a.  Temperature. The observer shall read the thermometer (attached to the barrel) to the nearest 0.5°F.

b.  Adjusting the Cistern. The barometer shall be mounted in a vertical position in accordance with applicable instructions. The observer shall adjust the cistern as follows:

(1)  Tap the barrel near the top of the mercury column.

(2)  Turn the thumbscrew at the bottom of the barometer until the surface of the mercury in the cistern touches the tip of the ivory point (i.e., until the top coincides with its image in the mercury). If a dimple forms on the surface, indicating that the mercury has been raised too far, turn the thumbscrew in the opposite direction until the dimple disappears and the ivory point coincides with its image in the mercury. Contact of the mercury with the ivory point is more easily seen against a white background.

(3)  Set the vernier so that the base just cuts off light at the highest point of the meniscus (the curved upper surface of the mercury column (see Figure 17-5, Mercury Barometer Readings Obtained from Various Types of Scales and Verniers). A white background facilitates this setting.

(4)  Lower the mercury about 1/4 inch from the ivory point; do not change the setting of the vernier.

17-35.   READING THE MERCURY BAROMETER

The observer shall read the barometer to the nearest 0.001 inch in the manner appropriate to the vernier and scale in use (see paragraph 17-37, Mercury Barometer - Scale Reading).

17-36.   MERCURY BAROMETER - TOTAL CORRECTION

The observer shall determine the Total Correction (the "Sum of Corrections" from the barometer correction card and the temperature correction) and add it to the reading obtained in paragraph 17-35 above, to obtain station pressure. The observer shall use one of the tables listed below to obtain the proper temperature correction when computing station pressure from mercury barometers (unless otherwise directed by separate instructions pertaining to special types of barometers).

a.  TA 455-0-1, "Correction of Mercurial Barometer for Temperature, English Measures."

b.  TA 455-0-4 or TA 455-0-4A, "Barometer Total Correction Table (for Fortin barometers, scale true at 16.7°C)."

c.  Table 5.2.1, "Correction of Mercurial Barometer for Temperature (scale true at 16.7°C)" in the Manual of Barometry (FMH No. 8).

d.  Table 5.4.1 "Barometer Total Correction Table (for Fortin barometers, scale true at 16.7°C)" in the Manual of Barometry (FMH No. 8).

Figure 17-5. Mercury Barometer Readings Obtained from Various Types of Scales and Verniers

 

Figure 17-6. International Table for Mercurial Barometer Temperature Correction

Example of procedures using Figure 17-6, International Table for Mercurial Barometer Temperature Correction:

(1)  Given: Observed mercurial barometer (ML-512) reading: 29.783 inches.

(a)  Attached thermometer reading: 71.5°F.

(b)  Adjacent tabulated corrections in Table 5.2.3 Manual Of Barometry (FMH No. 8).

             29.5       30.0
71.5     -.105     -.107

(2)  Enter the interpolation table with the difference (.002), find the column which contains the pressure value (".626 to .874" for .783), and read the interpolated difference (.001).

(3)   Apply the interpolated difference to the first (left) tabulated correction (e.g., .105 + .001 = -.106 temperature correction).

(4)  This temperature correction plus the sum of corrections equals the total correction to be applied to the observed mercurial barometer reading; i.e., corrected station pressure.

17-37.   MERCURY BAROMETER - SCALE READING

Mercury barometers are provided with verniers (movable scales) as an adjunct to the primary stationary scale to facilitate reading the primary scale to thousandths of an inch. Verniers having several ratios are in use. These ratios represent the ratio of the number of lines on the scale to the number of lines in an equivalent distance on the vernier. The zero line of the vernier is also the index for primary-scale readings. Whenever the index coincides exactly with a line on the scale, the barometer reading corresponds exactly to the value of this line (see Figure 17-5, Mercury Barometer Readings Obtained from Various Types of Scales and Verniers). When the index is intermediate between two lines of the scale, the reading is the sum of several increments made up of a primary-scale reading and a vernier reading obtained as follows:

a.  The primary-scale reading is the value corresponding to the line on the scale immediately below the index.

b.  The vernier reading is determined as follows:

(1)  When the line of the vernier, other than the index and top lines, coincides exactly with a line on the scale, the vernier reading corresponds exactly to the value of the vernier line.

