Handbook of Meteorology/Mercury Barometers
THE MEASUREMENT OF PRESSURE: THE MERCURY BAROMETER
Two terms may be used to express the gravity of the air—weight and pressure. The weight of air is used more properly to express the gravimetric force of a given volume: thus, 1 cubic foot of air at normal pressure and temperature of 32° F weighs 1.29 troy ounces (1 cu. cm. = 0.0013 gram). In meteorology it is more convenient to consider the weight of a column of air throughout its whole extent, from sea level upward. At sea level such a column of air presses upon the surface with an average force of 14.7 pounds, a pressure empirically termed 1 atmosphere.
The pressure of such a column varies, however, not only from day to day, but from hour to hour. If a mass or a wave of air accumulates over a given locality the pressure increases; conversely, if a depression, or a trough, occurs, the pressure decreases. As the observer goes from sea level to a higher altitude the pressure decreases also. At an altitude of 19,000 feet, a height occasionally reached by airmen, the pressure is about half of that at sea level.
The pressure of the air may be determined by weighing it; it is much more convenient to compare it with a column of mercury which balances it. A balance constructed for this purpose is a barometer. The theoretical construction of the barometer is simple; it consists of a glass tube about 33 inches long, closed at one end, filled with mercury, and inverted with the open end in a cup of mercury. The column of mercury within the tube exactly balances a column of air of equal section. If air accumulates, the increased pressure on the mercury in the cup forces the column higher in the tube; conversely, a decrease in pressure causes the mercury column to shorten;
The mercury barometer. Sectional view showing construction of cistern
the decreased pressure cannot force so much mercury into the tube.
Construction of the Barometer.—A common form of the barometer is a siphon tube, the short arm of which is enlarged to a bulb, and this constitutes the mercury cistern. The barometric column is the distance between the upper and the lower surface of the mercury.
The Weather Bureau pattern is more complicated. The tube has a caliber usually about 0.25 inch. The cistern or tank containing the mercury consists of a short cylindrical glass tube. The cover is a piece of boxwood perforated to receive the barometer tube and flanged to fit the cylindrical glass section of the cistern. The lower part of the cistern is a broad ring, of boxwood, flanged and fitted to the lower edge of the glass cylinder. To the lower part of the boxwood ring a kidskin bag is attached. The mercury fills the bag and reaches nearly to the top of the glass cylinder. The cistern fits snugly into a cylindrical metal box. A plug within the cylindrical box, operated by a screw at the bottom, partly supports the bag of mercury. The construction may be likened to a glass tube projecting vertically from the mouth of a rubber bag filled with water. Pressure on the bag forces water up the tube; release of pressure causes it to lower in the tube.
The object of the leather bag and the plug is two-fold; it enables the observer to raise or to lower the mercury in the cistern so that the surface touches the ivory point which is the end of the scale, thereby giving a more accurate reading; it also enables the observer to close the cistern and tube, so that the barometer may be carried in any position without permitting the mercury to escape from the tube, or air to enter it.
The cistern of this pattern of barometer is commonly known as the Fortin cistern. It was improved by Henry J. Green. The improved pattern is used by the Weather Bureau. The Tuch cistern, also used by the Weather Bureau, has a piston bottom that can be raised and lowered by a thumb-screw. The Weather Bureau pattern of the barometer made by the Taylor Instrument Companies has a similar device. In each, a stopper pressed against the mouth of the tube secures the mercury so that the barometer may be transported with a minimum of risk.
Fixed-cistern barometers have not been favorably considered by meteorologists. The objection to them on the whole is not well founded. If the scale has been compensated a fixed-cistern barometer will meet all the requirements of accuracy demanded by ordinary meteorological measurements. It is more serviceable for use at sea than a cistern of the Fortin type. It is less likely to injury in transportation.
The necessity of a compensated scale may be understood from the following facts. The sectional area of the cistern is about fifty times the sectional area of the tube. If the atmospheric pressure increases, say, from 29.00 inches to 30.00 inches the rise of 1 inch in the tube is balanced by a fall of 0.02 inch in the cistern. The true height of the column therefore is 30.02 inches. When the sectional areas of both cistern and tube have been accurately determined, an empiric scale compensated for the instrument may be engraved to meet the requirements of accuracy. Should the tube be broken, however, a new scale will be required inasmuch as the caliber of tubes varies considerably.
