(b) All wire standard resistors and the more important section of resistance apparatus f use in d-c circuits should be wound on metal or ceramic supports, preferably in a single laye Electrical connections to the resistance material should be brazed in all cases in which the tot resistance is less than 1,000 ohms. The resistance material should be protected against ox dation and other chemical action and should be annealed or aged by baking after winding. (c) Precision standard resistors should be so adjusted as to give an accuracy of at lea 0.02 percent without corrections. Precision resistance apparatus should be adjusted with 0.05 percent of nominal value. (d) Because comparatively rapid changes in resistance take place in new apparatus, it not advisable to calibrate new or repaired apparatus until at least two months after the sistors have been annealed and adjusted. Precision apparatus known to be new will be he in the laboratory (in the absence of other instructions) for at least a month, when the measur ments will be repeated to determine the drift in value, if any. No extra charge is made f these later measurements. Occasionally during the course of calibration it is discovered th the standard or instrument under observation is defective and in need of repair. In such i stances the item in question will be rejected and a fee equal to the published fee, in whole or part, will be assessed commensurate with the effort expended before the calibration was halte (e) Unless otherwise stated, the tests listed are generally made using a direct current such magnitude as to cause only a negligible heating of the resistance material. Calibration of standard resistors, bridges, and decade resistors consist of determinations of the resistan of the standards or of the resistance of the elements of the bridges or similar apparatus fro which values corresponding to all possible readings can be computed. Precision standa resistors are ordinarily measured at a temperature of 25 °C, while resistance apparatus measured at room temperatures, usually from 22 to 25° C. (f) The Bureau does not calibrate portable self-contained test equipment having relative low accuracy such as portable potentiometers, resistance test sets, and double-bridge ohr meters. The accuracy of these devices is such that a complete detailed determination of co rections is not economically feasible. Apparatus of this type may be spot-checked by measuri known voltages or resistances with them. Adequate calibration services of this type can obtained from a number of commercial testing laboratories. 201.101 Precision standard resistors. Standards of 10 ohms and less of the precision ty provided with amalgamated current terminals and designed for oil immersion must be of t four-terminal type, that is, must have both current and potential terminals. The resistan of standards having nominal values in the range 0.0001 ohm to 100,000 ohms will usually given to the nearest 0.0001 percent in terms of the calibrating unit (the legal unit) maintain by the Bureau with a group of 1-ohm standard resistors. Each report of calibration will sta the uncertainty of the reported value at the time of calibration. This uncertainty will va from 0.0001 percent for Thomas-type 1-ohm standards to 0.002 percent for resistors of nomin value 0.0001 ohm. Additional information regarding standard resistors is included with t report of calibration. Determination of resistance in oil bath at 25 °C. For all standards 201.101a-1 Two-terminal measurements. (*) 201.101a-2 Four-terminal measurements 0.01 ohm to 10 ohms (except Thomas type)___ (*) 201.101a-3 Four-terminal measurements, Thomas-type 1-ohm and standards (*) Determination of resistance in oil bath at 25 °C. For odd-valued 201.101c-1 201.101c-2 201.101z Measurement of resistance in oil bath at 20, 25, and 30 °C, and de- Two-terminal measurements.. Four-terminal measurements. For special tests not covered by the above schedule, advance arrange- (*) (*) *For current fees, see Appendix C. 201.102 Precision resistance apparatus. Corrections pertinent to apparatus of suitable quality submitted under this section will ordinarily be reported to a number of significant figures so chosen that normal variation of ambient conditions within the stated bounds of test conditions will not affect the corrections by more than a few units in the last place reported. Calibrations will be made at room temperature, usually 22 to 25 °C. 201.102a Precision decade and plug boxes. For decades not exceeding 10,000 ohms per step. (1) First point in each box. (2) Each additional point in same resistance box_ 201.102b Megohm box, 10 sections each 100,000 ohms... 201.102c Megohm boxes, wire-wound, 10 equal sections. Calibrations with (*) (*) (*) 201.102h Double ratio set for Kelvin bridge, with double set of fixed and vari- (*) 201.1021-3 Six-dial precision resistive voltage divider or universal ratio set___ (*) 201.102j Quick resistance ratio test on any precision voltage divider. Linera- (*) 201.102z For special tests not covered by the above schedule, advance arrange- *For current fees, see Appendix C. 201.103 Multi-megohm resistance standards-except wire-wound. Measurements made on resistors submitted under this section are accurate to 0.1 percent at the time of test if nominal values are in the range 106 to 1012 ohms; for higher-valued resistors the accuracy is 0.5 percent. In order that the reported results be of significance it is necessary that standards submitted for tests be made of suitable materials processed in such a manner that resistance values do not change rapidly with time. They should be so constructed and treated that the effect of relative humidity is minimized. The resistance of these standards usually depends on the magnitude of the applied voltage; the test voltage should therefore be specified. Each resistor should have an identifying number engraved on or permanently attached to it. Item Description Fee 201. 