THE MEASUREMENT OF PRESSURE: THE ANEROID
BAROMETER
The aneroid[1] barometer has become an instrument of the greatest usefulness to the explorer, the meteorologist and the airman. To the last named it is indispensable; no other form of barometer to take its place has been devised. Its great virtue is its portability.
The Construction of the Aneroid Barometer.—The essential part of the aneroid barometer is the shallow metal box with thin corrugated top and bottom, usually of German silver, having a thickness of 0.004 inch. The corrugation gives a much greater degree of expansion and contraction than would be possessed by a plane surface.
The box is the vacuum chamber or cell. The top and bottom are so elastic that, when the air is exhausted, they collapse almost completely. To prevent permanent warping a stout steel spring attached to a stud in the top of the vacuum box pulls it into a normal position. This mechanism results in a surface that is very sensitive to atmospheric pressure.
A train of levers, a chain and a drum translate the movements of the vacuum chamber covers, caused by changing pressure, into a circular motion; an index pointer moves back and forth over an arc graduated to represent inches of mercury. A movable scale with a zero point that can be set at a desired position encircles the barometric pressure scale. This scale is graduated to express altitudes.
The steel service spring and the metal of the vacuum chamber are weakened by warmth, thereby impairing the accuracy of the readings. In some aneroid barometers this is offset to a considerable degree by the admission of a small portion of dry air into the vacuum chamber; in others, the long lever extending from the steel service spring is made of two strips—brass and steel— brazed together. The altered length of the lever is made to offset the weakening of the service spring. An aneroid of this construction is thereby compensated for temperature. A barometer bearing the name of the maker, and not a fictitious name, is pretty apt to be as it is represented. Among commercial aneroid barometers one may find instruments thus marked in which the compensation is far from perfect.
When the aneroid is to be used wholly to indicate weather conditions, compensation, although desirable, is not essential. On the other hand, if it is to be used mainly for measuring altitudes, compensation must be regulated with extreme care. A compensated aneroid requires no temperature correction; the compensation is for the purpose of eliminating such corrections.
The Goldschmidt type of aneroid differs from the type commonly known by dispensing with the train of levers. A micrometer screw working in the cover of the box measures on its graduated rim the amount of the movement of the corrugated top of the vacuum chamber. When operated by an expert trained to the use of micrometry, this type of aneroid possesses many merits. It is not well adapted to general use.
At the best, the aneroid is a delicate instrument requiring great care, especially if it is a part of an engineer’s equipment. The reading of an instrument having a large dial will change with any material change in its position. If the user watches its variations, however, such erratic changes—and they are small—will not result in erroneous readings.
Adjustment of Aneroid Barometers.—An aneroid barometer is sometimes blamed because it does not agree with the reading of a mercurial barometer at the same level. This may occur when rapid changes in pressure are taking place. An aneroid of the best type is very sensitive. It responds to changes in pressure far more quickly than does a mercurial barometer. Because of its complex mechanism it is easily put out of adjustment; moreover, it will get out of adjustment for causes that are not well known.
Adjustment to a correct reading should be made, if possible, when pressure is stationary. The adjustment is made by means of the small screw in the back of the case—usually the only screw-head in sight. The index hand moves in the same direction that the screw-driver turns. The adjusting should not be used to set the index more than three-tenths of an inch. If the error is materially greater than this, it is better to have the adjustment made by an expert. When this cannot be done without delay, the error may be temporarily reduced, for convenience in reading, to even tenths of an inch. If it is desirable to have the index read to sea level reduction, it may be lifted off the stud and replaced as nearly as possible in the correct position. Any slight difference may then be taken up by the adjusting screw.
In moving the adjustment screw, one must take into consideration the position in which the instrument habitually rests. If a barometer which has been adjusted to a hanging position is laid upon its back, the reading changes several hundredths of an inch, and vice versa. It therefore must be held in its habitual position when the adjustment is made.
If the error in reading is two- or three-tenths, the observer must watch the readings for several days to ascertain if the adjustment has changed. If the index has been set forward it is apt to “creep” forward still further; if backward, the creeping will be backward. The reason therefor is not known with certainty. An aneroid taken to a higher elevation is apt to respond quickly to the reduced pressure; taken to a level materially lower, it may not respond so quickly.[2]
If an aneroid is in a proper condition, tapping the case with the finger will cause an instantaneous vibratory movement of the index which will settle each time to the same position. If it fails to recover its normal position, bring the instrument rather sharply down upon a chair cushion or cane seat; if the index fails to recover its position, or does not vibrate, a binding at the lever joints exists, and the instrument should go to the repair shop. The best test as to whether or not it is in good working condition is its sensitiveness to slight changes in elevation—for instance, the difference in elevation between the adjacent floors of a house. The response of the index should be instantaneous. An aneroid of the best type should show the difference of elevation between the top of a table and the floor.