A barometer with a fixed cistern, made by Schneider Brothers, is highly regarded among officials of the Weather Bureau. A feature of this barometer is the facility with which the mercury can be made secure within the cistern and tube, so that the instrument will not lose its adjustment.
Fixed-cistern barometers may not meet the requirements of precision measurements so well as instruments of the Fortin type of cistern; but for marine purposes or for field work, their simplicity of construction and stability commend instruments of this character. When securely adjusted, they may be transported over rough wagon roads and carried in any position without especial care.
The tube of the barometer is inclosed in a case with the necessary openings which permit the height of the mercury column to be read. Weather Bureau barometers are of the “gun-barrel” type, tube and cistern being inclosed in a cylindrical metal case. Openings, or “windows” are cut in the sides so that the top of the column of mercury is always in sight.
The scale of Weather Bureau barometers is on the left side of the window. It is a strip of white metal, with slotted screw holes so that it may be adjusted to compensate corrections which are constant, especially capillarity. The inch is divided into tenths and subdivided into twentieths. The scales of commercial barometers are usually without compensation adjustments.
The vernier enables the observer to read the height of the column to two one-thousandths of an inch, and to estimate it to a one-thousandth part. On commercial barometers the vernier enables the observer to read accurately to the one-hundredth part of an inch. A rack and pinion moved by a milled screw enable the observer to adjust the vernier to the height of the mercury.
The thermometer set into the metal case of the Weather Bureau barometer is always a standard instrument, whether carrying a certificate or not. The scale is etched on the tube to single degrees, but it may be read to half-degrees in accordance with the temperature corrections which are calculated to half-degrees.
The Installation of the Barometer.—When a barometer is sent from the manufacturer, or is issued from the Weather Bureau, it is pretty certain to be in good order and ready for installation. It is an almost universal custom to wrap the instrument first in tissue paper, then in cotton flannel, and finally in stout wrapping paper. The packing case should be so large that the elasticity of the packing material will compensate any jar that may occur from ordinary handling. Barometers sent out by the Weather Bureaus are usually packed in cases designed for the purpose. Ordinary precaution suggests that the cover of the packing case should be fastened with screws and not with nails.
When a barometer is sent by messenger it should be sent either in a leather case or a box designed for the purpose, with the handle so placed that it must be carried cistern uppermost. It should not be allowed to rest with the end on the floor of a moving vehicle.
Before the barometer is removed from the packing case, the position most advantageous for it should be determined. A wall or partition that is easily shaken should be avoided. A position on a window frame or near the corner of a room is often the best available. A position where the temperature is not subject to sudden change is very desirable.
Weather Bureau barometers are provided with a glass-paneled containing box, the front and right side of which swing open. Many observers prefer a plain board mounting. Marine barometers are usually contained in a box with an outrigger which permits them to be removed to a position convenient for reading.
The box or supporting board must be mounted so as to be vertical in all meridians. Metal eyelets, or hangers, accompany the supporting box or board. When in place, there should be no “wiggle” or dead motion. The hangers will be found in such a position that the barometer swings in the middle of the lower ring.
When the barometer is removed from the packing case it should be lifted, cistern uppermost, and laid on a table or bench to be unwrapped. Until it is finally in position it should be moved about cistern uppermost. When the wrappings are removed, it should be carried cistern uppermost to the support and turned carefully top end up. The cistern end should be put within its supporting ring before it is hung upon the hook of the support. If the box or the supporting board has been accurately leveled, the cistern will swing freely in the supporting ring. The centering screws in the ring may then be turned until each barely touches the cistern box. In case the screws are lost, pegs of soft wood, whittled to the right size, will answer temporarily. The case of the barometer should turn freely on the swivel, but there should be no dead motion.
The Care of the Barometer.—Except in unusual cases, a mercurial barometer should be kept indoors in a position where the temperature is as nearly uniform as possible. At temperatures materially below 10° F the readings of barometers side by side may vary enough to give concern to a conscientious observer. When the temperature is materially below zero, F, at an altitude of 5000 feet, more or less, the readings are often of uncertain value. The moral is obvious. Uniform and constant conditions are necessary for uniform results.