103a 201. 103b 201. 103c Determination of resistance of a resistor at one voltage (1.5 to 250 V) Determination of resistance of a resistor at one voltage (1.5 to 250 V) Determination of resistance of a resistor at each voltage (1.5 to 250 V) (*) (*) (*) 201. 103z For special tests not covered by the above schedule, advance arrange- *For current fees, see Appendix C. Inductance and Capacitance Measurements NOTE: Tests at radio frequencies are performed at the NBS Boulder Laboratories, Boulder, Colo., 80301. 201.104 Standard inductors. (a) Inductors for use in a-c bridges are ordinarily tested at 100, 400, 1,000, or 10,000 Hz (c/s) at a room temperature of 23° C and a relative humidity of 50 percent or less. Measurements at 10,000 Hz are limited to standard inductors of 0.1 henry or less. Most inductors used at 60 Hz can be tested at 100 Hz since the variation of inductance with frequency in this range is usually negligible. Purchase orders should state which frequency or frequencies are to be used for calibration purposes. A metal-encased standard is calibrated with the case connected to the "low" terminal of the inductor unless other conditions are specified. Variable inductors used as circuit elements in laboratory setups are low-accuracy devices which do not come within the purview of this schedule and should not be submitted for calibration. Q values are not supplied for inductors calibrated under this schedule. Inductors intended for use as Q standards at radio frequencies should be referred to the Boulder Laboratories. Mutual inductors used in magnetic testing for calibrating ballistic galvanometers should be calibrated with direct curren under item 201.403a. (b) Accuracy: Inductance values and accuracy statements given in reports of calibration depend upon two factors: (1) the accuracy of the comparison of the client's inductor with th NBS working standards of inductance; (2) the uncertainty in the derivation of the unit o inductance which is embodied in the NBS working standards. In general, inductance value will be given to as many significant figures as are justified at the time of measurement. Th uncertainty figure given in each report of calibration takes into account factors (1) and (2 stated above and will vary from 0.02 percent to 0.2 percent depending upon the nominal valu of the inductor and the frequency of the test current employed. (c) Inductors can usually be shipped safely by express but should be carefully packed t avoid damage to the coil fastenings and terminals. Item Description Fee 201.104a Determination of self or mutual inductance of a fixed inductor with (*) 201.104b Determination at an additional frequency 100, 400, 1,000, or 10,000 (*) 201.104z For special tests not covered by the above schedule, advance *For current fees, see Appendix C. 201.105 Standard capacitors. (a) Calibrations are ordinarily performed at 65, 100, 400, 1,000, and 10,000 Hz (c/s) with an ambient temperature of about 23 °C and a relative humidity of 50 percent or less. (b) The accuracy stated in the report of calibration is determined in part by the accuracy of the NBS measurements and in part by the performance characteristics of the capacito itself and is sufficiently broad to allow for variations in the stray capacitance at the connectors variations in temperature of a few degrees Celsius, considerable variation in relative humidity and atmospheric pressure, and frequency deviations of a few percent from the stated tes conditions. Over the above frequency range, and in the capacitance range from 0.001 pF to 100 μF, the available accuracy usually lies in the range 0.002 to 0.5 percent. (c) The capacitance value given is the equivalent parallel capacitance. In general determination of the equivalent parallel conductance with high accuracy is not feasible; how ever, for solid dielectric capacitors an approximate value is given without additional charge. (d) Continuously adjustable ("variable") capacitors are no longer calibrated by th Bureau. (e) In applying the following schedule to decade capacitance boxes the first entry and fe (201.105a) apply to a determination of the zero capacitance and conductance of the box (al dials set at zero). The second entry and fee apply to the determination of the capacitance and conductance added to the circuit when any one dial is advanced from zero to a specified setting and at the frequency used in determining zero capacitance. For measurements at additiona frequencies the schedule is applied in the same manner, i.e., the higher fee is used for the firs point (zero calibration) at the new frequency, and the lower fee applies to additional point at that frequency. |