At locations with a mercury barometer, commissioned ASOS, AWOS, or digital altimeter setting indicator (DASI) designated as the station pressure standard, analog aneroid barometers and analog altimeter setting indicators shall be standardized. Those analog instruments shall be routinely compared to the station pressure standard to ascertain continued reliability and to determine corrections to readings of the analog aneroid instruments. Comparison data shall be entered on MF1-13, using the guide for preparation on the back of the form. Figure 17-10, Sample of MF1-13 Barometer Comparison (Front), presents an illustration of the front side of Form MF1-13. Figure 17-11, Sample of MF1-13, Barometer Comparison (Back), presents an illustration of the back side of Form MF1-13. Station pressure standards shall be compared with travelling standards that are directly traceable to mercury standards at the National Institute of Standards and Technology (NIST).

17-49.   REQUIREMENTS FOR STANDARDIZING INSTRUMENTS

A series of comparative readings, as specified in paragraph17-50, Standardizing Procedures, shall be accomplished whenever:

a.  A station pressure standard is replaced.

b.  An aneroid barometer or altimeter setting indicator is installed, reset, or relocated.

c.  Erratic or excessive corrections are observed (see paragraph 17-53, General).

When standardization of an aneroid instrument is required (see Section 6, Performance of Aneroid Instruments, of this chapter), the observer shall make comparisons and determine mean corrections as follows:

a.  Make 10 comparisons of the aneroid instrument with the mercury barometer. If practicable, make these readings at hour intervals but, in no case, should the interval be less than 15 minutes.

b.  During the time the 10 comparisons are being made, the station pressure and altimeter setting for hour observations shall be based upon the readings of the mercury barometer. The aneroid instrument, using the correction from the preceding hour comparison, may be used to determine pressure values for SPECI observations and local needs.

c.  After the 10 comparisons have been made, compute the mean of the column 10 entries. Check the reliability of the instrument by verifying that the difference between corrected readings does not exceed allowable differences (see paragraph 17-65, Sling Psychrometer Ventilation). If determined reliable, enter and use the mean correction in column 12 as the "posted correction" to be applied to reading of the aneroid instrument; i.e., until the mean correction is redetermined.

d.  On each of the next 5 workdays, make two additional comparisons at a 6-hour interval and determine the mean correction as in c above, using the last 10 entries in column 10; e.g., using comparisons 3 through 12 on the first of the 5 days, 5 through 14 on the second day, etc.

Figure 17-10. Sample of MF1-13, Barometer Comparison (Front)

e.  After the comparisons above have been completed, and none of the differences between corrected readings exceed allowable differences specified in paragraph 17-54, Aneroid Barometer or Altimeter Setting Indicator, the aneroid instrument may be considered standardized and used for routine pressure readings.

f.  When an instrument has been standardized in accordance with the above instructions, the adjustment screw (aneroid barometer) or elevation scale (altimeter-setting indicator) shall not be reset without authorization from the appropriate headquarters or maintenance shop.

After an aneroid instrument has been standardized and accepted as reliable in accordance with paragraph 17-50, Standardizing Procedures, the observer shall make two comparisons at least six hours apart on the same day of each week, and:

a.  Following each comparison, shall conduct a check for reliability according to procedures in paragraph 17-49, Requirements for Standardizing Instruments.

b.  At the time of the second comparison, shall determine a mean of the last 10 correction values (column 10).

c.  Shall enter the mean correction in column 12 and check for excessive drift (see paragraph 17-50).

d.  Provided the instrument is determined reliable, shall use the mean (column 12) as the new posted correction to be applied to the aneroid instrument readings; i.e., until the mean correction is redetermined the following week.

a.  Disposition at LAWRS (ONLY LAWRS). MF1-13 forms shall be retained on station until reviewed by a representative from NWS Regional Headquarters.

b.  Disposition At Stations Other Than LAWRS. (NA LAWRS). Unless advised otherwise by the regional headquarters, the station shall retain the completed forms on station until they are reviewed by a representative from the regional headquarters. Forms more than 18 months old that have been reviewed may be destroyed.