The compensation for capillarity is usually corrected by adjustment of the scale. Mercury does not “wet” glass; therefore the surface of the tube not only tends to retard the rise of the column, but prevents the mercury from assuming a level surface at the top. The rounded surface is the meniscus, the shape of which changes from time to time, as pressure varies.
With a rising column the convexity is visibly greater than with a falling column. The larger the bore of the tube the less the correction for capillarity. A tube with a bore of less than 0.25 inch should be avoided. Inasmuch as the meniscus of the larger tube has a narrower range, the readings during changes are a little more accurate with a tube of larger bore.
When a barometer is new, the surface of the mercury is very bright. In the course of two or three years—or less—the surface of the mercury in the cistern may become oxidized, turning gray. Although unsightly, this condition offers no material interference with accurate reading. In time, also, the vacuous part of a poorly constructed barometer may acquire a gray tint owing to the use of impure mercury in filling the tube and cistern.[1] Although this may not affect the reading appreciably, it is a mark of careless workmanship, and such an instrument should be sent to a reputable maker to be refilled with clean mercury.
After a few years of service the film on the surface of the mercury may require cleaning. Emptying, cleaning, and refilling a barometer tube is a delicate task even for trained experts; it should not be attempted by one without experience.
A clean room, free from dust, is desirable for barometers. Dust is not preventable, but it should not be permitted to accumulate on instruments. A soft, damp—not wet—cloth will remove and gather it without scattering; a camel’s hair brush will remove it from corners and crevices which the cloth does not reach. If glass cylinder and tube are clean and bright there need be but little error in setting the mercury to the scale, or in cutting the top of the meniscus sharply by the sliding windows.
If a barometer is to be removed from its support the mercury in the cistern should first be raised until the mercury in the tube is flush with the opening near the top of the case. If it is to be removed to a position materially lower, allowance should be made for the increase in pressure. Unless this precaution is observed, the pressure may be great enough to force mercury through the joints of the cistern.
The following instructions concerning the removal from position are issued by the Weather Bureau: “When moved about, the cistern end should be carried uppermost. To turn the barometer tube-end up, bring it gradually to a horizontal position, watching for a small bubble at the cistern. This should not be large, nor should it be absent, in which case there may be serious pressure from within, tending to force the mercury out of the cistern. If necessary the adjusting screw should be turned so that the bubble is not larger than the space within which a dime can be placed. If there is an air vent, as in the Tuch cistern, as soon as the mercury is raised to the top of the cistern, close the air vent tight and continue screwing up the cistern until the top of the column reaches the summit of the opening in the metal tube. Avoid raising the mercury in the cistern until the tube is entirely filled with mercury. Do not strain the screw if it turns hard; mercury may have leaked from the cistern and there may not be enough to fill the tube." If no air has entered the upper end of the tube, when the barometer is inclined about half way the mercury will rise to the top of the tube with a slight but distinct click; and when the instrument is nearly horizontal a bubble should appear at the cistern.”
The foregoing cautions apply to Fortin type barometers chiefly, but will apply in some respects to other types. In the installation, removal, and care of other barometers, the directions of the makers should be followed. Marine barometers are provided with tubes a considerable portion of which is constricted to prevent the “pumping” of the mercury which the motion of the vessel would otherwise cause. The constriction prevents the vacuous part of the tube from filling quickly. The barometer must be inclined gradually, waiting until the flow ceases. By the time it is inclined about 40 degrees the mercury will have filled the tube. It can then be inverted and moved about in that position. Because the cistern is partly filled only, a marine barometer is easily put out of adjustment during transportation.
When a barometer is inclined so that the mercury is near the top of the tube, a slight lengthwise movement will cause it to flow to the top, striking it with an audible “click.” There is a tradition that the character of the vacuum can be determined by the character of the sound; but inasmuch as trained experts are sometimes deceived, the value of the click as a test is uncertain; and inasmuch as such a practise is likely to break the tube, the negative value is pretty certain.
