Watt barometer

In barometers, Watt barometer, aka “Steam gauge” (Watt, 1818) or “barometer gauge” (Farey, 1827), or “manometer” (Cardwell, 1971), refers to []

In 1765, James Watt, in the development of his steam engine (see: Watt engine) used a standard Torricellian barometer, which he attached to his condenser, so to ascertain a reading of the pressure of the steam in the piston and cylinder.

In 1818, Watt described his pressure-gauge, shown adjacent, as follows:

“The steam-gauge is a short glass tube with its lower end immersed in a cistern of mercury, which is placed within an iron box screwed to the boiler steam-pipe, or to some other part communicating freely with the steam, which, pressing on the surface of the mercury in the cistern, raises the mercury in the tube (which is open to the air at the upper end), and its altitude serves to show the elastic power of the steam over that of the atmosphere.”
James Watt (1818), “Appendix Notes on John Robinson’s Steam-Engine Articles” [2]

In 1827, John Farey, in his “Barometer gauge” section, of his A Treatise on the Steam Engine, gives the following summary of the pressure gauge: [3]

Barometer gauge. This is an instrument to indicate the degrees of exhaustion within the condenser; it is constructed like a weather-glass or barometer, except that the upper end of the glass tube communicates by a small copper pipe with the condenser: the lower end of the glass tube is open, and is immersed in mercury contained in a small cistern or basin, the surface being exposed to the atmospheric pressure. The glass tube, which is about 32-inches long, is placed in a perpendicular direction, and the copper pipe from the condenser is very closely cemented to the glass at the upper end. When the engine is at work, and a vacuum is formed in the condenser, the barometer tube is exhausted, so that the surface of the mercury in that tube is relieved from pressure, and the weight of the atmosphere which presses upon the surface of the mercury in the basin, causes a column thereof to mount up in the tube, to a greater or less height according as the vacuum in the condenser is more or less perfect, and according to the weight of the atmosphere, which will be shown by a common Torricellian barometer or weather-glass.

The pipe which leads from the condenser to the top of the barometer tube must be provided with a cόck , which should be only very partially opened, or else the variations of the vacuum at every stroke of the engine, would occasion such continual oscillations of the height of the mercury in the tube, as to prevent any accurate observation. A ruler or scale divided into inches, and numbered up. wards from the surface of the mercury in the bason, is fixed behind the tube to measure the height to which the mercury rises therein, in the same manner as the scale of a common weather-glass.

When an engine is in good order, free from leakage and with cold injection water, the cόck of the barometer being opened very gradually, the mercury will ascend in the tube to 28 inches above the level of the surface of the mercury in the basin, the ordinary atmospheric barometer at the same time standing at 30 inches. This shows that the elasticity of the rare steam which remains uncondensed in the condenser, is equal to the pressure of a column of two inches of mercury. According to the table of elasticities, the temperature of that steam would be near 102 deg. Fahrenheit, supposing that no atmospheric air is present; but as some air is always mixed with the rare steam, and augments its elasticity, the temperature of the water discharged into the hot well will be about 100 degrees.

The mercury will be observed to rise gradually, and fall suddenly, at every stroke of the engine. The vacuum is most perfect towards the end of the returning stroke, when the piston has risen to the top of its course, because the greatest cold has been then produced by the injection, and also the air-pump has just taken its stroke, and exhausted part of the air, water, and steam, from the condenser; at this period, the mercury will often stand at 28 or 28} inches, but when the exhausting valve is opened, and the steam is admitted into the condenser, the mercury will fall rapidly to 27 or 26, inches; but before the piston has completed its stroke, the mercury will become stationary, because the cylinder being exhausted, no more steam is let into the condenser; the mercury will even begin to rise again slowly, because the injection-water continues to flow and condense the steam. After the injection is shut, and whilst the engine makes its returning stroke, the mercury will continue to rise with more rapidity, in consequence of the condenser being further exhausted by the ascent of the air-pump bucket; and the mercury will have regained its original height of 28 or 28; inches, by the time the piston reaches the top of the cylinder.

The cόck of the barometer tube should be shut when the observation is finished, and should always remain so. It must be shut when the engine is blowing through, to avoid throwing the mercury out of the bason. The basin for the mercury must be large enough to contain all the mercury, because when the engine is not at work, the air will leak in, and the mercury will descend into the basin.

As glass tubes are liable to be broken, the barometer tube may be made of iron, in the form of an inverted syphon, like the steam-gauge, before described (pg. 872), one leg communicating with the condenser, and the other left open to the air, the rise of the mercury in one leg, will produce a corresponding fall of the mercury in the other; and the scale being applied to only one leg, the divisions must be half inches, that is, provided the two legs are of the same bore; but if they are different, the scale must not be half inches, but must be divided by trial, in a proper proportion to show the true difference of level between the surfaces of the mercury in the two legs.

The inverted syphon for this barometer may be made of glass; but both parts of the tube, must be correctly of one diameter, or else the result will be erroneous; it is difficult to graduate a scale by experiment in an iron tube, because the difference of level of the mercury in the legs cannot be seen. This barometer tube must communicate with the condenser by a small copper pipe, and a stop-cόck. The index is a light wooden rod, which is put into the open leg of the tube, the same as in the steam-gauge; the mercury being poured into the tube to within a few inches of the open end, the rod floats on the surface of the mercury, and its upper end points to the divisions on the scale ; the numbers on that scale must be inverted with respect to those of a single tube.

The engine barometer shows the perfection of the vacuum, or the pressure of the atmosphere to enter into the o space; the steam-gauge shows the pressure of the steam to escape into the air, and the height of these two columns added together, shows the utmost force which the steam is capable of exerting on the piston, supposing that the steam met with no resistance whatever, in passing from the boiler into the cylinder, or in passing out of the cylinder into the condenser.”

In c.1785, Watt later tried to affix a mercury barometer directly to the piston and cylinder, but the vibrations or something made this untenable; the following is one statement on this:

“The barometer being adapted only to ascertain the degree of exhaustion in the condenser where its variations were small, the vibrations of the mercury rendered it very difficult, if not impracticable, to ascertain the state of exhaustion of the cylinder at the different periods of the stroke of the engine.”
— James Watt (c.1675), Publication [1]
In 1790, Watt, working with his employee Matthew Boulton, supposedly (check), had invented the spring-style barometer, as seen used in the indicator.
See also
● Torricellian barometer
● Guericke barometer

1. Miller, David. (2015). James Watt, Chemist: Understanding the Origins of the Steam Age (pg. #). Routledge.
2. Robinson, John. (1818). The Articles Steam and Steam-Engines: Written for the Encyclopedia Britannica, by the Late John Robinson, with Notes and Additions by James Watt (§: Letter from Mr Watt to Dr Brewster on the History of the Steam Engine, pgs. iii; §: On Steam, pgs. 1-45; §: On the Steam Engine, pgs. 46-151; §: Appendix by Mr Watt, pgs. 152-80). Edinburgh: Murray.
3. Farey, John. (1827). A Treatise on the Steam Engine: Historical, Practical, and Descriptive (§: Barometer gauge, pgs. 377-79). Longman.

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