Rapid changes of temperature or exposure to direct heat or sunlight may cause erratic performance of aneroid instruments. Jarring of instruments may dislocate elements of the linkage system. Excessive friction or leaking pressure cells may result in erroneous pressure readings. After verification of erratic or excessive corrections which exceed the limits given in paragraph 17-54, below, the observer shall discontinue use of the unreliable instrument and notify the appropriate headquarters or maintenance shop. However, questionable corrections noted infrequently at the time of strong gusty winds are discouraged.

17-54.   ANEROID BAROMETER OR ALTIMETER SETTING INDICATOR

Whenever the difference between the station pressure derived from the station pressure standard and the corrected reading of the aneroid instrument (preceding column 12 entry applied to column 9) exceeds 0.010 inch Hg (0.3 hPa), the difference should be immediately verified by making a second comparison, preferably by another observer.

a.  If the difference between the second set of corrected readings does not exceed 0.010 inch Hg (0.3 hPa), the observer shall disregard the first set of readings and use the second set in computing posted corrections.

b.  If verified differences between corrected readings for both the first and second comparisons are 0.011 to 0.015 inch Hg (0.31 to 0.5 hPa), the instrument shall be restandardized using procedures in paragraph 17-50, Standardizing Procedures.

c.  If verified differences between corrected readings exceed 0.015 inch Hg (0.5 hPa), the observer shall discontinue use of the aneroid instrument and notify the designated organization or maintenance shop.

17-55.   EXCESSIVE DRIFT

Occasionally, a defect in an instrument may cause the correction to increase progressively at an abnormal rate. If the posted correction (column 12) of a standardized aneroid instrument has changed more than 0.020 inch Hg (0.7 hPa) since the instrument was reset, the observer shall discontinue use of the aneroid instrument and notify the designated headquarters or maintenance shop.

17-56.   INSPECTION COMPARISONS

Comparisons of station pressure standards with instruments from a headquarters or maintenance shop are made during visits by a representative of that headquarters or shop. Whenever there is doubt as to the accuracy or reliability of the station mercury barometer, the person in charge of the station should notify the next higher headquarters and ask that such a comparison be made.

17-57.   ELECTRONIC PRESSURE TRANSDUCERS

The highly accurate and reliable electronic pressure transducers, including the digital altimeter setting indicator (DASI), the ASOS, the AWOS, and other approved new technologies, are compared by highly reliable travelling standards that are continuously checked against national mercurial standards.