Barometer Scales and Standards.—For many years barometric pressure was expressed in the linear units of the country. The adoption of the metric system in several states of Europe changed the use of local units to metric units. The metric system has been authorized to be used in the United States, but the use has not been made compulsory. It is employed in laboratories and in certain scientific work, but not in the manufacture of precision machinery unless definitely ordered. It is not used for commercial purposes in English-speaking countries. In the latter, barometric pressure is expressed in inches. Metric scale barometers are furnished on order by the makers.
So far as choice between the two scales is concerned there is not much difference. Each is intelligible in the locality where it is used. So far as the keeping of records is concerned there is neither gain nor loss; each requires four figures and a decimal point.
Physicists who use the metric system of measurements find it convenient to use the dyne—a force that will impart to one gram an acceleration in velocity of one centimeter per second—as the unit of pressure. The pressure base proposed for barometric measurements is 1,000,000 dynes. This value is not sea level pressure, but the average pressure at a height of 106 meters (348 feet) above sea level. The unit is the kilobar, or 1000 millibars. The conventional atmosphere of 29.92 inches is 1013.2 millibars.
To the great majority of observers any barometer scale is more or less empiric. By long training and habit one gradually acquires a mental value of the figures which express pressure and these become visual proportions that can be compared in the mind. It is difficult to change the results of this education; it likewise requires time. So far as expression of barometric terms of pressure are concerned, there is not the slightest gain in the substitution of the metric for the inch scale or vice versa.[2] When records have covered considerable periods of time a change of either to the other results not only in confusion but in positive loss.
Barometer Observations and Records.—Weather Bureau barometer records are made at 8:00 a.m. and 8:00 p.m.. Observers usually note any changes that may have occurred during the day. Making an observation for record that shall meet the demands for reasonable accuracy requires a certain amount of experience and familiarity with the barometer.
Inasmuch as temperature, pressure, humidity and wind observations are to be taken at clock time, and all these require about ten minutes in the aggregate, it is sometimes necessary to decide quickly as to preference of order. During heavy storms a variation in barometric pressure may change visibly; and in winter weather, temperature may rise more than 1 degree between 8:00 a.m. and 8:10 a.m. Judgment and experience must determine. As a rule, however, two minutes will be a generous allowance of time for temperature observation. To make such observations habitually out of the established time should be a good reason for looking with suspicion upon the records thus made; if for any reason an observation is made out of time, the fact and the time should be noted. Slipshod practise in the time of making observations may not impair the results, but they certainly impair the character of the observer.
Because body warmth may affect the attached thermometer, the temperature should first be noted. It is best to record the temperature to the nearest half-degree. In field work, especially, if the temperature is within a few degrees of the freezing point, reading the temperature to the nearest degree will be sufficient for ordinary determination. Below 29° F the temperature corrections are additive; above 28° F they are subtractive.
The milled screw at the bottom of the case raises or lowers the mercury to the scale. When the surface of the mercury in the cistern touches the ivory point it is at the zero of the scale and the distance to the top of the column is the observed height.
Adjusting the surface of the mercury to the ivory point is best accomplished in many instances by the use of artificial light. Where convenient an extension socket to the nearest light plug is the best method; a flashlight will answer all purposes. There are several ways to determine tangency of the ivory point and the mercury:
Contact between the point and its shadow on the surface of the mercury.
Making a visible dent in the mercury with the ivory point; then lowering the surface until the dent disappears.
With the eye in the horizontal plane of the end of the ivory point, noting the position when the light space between the point and the mercury ceases to appear.
Observers usually prefer the last method. In practise, the first method is associated with it. The second method is fairly safe when the surface of the mercury is bright, but it is not easy to discern the dent if the mercury is tarnished. Experience will usually determine the method by which the observer will obtain the most accurate results.
Setting the vernier scale exactly to the meniscus, or rounded top of the mercury in the tube, is not always easy. The first requisite is a clean tube. The film that gathers upon the outside of the tube in damp weather catches dust and interferes with the transparency of the glass. The moral is obvious: the tube should be clean. The refraction of the glass has a tendency to produce a “drop,” making it slightly difficult to adjust the two edges of the vernier shutter so that the line of sight is precisely tangent to the meniscus. The eye, of course, must be in a line with the edges of the windows.
