General In the manufacture of calibrated Magnetometers, Gaussmeters, magnetic field standards, and search coils, it is necessary to have magnetic field sources of a higher order of accuracy to be able to check and calibrate such products. Calibrating System Description There are two basic types of uniform magnetic field sources in which the magnetic field intensity can be calculated with high accuracy as long as their respective construction geometry is carefully observed. A uniform magnetic field source is highly desirable for use as a standard due to the fact that precise positioning of articles being checked is not required. The long, single layer solenoid and the double coil Helmholtz arrangement are both classic examples of basic instrumentation devices, which have not been changed or improved upon in over fifty years. Magnetic Field Standards Magnetic field intensity in a properly constructed long solenoid can be calculated by the formula H = 1.257 NI / L cm. The magnetic field intensity in a properly constructed Helmholtz Coil arrangement is calculated by the formula H = 0.8992 NI / R cm. Both the geometric construction details and the field formulas are given in many textbooks as well as in the “Standard Handbook for Electrical Engineers”. The only requirement other than these formula constructed coils is that a known electrical current in standard DC amperes be circulated through the coil windings to create a very uniform, known magnetic field in the central area of such coils. Knowing the current in amperes the use can readily and accurately calculate the magnetic field intensity, in oersteds. Such formula built uniform magnetic field sources can be replicated anywhere, they are stable – no moving parts, no saturable magnetic material in their construction- therefore they display a very desirable linear characteristic, a necessity when being used as reference standard. Annis Helmholtz Coil Serial No. 65-M-10 was calculated and constructed according to formula so that the central uniform field area would be exactly 10 oersteds per standard, DC ampere of current flowing, through the coil windings. This particular coil was also checked by the National Bureau of Standards prior to their discontinuation of making magnetic calibration tests (N.B.S. Test No. 313.04/206875). In addition to the N.B.S. check, this Helmholtz Coil is periodically crosschecked against formula built long solenoid Serial Number 62-M-6, a still more basic type of reference magnetic field standard, but not as convenient to use. An identical Helmholtz Coil, Serial Number 65-M-11 was constructed at the same time and is maintained as a back up in the event of damage or loss of the N.B.S. checked coil. Current Measurement Four laboratory type Weston DC ammeters, one digital and three analog type, are maintained and used to accurately measure the calibrating current flowing in these standard coils. The digital and one analog ammeter are always connected in series to measure the calibrating current. These instruments are constantly checked against each other during the daily calibration of instruments. All four instruments are periodically crosschecked against each other to detect any significant deviation. Two of these four ammeters are sent to a qualified outside laboratory annually to have their calibration checked and certified traceable to N.B.S. Data sheets are logged and maintained showing correlation between current readings of the various instruments. Any significant discrepancy in readings calls for immediate remedial action and an independent calibration check of the suspected instrument. Environmental Conditions Two environmental conditions can effect magnetic field calibration. One is the ever-present local magnetic field, primarily the earth’s field, and the other is the temperature at which calibration checks are made. There are two approaches for eliminating the effect of the local ambient magnetic field when performing calibration checks. One is to create work in a zero field area and the other is to carefully orient the calibrating field source and tip the instrument being checked to that one angle where the local ambient field has zero effect on the instrument or probe being calibrated. This angle is readily determined by use of a magnetic compass and balanced dip needle, at the test location. By carefully orienting the plane of the Helmholtz Coils, so as to be parallel with local magnetic north-south direction, then adjusting the instrument platform located within the Helmholtz Coil so that the staff of the instrument being checked will be aligned exactly parallel with the dip of the local ambient field, at this one angle, any effect of the local magnetic field is entirely eliminated on the instrument being checked. At the same time, the instrument will be fully responsive to the calibrating field created by the Helmholtz Coil windings. These orientation azimuth and dip angles are regularly checked on a monthly basis in as much as the earth’s magnetic field, other local magnetic sources, do not remain constant either from an intensity or direction standpoint. Temperature is the other environmental condition that can effect calibration. Annis Model 25 Calibrated Pocket Magnetometers are quite stable and will withstand accidental exposure to strong magnetic fields as high as 400 oersteds without effecting calibration, but the barium ferrite magnet material in the instrument is somewhat temperature sensitive, amounting to 2% for every 18° F. shift in temperature from the normal 72° F. ambient. Therefore a critical table is used to correct the calibration current so as to compensate for any temperature deviation when the instrument calibrating temperature falls outs the noral 69° F. to 75°F. range. Thusly, both environmental conditions effecting instrument calibration are controlled. Annis Model 25 Calibrated Pocket Magnetometers are available in 8 different ranges, from the very sensitive _ gauss full scale to 100 gauss full scale. On order, these instruments can be serialized and furnished with a signed and dated certificate of calibration traceable to N.B.S. See attached sample certificate. Permanent written record is kept on all such certified instruments whether new or whenever they are sent in for repair or calibration check. Records are also kept on the periodic crosscheck between ammeters and between the uniform magnetic field standards. Of course, outside laboratory certification records of ammeter calibration are kept. Our magnetic field calibration facility has been surveyed a number of times and approved as meeting the calibration requirements of MIL-STD-45662. Proprietary manufacturing information and calibrating procedures have been written up and are maintained in a binder for in-house use. It is, of course, available to anyone making an official survey of our calibration facility. The above information is given as being true and correct. We feel that we can be justifiably proud of this stability and accuracy we have maintained over the years in the manufacture and calibration of magnetic instrument equipment that lies within our capability. Respectfully Submitted R.B. ANNIS CO., INC.
