little value in the use of this type of instrument as a reference standard.

or quality control will be accepted for test. Values will be furnished in letter report.

(2) Suggested test procedures for the verification of the ompliance of precision grade glassware with specifications and tolerances re available on request.

(b) The usual calibration procedure for metal volumetric pparatus consists of determining the value "to contain" or "to deliver" by ither gravimetric means or by the use of transfer standards. Normally the ureau will accept instruments for calibration which have values in the ange 1 gill to 1,000 gallons, which comply essentially with the specificaions contained in NBS Monograph 62, "Testing of Volumetric Volumetric Standards,' nd which are free from dents, bumps, or scratches.

(1) While it is preferred that the zero index or the gage cale be adjusted and scaled prior to calibration, these operations can be ncorporated into the calibration procedure if requested. Slicker ype standards should be adjusted by the manufacturer.

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Item

Description

12.312z Calibration of reference standard volumetric apparatus.

212.313 Density determinations of liquids and solids.

(a) The usual procedure for determining the density of solids is ydrostatic weighing. At the Bureau liquid densities usually are deermined by gravimetric methods. The picnometer can be filled at a pecified temperature between 0 and 70 °C. Other methods are available epending on the requirements. The Bureau will accept requests for density eterminations if the need is critical, as in the support of scientific tudies or standard sample programs. Limitations on the mass, physical imensions, or volume of the sample are available on request.

(b) Specific gravity hydrometers covering the range 0.62 to 3 nd proof spirit hydrometers in the range 0 to 200 proof, proof, which are esignated reference standard hydrometers (used to test other hydrometers), ormally are accepted for calibration. A limited number of other types of eference standard hydrometers will be accepted for multipoint calibration ubject to a discussion of detailed requests. Instruments accepted must omply essentially with the requirements of NBS Circular 555, "Testing of ydrometers."*

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12.313z Density determination of liquids or solids or the calibration of reference standard hydrometers.

Available from Superintendent ffice, Washington, DC 20402.

of Documents, U.S. Government Printing

213.010 General.

Acoustic Measurements

(a) Direct inquiries to, and mark shipments for the attention of: Sound Section, FW - 222, National Bureau of Standards, Washington, DC 20234. Telephone: 202-EM 2-4040-7083.

(b) Fees for the regular services described in these schedules are listed in Part 8 of this publication. Fees for z items and other services have not been fixed and charges will be made for actual costs incurred. Upon request, estimates will be furnished for specific tasks which should provide a close approximation of actual cost.

phones

213.011 Microphones.

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(a) Pressure calibrations are performed only on type-L microsatisfying the requirements of USA Standards Institute Publication S1.12.1967, Specifications for Laboratory Standard Microphones. In dition, microphones must be suitable for use with the calibrating couplers shown in Figures 6 and 10 of Publication S1.10-1966, Calibration of Microphones. Copies of these Standards can be purchased from the Institute at 10 East 40th Street, New York, New York 10016.

(b) Pressure calibrations are reported in terms of open-circuit voltage per unit sound pressure applied uniformly to the diaphragm. The open-circuit voltage at the electrical terminals of a microphone depends on the stray capacitance connected across these terminals. The stray capacitance is defined by the geometrical configuration of the ground shield shown in Figures 6 and 13 of S1.10. If the ground shield shield dimensions are not adhered to in making use of the response levels reported by the Bureau, errors may result. Methods for the measurement of open-circuit voltage are described in Clause 2.1 and in Figures 1 and 7 of S1.10.

(c) The free-field response levels of Western Electric Company Type 640AA condenser microphones, or equivalent, can be computed with good accuracy from the pressure response levels reported by the Bureau, using Table Al of S1. 10, provided the microphone is mounted on a preamplifier of size and shape illustrated in Figure 12 of S1.10. However, the use of manufacturers' response levels in conjunction with Table Al might result in errors unless the manufacturer has measured pressure response levels in accordance with S1.10.

(d) Associated preamplifiers or power supplies should not be forwarded to the Bureau. The response levels reported are independent of the electronic equipment used in the test.

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213.011a

Pressure calibration of Western Electric Co. Type 640AA condenser microphone or equivalent from 50 to 10,000 Hz at discrete frequencies. Response given in terms of open-circuit voltage per unit sound pressure applied to the microphone.

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