A very great part of the value of barometer observations consists of the knowledge that may be obtained by comparisons. In order to compare observations they must be reduced to a common base; namely, a temperature of 32° F and sea level. The temperature correction, except as noted, is subtractive; the altitude correction is additive, except as the station may be below sea level. Death Valley and Imperial Valley, California, are stations in the United States to which this exception applies.
A correction for latitude is required at Weather Bureau stations. This correction in the United States, Alaska excepted, varies from nothing at Lat. 45° to 0.05 inch in the southern part of the country. It is additive in latitudes higher than 45° and subtractive in latitudes lower. Being a constant, it may be included in the sea level reduction.
Except for weather bureau records, or for comparison with sea level records, reduction to sea level is not necessary. In general, station-altitude readings and the oscillations in pressure are of greater value to the observer than reduced readings. This is notably the case in aviation. It is often necessary to know whether one is entering a region of increasing or of decreasing pressure. The difference involves not only questions of plane support; it is also the distinction between clearness and cloudiness.
Obtaining Station Altitudes.—The altitude of a permanent station should be determined as closely as is possible with ordinary facilities. Two points, a “plane of reference” and a station fixed point are required. The first should be, if possible, a bench mark of the United States Coast Survey, the Lake Survey, the Mississippi River Commission, the Engineer Corps, or the United States Geological Survey. Of less precision are railway levels and city bench marks, and other surveys made by engineers; they will be found useful for reference even when their precision is doubtful. Railway station levels are reasonably precise. The top of the rail at a designated point within yard limits may be taken as a plane of reference.
If a precisely determined elevation is required it can be obtained best by a survey from the most accessible established bench mark. The station fixed point should be made on some object that is both fixed and durable. A young and rapidly growing tree is not a desirable object for a station mark; but a mark made on a full-grown tree is not subject to material change. A stone post set firmly in the ground, or a piece of painted scantling attached firmly to the corner of a building will answer the purpose. The mark should be of such a character that it will resist ordinary weathering. The final point in the determination is the station barometer, that is, the chain of determinations which begin at an established plane of reference and end with the ivory point within the cistern of the barometer.
The term “sea level” is differently interpreted in different localities, “Mean tide,” “mean low tide,” “mean high tide,” and “mean sea level” are used. If the local usage does not conform to that of established Federal usage, the nearest established Plane of Reference[3] practicable should be sought as a starting point. The Weather Bureau has established a specific elevation for each of its stations; the nearest station practicable therefore may be taken as an initial point.
Altitudes by Comparative Barometric Observations.—Reasonably correct altitudes may be established by synchronous observations, one series at an established altitude, the other at the place whose altitude is to be determined. For this purpose the position of known altitude should be a Weather Bureau station or an observatory having a standard barometer and an observer of experience. If a mercurial barometer is used at the location whose altitude is to be determined, it should be allowed to “rest”—that is, to adjust itself to the altitude—for a few days, if possible.
The readings may be made hourly at the same time at both stations, the height of the mercury, time and temperature correction being noted. This may be repeated for several days until the reduced readings are constant. If the stations are not far apart, a single series of observations may suffice; if they are more than 25 miles apart differences in pressure other than those due to altitude may interfere.
For instance, six consecutive observations between the stations show a constant difference, and the lower pressure at the upper station may be assumed as a difference in pressure due to altitude.
- ↑ Pure mercury in a saucer made chemically clean leaves no stain or metallic film when shaken about. If it contains even a trace of other metal—lead, zinc, or tin—spots and streaks will adhere to the saucer. If the mercury is not freshly distilled it may contain moisture. Mercury may be freed of its own oxide by filtering through a clean paper funnel with a pin-hole perforation at the bottom. The mercury used in Weather Bureau barometers is chemically pure.
- ↑ If a change from the English mercury-inch system should become desirable, the millibar scale would be considered preferable to any other so far proposed. At Greenwich, where the acceleration is 981.17 centimeters, the standard of pressure is 1,013,800 dynes; at Paris it is 1,013,600 dynes; in the United States (U. S. Coast Survey determination for Lat. 45°) the standard is 1,013,200 dynes, acceleration 980.62 centimeters or 32.16 feet.
- ↑ Planes of Reference established by the U. S. Geological Survey are established with reference to mean sea level.