(2)  When none of the lines of the vernier coincides with lines of the scale, locate the two lines of the scale that include a successive pair of vernier lines between them and are also the scale lines most nearly adjacent to this pair of vernier lines. The vernier reading in this case is the sum of two increments, obtained as follows:

(a)  The first increment is the value of the lower of the successive pair of vernier lines selected above.

(b)  The second increment is interpolated. It is the fractional value of one vernier division represented by the displacement of the vernier from a setting corresponding exactly to the value selected in (a) and toward a value exactly one vernier division greater than (a) (see Figure 17-5).

(c)  The vernier reading is the sum of the first and second increments.

c.  The barometer reading is the sum of the primary-scale reading in a and vernier reading in b(1) or b(2).

Readings of mercury barometers should be corrected for scale error and capillarity, gravity, removal (i.e., the difference between the actual elevation of the barometer and the assigned station elevation), and any known residual errors. The sum of these corrections should be obtained from the barometer correction card as issued or verified by the pertinent regional headquarters or maintenance shop.

17-39.   READING BAROGRAPHS FOR STATION PRESSURE

The observer shall obtain readings from barographs as follows:

a.  Lightly tap the top of the instrument case.

b.  Read current chart value to the nearest 0.005 inch or 0.1 hPa, interpolating for values lying between the printed ordinates.

c.  Add the correction determined in accordance with the instructions for Barograph Corrections, column 38, MF1M-10C, to the value obtained in b, above, to obtain the station pressure.

17-40.   BAROGRAPH TIME CHECK

Immediately after the 6-hour correction has been determined, the observer shall enter a time-check line and the time (LST) of the check on the barogram. The line should be about equal in length to the width of two divisions on the chart and should be made carefully to avoid injury to the delicate mechanism of the barograph. The observer shall not make a time check line whenever the instrument is cold enough that the pen might not return readily to the pressure trace because of increased viscosity of the fluid in the damper or dashpot.

17-41.   BAROGRAPH CLOCK AND CHART SCALE

The observer shall determine that the clock is running and the ink is flowing properly, and note the position of the pen on the chart. Whenever it appears that the pen will pass off the printed divisions of the chart, the observer shall set the pen up or down equivalent to one full inch of pressure by means of the adjusting screw and renumber the lines accordingly. The observer shall indicate on the chart the time of adjustment (see Figure 17-7, Barogram).

17-42.   BAROGRAMS

The observer shall change charts at 6-hour times closest to noon LST. If changing the chart must be delayed, the observer shall change it at the time of the next 3-hourly observation in order that the pressure-tendency record will be uninterrupted. On barographs with 12-hour gears, the observer shall change the chart daily.

a.  On barographs with 4-day gears, the observer shall change the chart on the 1st, 5th, 9th, etc.

b.  Before placing a chart on the barograph, the observer shall use a typewriter, rubber stamp, or ink pen to enter the following data:

(1)  In the upper left corner, or in spaces provided, enter the name of the station and type, state, meridian of Local Standard Time, and elevation of station (H_p) to the nearest foot (as shown on the barometer correction card). Where provision was not made for this entry, identify the value with the prefix "H_p =" (e.g., H_p = 317 feet).

Figure 17-7. Barogram

(2)  In the spaces provided, or above the appropriate noon time lines, enter the date of beginning and ending of the trace.

(3)  Immediately preceding the printed figures along the first and last time arcs, enter the appropriate figures to indicate the chart range (e.g., 28 preceding the printed 00 on the 28.00 inch line).

(4)  In the spaces provided; otherwise, near the point where the trace will begin, enter ON, time to the nearest minute (LST), and the current station pressure from block 36, MF1M-10C.

c.  After adjustments or removal of a completed barogram the observer shall:

(1)  Enter the time of each adjustment and an arrow to indicate the point of adjustment.