Engineers’ Aneroid Barometers.—Within the past few years material improvements in the construction of aneroids have removed the defects noted in the preceding paragraphs. On ordinary aneroids the divisions of the pressure scale are equal, while those of the altitude circle gradually diminish. It is evident therefore that a vernier could not be used on such a scale.
In the scale graduation of the engineer’s type of aneroid, the arrangement is reversed; the pressure scale divisions diminish while those of the altitude circle are made equal. If the scale divisions represent 20 feet, the vernier subdivides them into 2-foot divisions. Many of the newer instruments have these values in scale construction; on others the scale divisions are 50 feet and 5 feet.
Although the engineer’s aneroid is compensated for temperature a slight temperature correction is advisable where the difference in altitudes is considerable. P. R. Jameson has deduced the following rule: If the sum of. the number of degrees at the two stations is greater than 100° degrees F (55° degrees C), increase the height by one one-thousandth part for each degree F in excess of 100° degrees F; if the sum of the number of degrees is less than 100° degrees F, diminish the altitude by one one-thousandth part for each degree F.
In using the engineer’s aneroid for determining altitudes the zero point may be set at the station of known altitude. For reasons explained in a preceding paragraph, such a proceeding will not do with an aneroid whose scale divisions on the altitude circle are unequal. In using such an instrument the zero point should be set at a designated position and the correction made for the variation which the reading reduced to sea level shows to be necessary.
Pocket Aneroid Barometers.—This term is applied to small instruments about 2 inches in diameter. In quality they vary from good to poor. The chief virtue about them is convenience and portability. In spite of the name, the pocket is not a suitable receptacle in which to carry such an instrument. The moisture from the body sooner or later affects the metal mechanism; moreover, the knocking and banging which it is likely to receive if carried in an overcoat pocket are equally hurtful. In traveling, it is carried most safely in a grip sack.
A very good place for the pocket aneroid is the observer’s desk and it is advisable to set the zero mark of the altitude scale daily at the index position. Thereby is inculcated a habit of watching the barometric changes far more closely than is the custom when one must go across the room to set the mercurial barometer for a reading. The close observer gets a much better view-point of daily variations than does the casual observer.
Specific Uses of the Aneroid Barometer.—A custom that is wellnigh universal makes the mercurial barometer the standard instrument for the measurement of pressure at Weather Bureau stations. There is no doubt of the wisdom of this practise. For a substantial instrument not easily getting out of order, and susceptible of close reading, no other form of barometer approaches it. Such instruments as the Marvin normal barograph represent the highest skill in precision instruments.
The modern aneroid is quite as essential as the mercury barometer in the equipment of a Weather Bureau station, a maritime observatory, or a meteorological laboratory. For use on vessels it has many advantages over the mercurial barometer, Extras may be carried on various parts of the ship where convenience suggests. Not the least virtue of the ship’s aneroid is the fact that it responds to pressure changes more quickly than does the mercurial barometer.
Aneroid Recording Barometers.—Recording aneroids are sold under various copyrighted names; in Weather Bureau cant such an instrument is known as a barograph. The essential feature is one or more vacuum chambers, a drum moved by a clock, and a ruled sheet of paper on which the record is made. The better type of barograph has a battery of eight vacuum chambers, one upon the top of another. This arrangement permits the movement due to pressure to be multiplied eight-fold. A finer adjustment and a more accurate record is gained thereby. Temperature compensation is effected by the admission of a measured quantity of dry air into one of the chambers.
The long lever, the pen arm, carries a pen which presses lightly on the ruled paper wrapped around the drum. A milled screw adjusts the pressure of the pen on the paper, and a switch enables the observer to throw the pen off or on at pleasure. The drum makes one revolution per week. The record sheets are ruled with horizontal lines representing inches, halves, and twentieths. Arcs of circles, having radii equal to the length of the pen arm, divide the record sheet into midday and midnight periods, and two-hour intervals. Eight o’clock Monday morning is the normal time for changing the record sheets used by most observers. These contain day and month spaces for
Barograph.
recording dates. In Weather Bureau practise the sheets are changed on the 1st, 8th, 15th, 22d and 29th days of the month.
