The Barometer

A barometer is an instrument for denoting the pressure or weight of the atmosphere, which it does by means of a column of mercury inclosed in a tube, in which there is a vacuum, which may be produced as follows:

A tube having a parallel bore and closed at one end is filled with mercury and while the finger is placed over the open end of the tube, it is turned upside down and inverted in a cup of mercury that is open to receive the pressure of the atmosphere.

The finger is then removed from the end of the tube and the mercury will fall, leaving a vacuum at its upper end.

The pressure of the atmosphere on the surface of the mercury in the cup forces the mercury up the tube, because the surface of the mercury in the tube has no atmospheric pressure on it, the action being the same as that already described with reference to the principles of action of a pump.

The weight of the atmosphere is equal to the weight of that part of the column of mercury that is above the surface of the mercury in the cup, hence lines may be drawn at different heights representing the weight of the atmosphere, or of any other gas, when the column of mercury stands at the heights denoted by the respective lines.

But as mercury expands by heat, a definite degree of temperature must be taken in marking a column, to represent the weight, this temperature being 32° Fahrenheit.

Similarly, as the weight of the atmosphere varies, according to the height at which it is taken from the surface of the earth, a definite height must be taken.

The sea level is that usually taken, the mean or average atmosphere (at that level) being 14.7 lbs. per square inch.

For higher altitudes, the mean atmospheric pressure in lbs. per square inch may be found by multiplying the altitude or height above sea level by .00053, and subtracting the product from 14.7.

Each pound on the square inch is represented by a height of 2.036 inches of mercury, hence the height of a column of mercury at a temperature of 32° that will balance the mean weight of the atmosphere is 29.92 inches, and to avoid fractions, it is usual (for purposes not requiring to be very exact) to say that the atmospheric pressure at sea level is represented by 30 inches of mercury.

The atmospheric pressure is also, to avoid using fractions, taken roughly at 15 lbs. per square inch at sea level.

Each 2 inches of mercury will, under these conditions, represent 1 lb. of pressure.

Vacuum gauges are based upon the same principles and subject to the same variations as to altitude as mercury gauges or the barometer.

To find the absolute pressure, or pressure above zero, or a perfect vacuum, we may add the pressure of the boiler steam gauge to that shown by the mercury gauge or barometer.

In Fig. 3364 the line of no pressure is marked at 15 lbs. per square inch below the atmospheric line of the diagram, the atmospheric pressure being for convenience taken as 15 lbs. above a perfect vacuum.

The line of no pressure serves as a guide in showing the effectiveness of the condenser, as well as for computing the volume of steam used, but is not necessary in computing the horse power of a non-condensing engine, because the gauge pressure has its zero marked to correspond with the atmospheric pressure.

In computing the consumption of steam or water from the diagram, therefore, both the clearance line and the line of no pressure must be marked on the diagram, and lines of the diagram extended so as to include them, thus accounting for all the steam that leaves the steam chest from the piston stroke.

Indicator springs are varied in strength to suit the pressure of steam they are to be used for.

The scale of the spring is the number of lbs. pressure per square inch represented by a vertical motion of the pencil; thus, a 40 lb. spring is one in which a pressure of steam of 40 lbs. per square inch would cause the piston to rise an inch above the atmospheric line of the diagram.

The strength or tension of the spring is so adjusted as to cause the diagram to be about 2⁄2 inches high, let the steam pressure be what it may. The following are the scales of springs of the Thompson and Tabor indicator.

Thompson Indicator.

Scale of
spring. Used for pressure above
atmosphere if not more than

15 lbs. 21 lbs. per square inch.

20 „ 38 „ „ „ „

30 „ 94 „ „ „ „

40 „ 90 „ „ „ „

60 „ 143 „ „ „ „

Tabor Indicator.

10 lbs. 14 „ „ „ „

12 „ 20 „ „ „ „

16 „ 30 „ „ „ „

20 „ 40 „ „ „ „

24 „ 48 „ „ „ „

30 „ 60 „ „ „ „

32 „ 64 „ „ „ „

40 „ 80 „ „ „ „

48 „ 96 „ „ „ „

50 „ 100 „ „ „ „

60 „ 120 „ „ „ „

64 „ 128 „ „ „ „

80 „ 160 „ „ „ „

A spring that is strong enough for a given pressure may be used for any less pressure.

