The provisions of this section are applicable only at LAWRS.

The only pressure related measurement required at LAWRS is the altimeter setting.

14-50.   DETERMINING ALTIMETER SETTING (ONLY LAWRS)

The altimeter setting shall be determined by use of an altimeter setting indicator (ASI), a digital altimeter setting indicator (DASI), ASOS, AWOS, electronic pressure transducer, or a mercury barometer. These instruments shall be routinely compared and corrected as described in this section. The correction for each instrument used to report altimeter setting shall be determined. The latest correction, even if zero, shall be displayed on the instrument. The posted correction shall be added to the instrument's reading before reporting the altimeter setting. Do not change the value of the altimeter setting in the ASOS one-minute page to add a correction. This will turn off report processing, which can only be turned back on by an NWS technician.

14-51.   ALTIMETER SETTING FROM ALTIMETER SETTING INDICATORS (ONLY LAWRS)

Altimeter setting indicators may be digital or analog. If a digital instrument is used, the altimeter setting shall be determined by adding the posted correction to the reading. Some digital altimeter setting indicators including electronic pressure transducers do not require any corrections. If an analog instrument is used, the following procedures shall apply:

a.  Tap the face of the instrument lightly with the finger to reduce the effect of friction on the pointer mechanism.

b.  Read the pressure scale of the indicator at the pointer, to the next lower 0.005 inch.

c.  Determine the correct altimeter setting by adding the posted correction to the reading and rounding the sum down to the nearest inch and hundredths of inches.

14-52.   DETERMINING THE RELIABILITY OF ALTIMETER SETTING INDICATORS (ONLY LAWRS)

The reliability of each altimeter setting indicator (ASI) shall be verified as follows:

a.  Compare the ASI readings daily with the altimeter setting obtained from an adjacent FSS, LAWRS, or NWS office meeting the criteria of paragraph 14-53, Daily Comparison with an Adjacent Station (Only LAWRS), or weekly with an approved commissioned pressure standard at the facility as described in paragraphs 17-53 through 17-56.

b.  If the difference between altimeter settings does not exceed .02 inch at precision approach locations or .05 inch at other locations, the ASI is considered reliable and may be used, with the posted correction, to report altimeter setting values.

c.  If all ASIs at the facility exceed limits, the altimeter setting shall not be reported.

d.  Instruments which exceed the stated limits shall be reported to maintenance personnel and, for NWS maintained commissioned ASOS’s, or other NWS maintained pressure instruments, to the appropriate NWS office.

e.  At locations with a commissioned ASOS, or commissioned dual transducer AWOS units, the ASOS/AWOS becomes the pressure standard. At these commissioned locations, mercurial barometers are no longer required. Per FAA Order 7210.3, an ASI shall be compared to the pressure standard daily and a DASI shall be compared to the pressure standard at least monthly. When the difference is less than the tolerances specified in 14-52, the value (+ or - ) is posted on or near the ASI/DASI and applied as the correction factor to determine the operational altimeter setting.

Locations without an approved pressure standard may compare their altimeter device against values obtained from an adjacent NWS office, FSS, or a LAWRS with an approved pressure standard provided:

a.  At locations where precision approaches are conducted, the weather station is not more than 10 nautical miles away and, at both locations, the wind speed is 12 knots or less with no gusts above 15 knots.

b.  At all other locations, the distance does not exceed 25 nautical miles and, at both locations, the wind speed shall be 15 knots or less with no gusts above 20 knots.

c.  The difference in elevation does not exceed 100 feet at precision approach locations and 200 feet at all other locations.

d.  The station's temperature, at both locations, is within 30° F (16ºC) of the Standard Atmosphere Temperature for the station's elevation.

The observer shall not use altimeter setting values from aneroid instruments when the difference exceeds .02 at precision approach locations or .05 at all other locations. (Note: LAWRS locations using an automated sensor, such as ASOS or AWOS, are not required to conduct a daily comparison of the sensor output with an adjacent station.)

Altimeter settings may be obtained from a mercury barometer. The mercury barometer may be used to determine station pressure from which the altimeter setting may be determined. The procedures for using the mercury barometer to determine station pressure are given in paragraphs 17-34 through 17-38. The altimeter setting may be determined from station pressure by use of a pressure reduction computer, reduction constant, or table.

14-55.   ALTIMETER SETTING FROM A DASI

Altimeter settings may be obtained from a DASI through direct readings. Some DASI’s may have a correction posted from a travelling standard.

14-56.   ELECTRONIC PRESSURE TRANSDUCERS OR OTHER DASI’S

A DASI may be used to determine station pressure from which the altimeter setting may be determined. The altimeter setting may be determined from station pressure by use of a pressure reduction computer, reduction constant, table, or by direct readout.