The clocks used in most barographs are watch movements of the finest type. It is desirable to have the clock oiled and cleaned once a year. The clock is practically dust-proof, being within the drum, and protected also by the glass case which incloses the mechanism of the barograph.
In replacing the record sheet the observer must look carefully to two things; the lower edge of the record sheet must fit closely to the collar at the lower edge of the drum; the lines of equal pressure must match precisely. A failure to conform to these requirements leads to incorrect records.
The ink used consists usually of glycerine, water and color pigment. If a permanent record is required black ink is preferable; green or blue is fairly permanent; red and purple fade in the course of a few years. A red ink with a madder or a genuine carmine base will not fade. Aniline red inks are marketed as “carmine,” however, and they are not at all permanent.
The pen of the barograph has a sleeve which slips over the end of the pen arm. It is not always easy to remove it when the pen requires cleaning. If it cannot be removed from the pen arm, lift the drum off the spindle; hold the pen firmly and clean with a small camel’s hair brush and water. Before filling the pen, draw a narrow strip of paper or a very thin spatula blade through the prongs of the pen in order that the ink may flow freely. With reasonable care for its cleanliness the pen will make a sharply-cut line; a foul pen leaves its own record.
The milled screw in the bar directly over the vacuum chamber adjusts the pen so that it has the correct position on the record sheet. The pen may be adjusted to record sea level pressure; but, as a rule, it is better to keep local pressure. The record sheets are usually lithographed with figures ranging from 28 to 31 inches. They are lithographed for other altitudes and also without any altitude marks. A “long-range” barograph, registering from 25 to 31 inches, is also made. Metric charts may be obtained from dealers in meteorological supplies.
When a barograph is to be moved—if carried otherwise than by hand—the drum and the ink bottle should be removed and packed separately. The pen arm should be fastened loosely to the switch rod. A dozen thicknesses of tissue paper or of soft cloth should be wrapped around the glass case and stand; they should be fastened so firmly that the parts cannot jostle. With an additional protective wrapping of heavy paper the instrument will ride safely in the packing case. There should be no packing of any sort around the vacuum chambers and levers.
If the barograph is carried by hand, the drum need not be removed from the spindle, but the pen arm should be thrown from the drum. A handle should be fixed to the package so that it may be carried in its proper position.
Interpretation of Barograph Records.—The usual eight o’clock observations of pressure furnish nothing more than changes in pressure at twelve-hour intervals. For the purpose of scientific study the continuous barogram is the best object lesson. The West India hurricane, the ordinary storm, the heavy downpour, the thunder-storm and the cold wave—each leaves its individual earmarks on the record sheet. In no other way can the observer work out his position and time of diurnal inequality so well as by the use of the record sheet.
A discussion of the specific features made by the barograph pen, noted in the preceding paragraph, is not necessary. Each should be noted on the record sheets, at the time of its occurrence. At times the specific features in a record sheet enable an observer to make forecasts that would escape notice if the observations were made with an ordinary barometer. Thus, if the trace of the pen in the progress of a falling barometer is convex, an increasing violence of an approaching storm is indicated. In the same manner, a concave trace of the pen with a rising barometer indicates an increasing force of the wind—frequently the onset of a cold wave.
A close study of the trace of the barograph pen will enable an observer to discern conditions, leading to fairly certain predictions, which otherwise would pass unnoticed. Continuity of record constitutes, to a great degree, the value of pressure records, and while the records of hourly observations intelligently graphed, might lead to similar conclusions, few meteorological laboratories are so equipped as to make such observations possible. But the barograph catches and records minute pressure alterations that might escape the notice of the most careful observers even if hourly observations were recorded.
There is not much difficulty in comparing the graphs of metric charts with those of inch charts. A base line for either may be drawn on the other, and distances or ordinates are readily measured with dividers or with a graduated scale. A record sheet ruled on tracing cloth affords a convenient method of comparison. The best method of comparison is the one which best suits the observer.
- ↑ The name is not connected with the Greek word meaning “air”; it is formed of two Greek words meaning “without a liquid.”
- ↑ An aneroid taken by the author from sea level to stations in Colorado varying from 8000 to 13,000 feet, responded promptly to the decrease in pressure on the outward trip. After it had been brought back to sea level, it registered an altitude of about 2000 feet. At the end of three weeks it still varied by nearly 0.3 inch from normal pressure. It was therefore sent to the manufacturer to be put in order. This illustration will apply in many instances; it does not apply to aneroids of the better class made at the present time.