The height of the diagram will, however, be less, and accuracy is best secured by having the diagram up to the limit of about 2⁄2 inches, using a spring that is light enough to secure this result.

Diagrams of high speed engines, however, will have their lines

more regular in proportion as a stronger spring is used.

This occurs because the spring, being under more tension, is less liable to vibration.

An indicator requires careful cleaning and oiling with the best of oil, as the slightest undue friction seriously impairs the working of the instrument.

Instructions upon the care of the instrument, and how to take it apart, etc., are usually given by the makers of the indicator.

There are various methods of giving to the paper drum of the indicator a motion coincident with that of the engine piston, but few of them give correct results.

Reducing levers, such as shown in Fig. 3365, are constructed as follows:

Fig. 3365 represents a reducing lever with the indicators attached. a c is a strip of pine board three or four inches wide and about one and one-half times as long as the stroke of the engine. It is hung by a screw or small bolt to a wooden frame attached overhead. A link c one-third as long as the stroke is attached at one end to the lever, and at the other end to a stud screwed into the cross head, or to an iron clamped to the cross head by one of the nuts that adjust the gibs, or to any part of the cross head that may be conveniently used. The lever should stand in a vertical position when the piston is at the middle of the stroke. The connecting link c, when at that point, should be as far below a horizontal position as it is above it at either end of the stroke. The cords which drive the paper drums may be attached to a screw inserted in the lever near the point of suspension; but a better plan is to provide a segment, a, b, the centre of which coincides with the point of suspension, and allow the cord to pass around the circular edge. The distance from edge to centre should bear the same proportion to the length of the reducing lever as the desired length of diagram bears to the length of the stroke. On an engine having a stroke of 48 inches, the lever should be 72 inches, and the link c 16 inches in length, in which case, to obtain a diagram 4 inches long, the radius of the segment would be 6 inches. It is immaterial what the actual length of the diagram is, except as it suits the operator’s fancy, but 4 inches is a length that is usually satisfactory. It may be reduced to advantage to 3 inches at very high speeds. The cords should leave the segment in a line parallel with the axis of the engine cylinder.

The pulleys over which they pass should incline from a vertical plane and point to the indicators wherever they may be located. If the indicators and the reducing lever can be placed so as to be in line with each other, the pulleys may be dispensed with and the cords carried directly from the segment to the instruments, a longer arc being provided for this purpose. The arm which holds the carrier pulleys on each indicator should be adjusted so as to point in the direction in which the cord is received.

In all arrangements of this kind the reduced motion is not mathematically exact, because the leverage is not constant at all points of the stroke.

Pantagraph motions have been devised for overcoming these defects. Two forms have been successfully used, which, if well made, well cared for, and properly handled, reproduce the motion on the reduced scale with perfect accuracy. They are shown in working position in Figs. 3366 and 3367.

Fig. 3366 represents the manner of attaching the pantagraph motion, or lazy tongs, as it is sometimes called, when the indicators are applied to the side of the cylinder. It works in a horizontal plane, the pivot end being supported by a post b erected in front of the guides, and the working end receiving motion from an iron attached to the cross head.

By adjusting the post to the proper height and at a proper distance in front of the cross head, the cords may be carried from the cord pin c to the indicators, without the intervention of carrier pulleys.

If the indicators are attached to the side of the cylinder, the simplest form of pantagraph shown in Fig. 3367 may be used. The working end a receives motion from the cross head, and the front piece b is attached to the floor. The cord pin d is fixed in line between the pivot and the working end, and the pulleys e, attached to the block c, guide the cords to the indicators.

The indicator rigging that gives the best results at high speeds is a plain reducing lever like that first described, provided at the lower end with a slot that receives a stud, screwed into the cross head. The length of the lever should be one and one-half times the engine stroke, as given on the preceding page.

Whatever plan is followed, it is desirable to avoid the use of long stretches of cord. If the motion must be carried a long distance, strips of wood may often be arranged in their place and operated with direct connections. Braided linen cord, a little in excess of one-sixteenth of an inch in diameter, is a suitable material for indicator work.

To take a diagram, a blank card is stretched smoothly upon the paper drum, the ends being held by the spring clips. The driving cord is attached and so adjusted that the motion of the drum is central.

For convenience two diagrams, one from each end of the cylinder, may be made on the same card, as shown in Fig. 3368.