DIFFERENTIAL PRESSURE INDICATORS FOR DETERMINING THE POWER OF STEAM ENGINES OR OTHER MOTORS."
Year: 1879

page_text: PROVISIONAL SPECIFICATION left by Robert Willis at the Office of the
Commissioners of Patents on the 24th April 1879.
"IMPROVEMENTS IN
ROBERT WILLIS, of No. 56, Leadenhall Street, London.
DIFFERENTIAL PRESSURE INDICATORS FOR DETERMINING THE POWER OF STEAM
ENGINES OR OTHER MOTORS."
The main object of my improvement is to indicate at all times, if desired, the
maximum, minimum, and mean pressure of steam in the cylinders of engines by a
pointer on a graduated dial, and when using varying pressures of steam from the
boiler, or when the pressure is varied in the cylinder by the use of any system of
cut off valves. It also shews any improper set or irregular working of the steam
valve or valves, particularly in cases where more pressure is admitted to one than
the other end of the cylinder.
The indicator in one of its forms is essentially composed of two chambers of any
shape. One of these chambers is kept in constant communication with the steam
end of the cylinder while in use by means of the automatic working of a double
valve placed in a suitable position between the chambers. The valve is worked in
such a manner that by the pressure of steam from the cylinder alternately acting
on each end of it, so that it closes the communication of the said chamber with the
exhaust end of the cylinder, and opens communication with the steam end of the
cylinder.
The body of the indicator composing the two chambers may be made of any form
desired, but it is usually made cylindrical from five to six inches in diameter, more
or less, and of the proper length, and of any suitable metal strong enough to resist
the required pressure. The ends of the indicator may be secured to it by screws,
bolts, or partially or wholly cast to the indicator. The said two chambers of the
body of the indicator is usually formed by a diaphragm across its internal diameter,
an opening is made through this diaphragm from side to side of the indicator,
admitting steam from the cylinder, and receiving the automatic double valve for
alternately admitting steam from each end of the cylinder, and closing communica-
tion with the exhaust end in the manner before stated.
Two small openings are usually made to place the lower chamber in the body of
tho indicator in steam communication with the cylinder by the valve and steam
passage in the diaphragm. The other chamber, which is generally the upper one in
the indicator when it is set in position for working, is put in communication with
rD...931

page_text: the lower chamber by means of an opening or openings made in any suitable part
of the diaphragm, and provided with an adjustable valve or valves made to open
into the upper or lower chambers for increasing or diminishing the area of the said
valve openings between the two chambers, for shewing the maximum, minimum,
and mean pressure of steam in the cylinder at any time or point of the stroke of
the piston. For obtaining the mean pressure the area of the valve opening is
contracted sufficient to cause the pointer on the indicator dial to vibrate but little,
or to come to rest, and the pressure marked on the dial between the two points of
vibration will be found the mean pressure in the cylinder. It is necessary to use
suitable mechanical means for moving and holding the valve in position, so that
the area of the valve opening may be increased or diminished to the smallest
possible degree, and retain it at any desired area by the proper working of the
valve.
The indicator is usually placed at an equal distance between the ends of the
cylinder, and put in communication therewith by means of pipes and cocks; when
not in use, or for other purposes, the communication is shut off by the cocks usually
placed at some convenient part in the pipes or indicator if desired.
In putting the indicator to work, the lower chamber is usually supplied with
water or other liquid for the steam pressure to act on; the water is forced from
the lower to the upper chamber through a pipe extending nearly to the bottom of
the lower chamber, and connected with the regulating valve opening, by which the
two chambers are put into communication in the manner before described. As the
water is forced to the upper chamber, air is trapped in the upper part of it, and
thereby gives an elastic resistance for the steam pressure to act on the indicating
mechanism for moving the pointer on the dial. If desired, the water or liquid may
be dispensed with in the indicator chamber, and the steam permitted to act direct on
the indicator mechanism for shewing the pressure. By the use of steam alone the
indicator would be more sensative to slight variations of pressure, and consequently
act quicker, and more likely indicate the true pressure of steam at all times. When
using steam alone in the indicator chambers, means must be employed to keep both
chambers free from condensed steam.
The use of my improved indicator readily detects any defect in the set or working
of the induction or eduction valves of the engine by shewing if more steam is
admitted to one end of the cylinder than the other, or if the exhaust is choked, or
the valve or valves set in any way to impede the proper working of the engine.
It may happen from the condensation of steam and loss of air that the chambers
may become too full of water; this is provided, for by cocks attached to suitable
parts of the chamber to draw off the water and resupply it with air. The indicator
may be constructed double or duplex in such a manner as to have a single pointer
to indicato steam pressure at one end of the cylinder only, or two independent
indicators may be used for the same purpose, one to indicate the pressure in one end
of the cylinder only.