(2)  In the spaces provided; otherwise, near the end of the trace, enter OFF, the time to the nearest minute (LST), and the current station pressure from block 36, MF1M-10C.

(3)  Enter above the time-check lines the appropriate corrections; e.g., from column 38, MF1M-10C.

(4)  When an adjustment for pressure is made, enter the current station pressure and corrections applying to both the preceding and following record; e.g., -.055/0 near the break in the trace (see Figure 17-3, Evaluation of Precipitation on Weighing Rain Gauge Chart).

(5)  On 12-hour barograms, whenever the traces for successive cycles of rotation intersect, identify intervening segments of the trace as necessary to preserve the identity of the trace for each rotation; e.g., when each rotation is on a different day, enter the day of the month in a circle of the trace for one rotation. (See Figure 17-8, Example of Intersecting Barogram Trace.)

Figure 17-8. Example of Intersecting Barogram Trace

d.  The observer shall forward completed barograms monthly to NCDC with the MF1M-10Cs.

17-43.   ADJUSTMENT OF THE BAROGRAPH PEN FOR PRESSURE

To adjust the position of the pen, the observer shall turn the knurled pressure-adjusting knob at the top of the cylindrical pressure-element housing until the pen is at the correct station pressure. The observer shall tap the case or chassis lightly to overcome any sticking in the linkage mechanism before checking the adjustment of the pen. The observer shall adjust the barograph to a zero correction when:

a.  The chart is changed and the correction exceeds 0.01 inch or 0.3 hPa, or

b.  The correction exceeds 0.05 inch or 1.5 hPa.

17-44.   BAROGRAPH ADJUSTMENT FOR TIME

To adjust the cylinder for time, the observer shall turn it counterclockwise until all slack motion in the drive mechanism is removed. If the pen position does not bear the proper relationship to the time-ordinate lines after the slack has been removed, the observer shall continue to turn the cylinder counterclockwise with sufficient force to override the friction drive until the timing error is eliminated. The observer shall adjust the instrument promptly if, at any time, the record trace is in error by more than 1/4 of a chart division.

17-45.   SELECTION OF BAROGRAM

At stations with 12-hour barographs, use WS Form 455-18. At stations with 4-day barographs, use either WS Form 1068C, with vertical scale mid value .250 inch, or WS Form 455-17, with mid value .750 inch, whichever is most appropriate for the station. The general rule is to select the barogram for which the mean-annual, station-pressure value would be closest to the middle of the chart. The mean-annual pressure can be estimated from climatological data for the station or the barometer correction card. If these are not available, use Figure 17-9, Selection of Barograms. If an extreme high or low system will cause the barograph pen to go above or below the chart, adjust the pen up or down an inch for the duration of the event.

a.  Remove the pen from the chart by means of the pen shifting lever.

b.  Open the barograph case (some Barograph cases must be opened to expose the pen shifting lever).

c.  Lift the cylinder vertically until it is free of the spindle and remove the chart from the cylinder. Avoid storing or handling charts in a manner that might smear the trace before it is dry.

d.  Fit the replacement chart smoothly and tightly on the cylinder, with the bottom edge of the chart uniformly in contact with the flange at the bottom of the cylinder, and replace the

clip. Inaccurately cut charts should be trimmed along a line parallel to and 1/4 inch below the lower, horizontal boundary ordinate.

e.  Wind the clock and lower the cylinder gently over the center spindle until the gears have fully meshed, holding the cylinder at the top and bottom to avoid disturbing the position of the chart.

f.  Fill the pen with ink and return it to the surface of the chart, adjusting it, if necessary, for pressure and time. Check the pen and clock for operation.

17-47.   MAINTENANCE OF PRESSURE REDUCTION COMPUTER

Maintenance procedures for the pressure reduction computer are the same as those for the psychrometric calculator (see paragraph 17-84, Maintenance of Psychrometric Calculator).

Figure 17-9. Selection of Barograms